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diff --git a/docs/tutorial/LangImpl7.html b/docs/tutorial/LangImpl7.html deleted file mode 100644 index 0b46ba5..0000000 --- a/docs/tutorial/LangImpl7.html +++ /dev/null @@ -1,2164 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> - -<html> -<head> - <title>Kaleidoscope: Extending the Language: Mutable Variables / SSA - construction</title> - <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> - <meta name="author" content="Chris Lattner"> - <link rel="stylesheet" href="../llvm.css" type="text/css"> -</head> - -<body> - -<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div> - -<ul> -<li><a href="index.html">Up to Tutorial Index</a></li> -<li>Chapter 7 - <ol> - <li><a href="#intro">Chapter 7 Introduction</a></li> - <li><a href="#why">Why is this a hard problem?</a></li> - <li><a href="#memory">Memory in LLVM</a></li> - <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li> - <li><a href="#adjustments">Adjusting Existing Variables for - Mutation</a></li> - <li><a href="#assignment">New Assignment Operator</a></li> - <li><a href="#localvars">User-defined Local Variables</a></li> - <li><a href="#code">Full Code Listing</a></li> - </ol> -</li> -<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM - tidbits</li> -</ul> - -<div class="doc_author"> - <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p> -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language -with LLVM</a>" tutorial. In chapters 1 through 6, we've built a very -respectable, albeit simple, <a -href="http://en.wikipedia.org/wiki/Functional_programming">functional -programming language</a>. In our journey, we learned some parsing techniques, -how to build and represent an AST, how to build LLVM IR, and how to optimize -the resultant code as well as JIT compile it.</p> - -<p>While Kaleidoscope is interesting as a functional language, the fact that it -is functional makes it "too easy" to generate LLVM IR for it. In particular, a -functional language makes it very easy to build LLVM IR directly in <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>. -Since LLVM requires that the input code be in SSA form, this is a very nice -property and it is often unclear to newcomers how to generate code for an -imperative language with mutable variables.</p> - -<p>The short (and happy) summary of this chapter is that there is no need for -your front-end to build SSA form: LLVM provides highly tuned and well tested -support for this, though the way it works is a bit unexpected for some.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="why">Why is this a hard problem?</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p> -To understand why mutable variables cause complexities in SSA construction, -consider this extremely simple C example: -</p> - -<div class="doc_code"> -<pre> -int G, H; -int test(_Bool Condition) { - int X; - if (Condition) - X = G; - else - X = H; - return X; -} -</pre> -</div> - -<p>In this case, we have the variable "X", whose value depends on the path -executed in the program. Because there are two different possible values for X -before the return instruction, a PHI node is inserted to merge the two values. -The LLVM IR that we want for this example looks like this:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.2 -} -</pre> -</div> - -<p>In this example, the loads from the G and H global variables are explicit in -the LLVM IR, and they live in the then/else branches of the if statement -(cond_true/cond_false). In order to merge the incoming values, the X.2 phi node -in the cond_next block selects the right value to use based on where control -flow is coming from: if control flow comes from the cond_false block, X.2 gets -the value of X.1. Alternatively, if control flow comes from cond_true, it gets -the value of X.0. The intent of this chapter is not to explain the details of -SSA form. For more information, see one of the many <a -href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online -references</a>.</p> - -<p>The question for this article is "who places the phi nodes when lowering -assignments to mutable variables?". The issue here is that LLVM -<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it. -However, SSA construction requires non-trivial algorithms and data structures, -so it is inconvenient and wasteful for every front-end to have to reproduce this -logic.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="memory">Memory in LLVM</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>The 'trick' here is that while LLVM does require all register values to be -in SSA form, it does not require (or permit) memory objects to be in SSA form. -In the example above, note that the loads from G and H are direct accesses to -G and H: they are not renamed or versioned. This differs from some other -compiler systems, which do try to version memory objects. In LLVM, instead of -encoding dataflow analysis of memory into the LLVM IR, it is handled with <a -href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on -demand.</p> - -<p> -With this in mind, the high-level idea is that we want to make a stack variable -(which lives in memory, because it is on the stack) for each mutable object in -a function. To take advantage of this trick, we need to talk about how LLVM -represents stack variables. -</p> - -<p>In LLVM, all memory accesses are explicit with load/store instructions, and -it is carefully designed not to have (or need) an "address-of" operator. Notice -how the type of the @G/@H global variables is actually "i32*" even though the -variable is defined as "i32". What this means is that @G defines <em>space</em> -for an i32 in the global data area, but its <em>name</em> actually refers to the -address for that space. Stack variables work the same way, except that instead of -being declared with global variable definitions, they are declared with the -<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p> - -<div class="doc_code"> -<pre> -define i32 @example() { -entry: - %X = alloca i32 ; type of %X is i32*. - ... - %tmp = load i32* %X ; load the stack value %X from the stack. - %tmp2 = add i32 %tmp, 1 ; increment it - store i32 %tmp2, i32* %X ; store it back - ... -</pre> -</div> - -<p>This code shows an example of how you can declare and manipulate a stack -variable in the LLVM IR. Stack memory allocated with the alloca instruction is -fully general: you can pass the address of the stack slot to functions, you can -store it in other variables, etc. In our example above, we could rewrite the -example to use the alloca technique to avoid using a PHI node:</p> - -<div class="doc_code"> -<pre> -@G = weak global i32 0 ; type of @G is i32* -@H = weak global i32 0 ; type of @H is i32* - -define i32 @test(i1 %Condition) { -entry: - %X = alloca i32 ; type of %X is i32*. - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - store i32 %X.0, i32* %X ; Update X - br label %cond_next - -cond_false: - %X.1 = load i32* @H - store i32 %X.1, i32* %X ; Update X - br label %cond_next - -cond_next: - %X.2 = load i32* %X ; Read X - ret i32 %X.2 -} -</pre> -</div> - -<p>With this, we have discovered a way to handle arbitrary mutable variables -without the need to create Phi nodes at all:</p> - -<ol> -<li>Each mutable variable becomes a stack allocation.</li> -<li>Each read of the variable becomes a load from the stack.</li> -<li>Each update of the variable becomes a store to the stack.</li> -<li>Taking the address of a variable just uses the stack address directly.</li> -</ol> - -<p>While this solution has solved our immediate problem, it introduced another -one: we have now apparently introduced a lot of stack traffic for very simple -and common operations, a major performance problem. Fortunately for us, the -LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles -this case, promoting allocas like this into SSA registers, inserting Phi nodes -as appropriate. If you run this example through the pass, for example, you'll -get:</p> - -<div class="doc_code"> -<pre> -$ <b>llvm-as < example.ll | opt -mem2reg | llvm-dis</b> -@G = weak global i32 0 -@H = weak global i32 0 - -define i32 @test(i1 %Condition) { -entry: - br i1 %Condition, label %cond_true, label %cond_false - -cond_true: - %X.0 = load i32* @G - br label %cond_next - -cond_false: - %X.1 = load i32* @H - br label %cond_next - -cond_next: - %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.01 -} -</pre> -</div> - -<p>The mem2reg pass implements the standard "iterated dominance frontier" -algorithm for constructing SSA form and has a number of optimizations that speed -up (very common) degenerate cases. The mem2reg optimization pass is the answer to dealing -with mutable variables, and we highly recommend that you depend on it. Note that -mem2reg only works on variables in certain circumstances:</p> - -<ol> -<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it -promotes them. It does not apply to global variables or heap allocations.</li> - -<li>mem2reg only looks for alloca instructions in the entry block of the -function. Being in the entry block guarantees that the alloca is only executed -once, which makes analysis simpler.</li> - -<li>mem2reg only promotes allocas whose uses are direct loads and stores. If -the address of the stack object is passed to a function, or if any funny pointer -arithmetic is involved, the alloca will not be promoted.</li> - -<li>mem2reg only works on allocas of <a -href="../LangRef.html#t_classifications">first class</a> -values (such as pointers, scalars and vectors), and only if the array size -of the allocation is 1 (or missing in the .ll file). mem2reg is not capable of -promoting structs or arrays to registers. Note that the "scalarrepl" pass is -more powerful and can promote structs, "unions", and arrays in many cases.</li> - -</ol> - -<p> -All of these properties are easy to satisfy for most imperative languages, and -we'll illustrate it below with Kaleidoscope. The final question you may be -asking is: should I bother with this nonsense for my front-end? Wouldn't it be -better if I just did SSA construction directly, avoiding use of the mem2reg -optimization pass? In short, we strongly recommend that you use this technique -for building SSA form, unless there is an extremely good reason not to. Using -this technique is:</p> - -<ul> -<li>Proven and well tested: llvm-gcc and clang both use this technique for local -mutable variables. As such, the most common clients of LLVM are using this to -handle a bulk of their variables. You can be sure that bugs are found fast and -fixed early.</li> - -<li>Extremely Fast: mem2reg has a number of special cases that make it fast in -common cases as well as fully general. For example, it has fast-paths for -variables that are only used in a single block, variables that only have one -assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc. -</li> - -<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html"> -Debug information in LLVM</a> relies on having the address of the variable -exposed so that debug info can be attached to it. This technique dovetails -very naturally with this style of debug info.</li> -</ul> - -<p>If nothing else, this makes it much easier to get your front-end up and -running, and is very simple to implement. Lets extend Kaleidoscope with mutable -variables now! -</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="kalvars">Mutable Variables in -Kaleidoscope</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>Now that we know the sort of problem we want to tackle, lets see what this -looks like in the context of our little Kaleidoscope language. We're going to -add two features:</p> - -<ol> -<li>The ability to mutate variables with the '=' operator.</li> -<li>The ability to define new variables.</li> -</ol> - -<p>While the first item is really what this is about, we only have variables -for incoming arguments as well as for induction variables, and redefining those only -goes so far :). Also, the ability to define new variables is a -useful thing regardless of whether you will be mutating them. Here's a -motivating example that shows how we could use these:</p> - -<div class="doc_code"> -<pre> -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -# Recursive fib, we could do this before. -def fib(x) - if (x < 3) then - 1 - else - fib(x-1)+fib(x-2); - -# Iterative fib. -def fibi(x) - <b>var a = 1, b = 1, c in</b> - (for i = 3, i < x in - <b>c = a + b</b> : - <b>a = b</b> : - <b>b = c</b>) : - b; - -# Call it. -fibi(10); -</pre> -</div> - -<p> -In order to mutate variables, we have to change our existing variables to use -the "alloca trick". Once we have that, we'll add our new operator, then extend -Kaleidoscope to support new variable definitions. -</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for -Mutation</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p> -The symbol table in Kaleidoscope is managed at code generation time by the -'<tt>NamedValues</tt>' map. This map currently keeps track of the LLVM "Value*" -that holds the double value for the named variable. In order to support -mutation, we need to change this slightly, so that it <tt>NamedValues</tt> holds -the <em>memory location</em> of the variable in question. Note that this -change is a refactoring: it changes the structure of the code, but does not -(by itself) change the behavior of the compiler. All of these changes are -isolated in the Kaleidoscope code generator.</p> - -<p> -At this point in Kaleidoscope's development, it only supports variables for two -things: incoming arguments to functions and the induction variable of 'for' -loops. For consistency, we'll allow mutation of these variables in addition to -other user-defined variables. This means that these will both need memory -locations. -</p> - -<p>To start our transformation of Kaleidoscope, we'll change the NamedValues -map so that it maps to AllocaInst* instead of Value*. Once we do this, the C++ -compiler will tell us what parts of the code we need to update:</p> - -<div class="doc_code"> -<pre> -static std::map<std::string, AllocaInst*> NamedValues; -</pre> -</div> - -<p>Also, since we will need to create these alloca's, we'll use a helper -function that ensures that the allocas are created in the entry block of the -function:</p> - -<div class="doc_code"> -<pre> -/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of -/// the function. This is used for mutable variables etc. -static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction, - const std::string &VarName) { - IRBuilder<> TmpB(&TheFunction->getEntryBlock(), - TheFunction->getEntryBlock().begin()); - return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0, - VarName.c_str()); -} -</pre> -</div> - -<p>This funny looking code creates an IRBuilder object that is pointing at -the first instruction (.begin()) of the entry block. It then creates an alloca -with the expected name and returns it. Because all values in Kaleidoscope are -doubles, there is no need to pass in a type to use.</p> - -<p>With this in place, the first functionality change we want to make is to -variable references. In our new scheme, variables live on the stack, so code -generating a reference to them actually needs to produce a load from the stack -slot:</p> - -<div class="doc_code"> -<pre> -Value *VariableExprAST::Codegen() { - // Look this variable up in the function. - Value *V = NamedValues[Name]; - if (V == 0) return ErrorV("Unknown variable name"); - - <b>// Load the value. - return Builder.CreateLoad(V, Name.c_str());</b> -} -</pre> -</div> - -<p>As you can see, this is pretty straightforward. Now we need to update the -things that define the variables to set up the alloca. We'll start with -<tt>ForExprAST::Codegen</tt> (see the <a href="#code">full code listing</a> for -the unabridged code):</p> - -<div class="doc_code"> -<pre> - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - <b>// Create an alloca for the variable in the entry block. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);</b> - - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->Codegen(); - if (StartVal == 0) return 0; - - <b>// Store the value into the alloca. - Builder.CreateStore(StartVal, Alloca);</b> - ... - - // Compute the end condition. - Value *EndCond = End->Codegen(); - if (EndCond == 0) return EndCond; - - <b>// Reload, increment, and restore the alloca. This handles the case where - // the body of the loop mutates the variable. - Value *CurVar = Builder.CreateLoad(Alloca); - Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar"); - Builder.CreateStore(NextVar, Alloca);</b> - ... -</pre> -</div> - -<p>This code is virtually identical to the code <a -href="LangImpl5.html#forcodegen">before we allowed mutable variables</a>. The -big difference is that we no longer have to construct a PHI node, and we use -load/store to access the variable as needed.</p> - -<p>To support mutable argument variables, we need to also make allocas for them. -The code for this is also pretty simple:</p> - -<div class="doc_code"> -<pre> -/// CreateArgumentAllocas - Create an alloca for each argument and register the -/// argument in the symbol table so that references to it will succeed. -void PrototypeAST::CreateArgumentAllocas(Function *F) { - Function::arg_iterator AI = F->arg_begin(); - for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) { - // Create an alloca for this variable. - AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]); - - // Store the initial value into the alloca. - Builder.CreateStore(AI, Alloca); - - // Add arguments to variable symbol table. - NamedValues[Args[Idx]] = Alloca; - } -} -</pre> -</div> - -<p>For each argument, we make an alloca, store the input value to the function -into the alloca, and register the alloca as the memory location for the -argument. This method gets invoked by <tt>FunctionAST::Codegen</tt> right after -it sets up the entry block for the function.</p> - -<p>The final missing piece is adding the mem2reg pass, which allows us to get -good codegen once again:</p> - -<div class="doc_code"> -<pre> - // Set up the optimizer pipeline. Start with registering info about how the - // target lays out data structures. - OurFPM.add(new TargetData(*TheExecutionEngine->getTargetData())); - <b>// Promote allocas to registers. - OurFPM.add(createPromoteMemoryToRegisterPass());</b> - // Do simple "peephole" optimizations and bit-twiddling optzns. - OurFPM.add(createInstructionCombiningPass()); - // Reassociate expressions. - OurFPM.add(createReassociatePass()); -</pre> -</div> - -<p>It is interesting to see what the code looks like before and after the -mem2reg optimization runs. For example, this is the before/after code for our -recursive fib function. Before the optimization:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - <b>%x1 = alloca double - store double %x, double* %x1 - %x2 = load double* %x1</b> - %cmptmp = fcmp ult double %x2, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: ; preds = %entry - br label %ifcont - -else: ; preds = %entry - <b>%x3 = load double* %x1</b> - %subtmp = fsub double %x3, 1.000000e+00 - %calltmp = call double @fib( double %subtmp ) - <b>%x4 = load double* %x1</b> - %subtmp5 = fsub double %x4, 2.000000e+00 - %calltmp6 = call double @fib( double %subtmp5 ) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>Here there is only one variable (x, the input argument) but you can still -see the extremely simple-minded code generation strategy we are using. In the -entry block, an alloca is created, and the initial input value is stored into -it. Each reference to the variable does a reload from the stack. Also, note -that we didn't modify the if/then/else expression, so it still inserts a PHI -node. While we could make an alloca for it, it is actually easier to create a -PHI node for it, so we still just make the PHI.</p> - -<p>Here is the code after the mem2reg pass runs:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - -then: - br label %ifcont - -else: - %subtmp = fsub double <b>%x</b>, 1.000000e+00 - %calltmp = call double @fib( double %subtmp ) - %subtmp5 = fsub double <b>%x</b>, 2.000000e+00 - %calltmp6 = call double @fib( double %subtmp5 ) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - -ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp -} -</pre> -</div> - -<p>This is a trivial case for mem2reg, since there are no redefinitions of the -variable. The point of showing this is to calm your tension about inserting -such blatent inefficiencies :).</p> - -<p>After the rest of the optimizers run, we get:</p> - -<div class="doc_code"> -<pre> -define double @fib(double %x) { -entry: - %cmptmp = fcmp ult double %x, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp ueq double %booltmp, 0.000000e+00 - br i1 %ifcond, label %else, label %ifcont - -else: - %subtmp = fsub double %x, 1.000000e+00 - %calltmp = call double @fib( double %subtmp ) - %subtmp5 = fsub double %x, 2.000000e+00 - %calltmp6 = call double @fib( double %subtmp5 ) - %addtmp = fadd double %calltmp, %calltmp6 - ret double %addtmp - -ifcont: - ret double 1.000000e+00 -} -</pre> -</div> - -<p>Here we see that the simplifycfg pass decided to clone the return instruction -into the end of the 'else' block. This allowed it to eliminate some branches -and the PHI node.</p> - -<p>Now that all symbol table references are updated to use stack variables, -we'll add the assignment operator.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="assignment">New Assignment Operator</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>With our current framework, adding a new assignment operator is really -simple. We will parse it just like any other binary operator, but handle it -internally (instead of allowing the user to define it). The first step is to -set a precedence:</p> - -<div class="doc_code"> -<pre> - int main() { - // Install standard binary operators. - // 1 is lowest precedence. - <b>BinopPrecedence['='] = 2;</b> - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; -</pre> -</div> - -<p>Now that the parser knows the precedence of the binary operator, it takes -care of all the parsing and AST generation. We just need to implement codegen -for the assignment operator. This looks like:</p> - -<div class="doc_code"> -<pre> -Value *BinaryExprAST::Codegen() { - // Special case '=' because we don't want to emit the LHS as an expression. - if (Op == '=') { - // Assignment requires the LHS to be an identifier. - VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS); - if (!LHSE) - return ErrorV("destination of '=' must be a variable"); -</pre> -</div> - -<p>Unlike the rest of the binary operators, our assignment operator doesn't -follow the "emit LHS, emit RHS, do computation" model. As such, it is handled -as a special case before the other binary operators are handled. The other -strange thing is that it requires the LHS to be a variable. It is invalid to -have "(x+1) = expr" - only things like "x = expr" are allowed. -</p> - -<div class="doc_code"> -<pre> - // Codegen the RHS. - Value *Val = RHS->Codegen(); - if (Val == 0) return 0; - - // Look up the name. - Value *Variable = NamedValues[LHSE->getName()]; - if (Variable == 0) return ErrorV("Unknown variable name"); - - Builder.CreateStore(Val, Variable); - return Val; - } - ... -</pre> -</div> - -<p>Once we have the variable, codegen'ing the assignment is straightforward: -we emit the RHS of the assignment, create a store, and return the computed -value. Returning a value allows for chained assignments like "X = (Y = Z)".</p> - -<p>Now that we have an assignment operator, we can mutate loop variables and -arguments. For example, we can now run code like this:</p> - -<div class="doc_code"> -<pre> -# Function to print a double. -extern printd(x); - -# Define ':' for sequencing: as a low-precedence operator that ignores operands -# and just returns the RHS. -def binary : 1 (x y) y; - -def test(x) - printd(x) : - x = 4 : - printd(x); - -test(123); -</pre> -</div> - -<p>When run, this example prints "123" and then "4", showing that we did -actually mutate the value! Okay, we have now officially implemented our goal: -getting this to work requires SSA construction in the general case. However, -to be really useful, we want the ability to define our own local variables, lets -add this next! -</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="localvars">User-defined Local -Variables</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p>Adding var/in is just like any other other extensions we made to -Kaleidoscope: we extend the lexer, the parser, the AST and the code generator. -The first step for adding our new 'var/in' construct is to extend the lexer. -As before, this is pretty trivial, the code looks like this:</p> - -<div class="doc_code"> -<pre> -enum Token { - ... - <b>// var definition - tok_var = -13</b> -... -} -... -static int gettok() { -... - if (IdentifierStr == "in") return tok_in; - if (IdentifierStr == "binary") return tok_binary; - if (IdentifierStr == "unary") return tok_unary; - <b>if (IdentifierStr == "var") return tok_var;</b> - return tok_identifier; -... -</pre> -</div> - -<p>The next step is to define the AST node that we will construct. For var/in, -it looks like this:</p> - -<div class="doc_code"> -<pre> -/// VarExprAST - Expression class for var/in -class VarExprAST : public ExprAST { - std::vector<std::pair<std::string, ExprAST*> > VarNames; - ExprAST *Body; -public: - VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames, - ExprAST *body) - : VarNames(varnames), Body(body) {} - - virtual Value *Codegen(); -}; -</pre> -</div> - -<p>var/in allows a list of names to be defined all at once, and each name can -optionally have an initializer value. As such, we capture this information in -the VarNames vector. Also, var/in has a body, this body is allowed to access -the variables defined by the var/in.</p> - -<p>With this in place, we can define the parser pieces. The first thing we do is add -it as a primary expression:</p> - -<div class="doc_code"> -<pre> -/// primary -/// ::= identifierexpr -/// ::= numberexpr -/// ::= parenexpr -/// ::= ifexpr -/// ::= forexpr -<b>/// ::= varexpr</b> -static ExprAST *ParsePrimary() { - switch (CurTok) { - default: return Error("unknown token when expecting an expression"); - case tok_identifier: return ParseIdentifierExpr(); - case tok_number: return ParseNumberExpr(); - case '(': return ParseParenExpr(); - case tok_if: return ParseIfExpr(); - case tok_for: return ParseForExpr(); - <b>case tok_var: return ParseVarExpr();</b> - } -} -</pre> -</div> - -<p>Next we define ParseVarExpr:</p> - -<div class="doc_code"> -<pre> -/// varexpr ::= 'var' identifier ('=' expression)? -// (',' identifier ('=' expression)?)* 'in' expression -static ExprAST *ParseVarExpr() { - getNextToken(); // eat the var. - - std::vector<std::pair<std::string, ExprAST*> > VarNames; - - // At least one variable name is required. - if (CurTok != tok_identifier) - return Error("expected identifier after var"); -</pre> -</div> - -<p>The first part of this code parses the list of identifier/expr pairs into the -local <tt>VarNames</tt> vector. - -<div class="doc_code"> -<pre> - while (1) { - std::string Name = IdentifierStr; - getNextToken(); // eat identifier. - - // Read the optional initializer. - ExprAST *Init = 0; - if (CurTok == '=') { - getNextToken(); // eat the '='. - - Init = ParseExpression(); - if (Init == 0) return 0; - } - - VarNames.push_back(std::make_pair(Name, Init)); - - // End of var list, exit loop. - if (CurTok != ',') break; - getNextToken(); // eat the ','. - - if (CurTok != tok_identifier) - return Error("expected identifier list after var"); - } -</pre> -</div> - -<p>Once all the variables are parsed, we then parse the body and create the -AST node:</p> - -<div class="doc_code"> -<pre> - // At this point, we have to have 'in'. - if (CurTok != tok_in) - return Error("expected 'in' keyword after 'var'"); - getNextToken(); // eat 'in'. - - ExprAST *Body = ParseExpression(); - if (Body == 0) return 0; - - return new VarExprAST(VarNames, Body); -} -</pre> -</div> - -<p>Now that we can parse and represent the code, we need to support emission of -LLVM IR for it. This code starts out with:</p> - -<div class="doc_code"> -<pre> -Value *VarExprAST::Codegen() { - std::vector<AllocaInst *> OldBindings; - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Register all variables and emit their initializer. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) { - const std::string &VarName = VarNames[i].first; - ExprAST *Init = VarNames[i].second; -</pre> -</div> - -<p>Basically it loops over all the variables, installing them one at a time. -For each variable we put into the symbol table, we remember the previous value -that we replace in OldBindings.</p> - -<div class="doc_code"> -<pre> - // Emit the initializer before adding the variable to scope, this prevents - // the initializer from referencing the variable itself, and permits stuff - // like this: - // var a = 1 in - // var a = a in ... # refers to outer 'a'. - Value *InitVal; - if (Init) { - InitVal = Init->Codegen(); - if (InitVal == 0) return 0; - } else { // If not specified, use 0.0. - InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0)); - } - - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - Builder.CreateStore(InitVal, Alloca); - - // Remember the old variable binding so that we can restore the binding when - // we unrecurse. - OldBindings.push_back(NamedValues[VarName]); - - // Remember this binding. - NamedValues[VarName] = Alloca; - } -</pre> -</div> - -<p>There are more comments here than code. The basic idea is that we emit the -initializer, create the alloca, then update the symbol table to point to it. -Once all the variables are installed in the symbol table, we evaluate the body -of the var/in expression:</p> - -<div class="doc_code"> -<pre> - // Codegen the body, now that all vars are in scope. - Value *BodyVal = Body->Codegen(); - if (BodyVal == 0) return 0; -</pre> -</div> - -<p>Finally, before returning, we restore the previous variable bindings:</p> - -<div class="doc_code"> -<pre> - // Pop all our variables from scope. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) - NamedValues[VarNames[i].first] = OldBindings[i]; - - // Return the body computation. - return BodyVal; -} -</pre> -</div> - -<p>The end result of all of this is that we get properly scoped variable -definitions, and we even (trivially) allow mutation of them :).</p> - -<p>With this, we completed what we set out to do. Our nice iterative fib -example from the intro compiles and runs just fine. The mem2reg pass optimizes -all of our stack variables into SSA registers, inserting PHI nodes where needed, -and our front-end remains simple: no "iterated dominance frontier" computation -anywhere in sight.</p> - -</div> - -<!-- *********************************************************************** --> -<div class="doc_section"><a name="code">Full Code Listing</a></div> -<!-- *********************************************************************** --> - -<div class="doc_text"> - -<p> -Here is the complete code listing for our running example, enhanced with mutable -variables and var/in support. To build this example, use: -</p> - -<div class="doc_code"> -<pre> - # Compile - g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy - # Run - ./toy -</pre> -</div> - -<p>Here is the code:</p> - -<div class="doc_code"> -<pre> -#include "llvm/DerivedTypes.h" -#include "llvm/ExecutionEngine/ExecutionEngine.h" -#include "llvm/ExecutionEngine/JIT.h" -#include "llvm/LLVMContext.h" -#include "llvm/Module.h" -#include "llvm/PassManager.h" -#include "llvm/Analysis/Verifier.h" -#include "llvm/Target/TargetData.h" -#include "llvm/Target/TargetSelect.h" -#include "llvm/Transforms/Scalar.h" -#include "llvm/Support/IRBuilder.h" -#include <cstdio> -#include <string> -#include <map> -#include <vector> -using namespace llvm; - -//===----------------------------------------------------------------------===// -// Lexer -//===----------------------------------------------------------------------===// - -// The lexer returns tokens [0-255] if it is an unknown character, otherwise one -// of these for known things. -enum Token { - tok_eof = -1, - - // commands - tok_def = -2, tok_extern = -3, - - // primary - tok_identifier = -4, tok_number = -5, - - // control - tok_if = -6, tok_then = -7, tok_else = -8, - tok_for = -9, tok_in = -10, - - // operators - tok_binary = -11, tok_unary = -12, - - // var definition - tok_var = -13 -}; - -static std::string IdentifierStr; // Filled in if tok_identifier -static double NumVal; // Filled in if tok_number - -/// gettok - Return the next token from standard input. -static int gettok() { - static int LastChar = ' '; - - // Skip any whitespace. - while (isspace(LastChar)) - LastChar = getchar(); - - if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]* - IdentifierStr = LastChar; - while (isalnum((LastChar = getchar()))) - IdentifierStr += LastChar; - - if (IdentifierStr == "def") return tok_def; - if (IdentifierStr == "extern") return tok_extern; - if (IdentifierStr == "if") return tok_if; - if (IdentifierStr == "then") return tok_then; - if (IdentifierStr == "else") return tok_else; - if (IdentifierStr == "for") return tok_for; - if (IdentifierStr == "in") return tok_in; - if (IdentifierStr == "binary") return tok_binary; - if (IdentifierStr == "unary") return tok_unary; - if (IdentifierStr == "var") return tok_var; - return tok_identifier; - } - - if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+ - std::string NumStr; - do { - NumStr += LastChar; - LastChar = getchar(); - } while (isdigit(LastChar) || LastChar == '.'); - - NumVal = strtod(NumStr.c_str(), 0); - return tok_number; - } - - if (LastChar == '#') { - // Comment until end of line. - do LastChar = getchar(); - while (LastChar != EOF && LastChar != '\n' && LastChar != '\r'); - - if (LastChar != EOF) - return gettok(); - } - - // Check for end of file. Don't eat the EOF. - if (LastChar == EOF) - return tok_eof; - - // Otherwise, just return the character as its ascii value. - int ThisChar = LastChar; - LastChar = getchar(); - return ThisChar; -} - -//===----------------------------------------------------------------------===// -// Abstract Syntax Tree (aka Parse Tree) -//===----------------------------------------------------------------------===// - -/// ExprAST - Base class for all expression nodes. -class ExprAST { -public: - virtual ~ExprAST() {} - virtual Value *Codegen() = 0; -}; - -/// NumberExprAST - Expression class for numeric literals like "1.0". -class NumberExprAST : public ExprAST { - double Val; -public: - NumberExprAST(double val) : Val(val) {} - virtual Value *Codegen(); -}; - -/// VariableExprAST - Expression class for referencing a variable, like "a". -class VariableExprAST : public ExprAST { - std::string Name; -public: - VariableExprAST(const std::string &name) : Name(name) {} - const std::string &getName() const { return Name; } - virtual Value *Codegen(); -}; - -/// UnaryExprAST - Expression class for a unary operator. -class UnaryExprAST : public ExprAST { - char Opcode; - ExprAST *Operand; -public: - UnaryExprAST(char opcode, ExprAST *operand) - : Opcode(opcode), Operand(operand) {} - virtual Value *Codegen(); -}; - -/// BinaryExprAST - Expression class for a binary operator. -class BinaryExprAST : public ExprAST { - char Op; - ExprAST *LHS, *RHS; -public: - BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) - : Op(op), LHS(lhs), RHS(rhs) {} - virtual Value *Codegen(); -}; - -/// CallExprAST - Expression class for function calls. -class CallExprAST : public ExprAST { - std::string Callee; - std::vector<ExprAST*> Args; -public: - CallExprAST(const std::string &callee, std::vector<ExprAST*> &args) - : Callee(callee), Args(args) {} - virtual Value *Codegen(); -}; - -/// IfExprAST - Expression class for if/then/else. -class IfExprAST : public ExprAST { - ExprAST *Cond, *Then, *Else; -public: - IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else) - : Cond(cond), Then(then), Else(_else) {} - virtual Value *Codegen(); -}; - -/// ForExprAST - Expression class for for/in. -class ForExprAST : public ExprAST { - std::string VarName; - ExprAST *Start, *End, *Step, *Body; -public: - ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end, - ExprAST *step, ExprAST *body) - : VarName(varname), Start(start), End(end), Step(step), Body(body) {} - virtual Value *Codegen(); -}; - -/// VarExprAST - Expression class for var/in -class VarExprAST : public ExprAST { - std::vector<std::pair<std::string, ExprAST*> > VarNames; - ExprAST *Body; -public: - VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames, - ExprAST *body) - : VarNames(varnames), Body(body) {} - - virtual Value *Codegen(); -}; - -/// PrototypeAST - This class represents the "prototype" for a function, -/// which captures its name, and its argument names (thus implicitly the number -/// of arguments the function takes), as well as if it is an operator. -class PrototypeAST { - std::string Name; - std::vector<std::string> Args; - bool isOperator; - unsigned Precedence; // Precedence if a binary op. -public: - PrototypeAST(const std::string &name, const std::vector<std::string> &args, - bool isoperator = false, unsigned prec = 0) - : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {} - - bool isUnaryOp() const { return isOperator && Args.size() == 1; } - bool isBinaryOp() const { return isOperator && Args.size() == 2; } - - char getOperatorName() const { - assert(isUnaryOp() || isBinaryOp()); - return Name[Name.size()-1]; - } - - unsigned getBinaryPrecedence() const { return Precedence; } - - Function *Codegen(); - - void CreateArgumentAllocas(Function *F); -}; - -/// FunctionAST - This class represents a function definition itself. -class FunctionAST { - PrototypeAST *Proto; - ExprAST *Body; -public: - FunctionAST(PrototypeAST *proto, ExprAST *body) - : Proto(proto), Body(body) {} - - Function *Codegen(); -}; - -//===----------------------------------------------------------------------===// -// Parser -//===----------------------------------------------------------------------===// - -/// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current -/// token the parser is looking at. getNextToken reads another token from the -/// lexer and updates CurTok with its results. -static int CurTok; -static int getNextToken() { - return CurTok = gettok(); -} - -/// BinopPrecedence - This holds the precedence for each binary operator that is -/// defined. -static std::map<char, int> BinopPrecedence; - -/// GetTokPrecedence - Get the precedence of the pending binary operator token. -static int GetTokPrecedence() { - if (!isascii(CurTok)) - return -1; - - // Make sure it's a declared binop. - int TokPrec = BinopPrecedence[CurTok]; - if (TokPrec <= 0) return -1; - return TokPrec; -} - -/// Error* - These are little helper functions for error handling. -ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;} -PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; } -FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; } - -static ExprAST *ParseExpression(); - -/// identifierexpr -/// ::= identifier -/// ::= identifier '(' expression* ')' -static ExprAST *ParseIdentifierExpr() { - std::string IdName = IdentifierStr; - - getNextToken(); // eat identifier. - - if (CurTok != '(') // Simple variable ref. - return new VariableExprAST(IdName); - - // Call. - getNextToken(); // eat ( - std::vector<ExprAST*> Args; - if (CurTok != ')') { - while (1) { - ExprAST *Arg = ParseExpression(); - if (!Arg) return 0; - Args.push_back(Arg); - - if (CurTok == ')') break; - - if (CurTok != ',') - return Error("Expected ')' or ',' in argument list"); - getNextToken(); - } - } - - // Eat the ')'. - getNextToken(); - - return new CallExprAST(IdName, Args); -} - -/// numberexpr ::= number -static ExprAST *ParseNumberExpr() { - ExprAST *Result = new NumberExprAST(NumVal); - getNextToken(); // consume the number - return Result; -} - -/// parenexpr ::= '(' expression ')' -static ExprAST *ParseParenExpr() { - getNextToken(); // eat (. - ExprAST *V = ParseExpression(); - if (!V) return 0; - - if (CurTok != ')') - return Error("expected ')'"); - getNextToken(); // eat ). - return V; -} - -/// ifexpr ::= 'if' expression 'then' expression 'else' expression -static ExprAST *ParseIfExpr() { - getNextToken(); // eat the if. - - // condition. - ExprAST *Cond = ParseExpression(); - if (!Cond) return 0; - - if (CurTok != tok_then) - return Error("expected then"); - getNextToken(); // eat the then - - ExprAST *Then = ParseExpression(); - if (Then == 0) return 0; - - if (CurTok != tok_else) - return Error("expected else"); - - getNextToken(); - - ExprAST *Else = ParseExpression(); - if (!Else) return 0; - - return new IfExprAST(Cond, Then, Else); -} - -/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression -static ExprAST *ParseForExpr() { - getNextToken(); // eat the for. - - if (CurTok != tok_identifier) - return Error("expected identifier after for"); - - std::string IdName = IdentifierStr; - getNextToken(); // eat identifier. - - if (CurTok != '=') - return Error("expected '=' after for"); - getNextToken(); // eat '='. - - - ExprAST *Start = ParseExpression(); - if (Start == 0) return 0; - if (CurTok != ',') - return Error("expected ',' after for start value"); - getNextToken(); - - ExprAST *End = ParseExpression(); - if (End == 0) return 0; - - // The step value is optional. - ExprAST *Step = 0; - if (CurTok == ',') { - getNextToken(); - Step = ParseExpression(); - if (Step == 0) return 0; - } - - if (CurTok != tok_in) - return Error("expected 'in' after for"); - getNextToken(); // eat 'in'. - - ExprAST *Body = ParseExpression(); - if (Body == 0) return 0; - - return new ForExprAST(IdName, Start, End, Step, Body); -} - -/// varexpr ::= 'var' identifier ('=' expression)? -// (',' identifier ('=' expression)?)* 'in' expression -static ExprAST *ParseVarExpr() { - getNextToken(); // eat the var. - - std::vector<std::pair<std::string, ExprAST*> > VarNames; - - // At least one variable name is required. - if (CurTok != tok_identifier) - return Error("expected identifier after var"); - - while (1) { - std::string Name = IdentifierStr; - getNextToken(); // eat identifier. - - // Read the optional initializer. - ExprAST *Init = 0; - if (CurTok == '=') { - getNextToken(); // eat the '='. - - Init = ParseExpression(); - if (Init == 0) return 0; - } - - VarNames.push_back(std::make_pair(Name, Init)); - - // End of var list, exit loop. - if (CurTok != ',') break; - getNextToken(); // eat the ','. - - if (CurTok != tok_identifier) - return Error("expected identifier list after var"); - } - - // At this point, we have to have 'in'. - if (CurTok != tok_in) - return Error("expected 'in' keyword after 'var'"); - getNextToken(); // eat 'in'. - - ExprAST *Body = ParseExpression(); - if (Body == 0) return 0; - - return new VarExprAST(VarNames, Body); -} - -/// primary -/// ::= identifierexpr -/// ::= numberexpr -/// ::= parenexpr -/// ::= ifexpr -/// ::= forexpr -/// ::= varexpr -static ExprAST *ParsePrimary() { - switch (CurTok) { - default: return Error("unknown token when expecting an expression"); - case tok_identifier: return ParseIdentifierExpr(); - case tok_number: return ParseNumberExpr(); - case '(': return ParseParenExpr(); - case tok_if: return ParseIfExpr(); - case tok_for: return ParseForExpr(); - case tok_var: return ParseVarExpr(); - } -} - -/// unary -/// ::= primary -/// ::= '!' unary -static ExprAST *ParseUnary() { - // If the current token is not an operator, it must be a primary expr. - if (!isascii(CurTok) || CurTok == '(' || CurTok == ',') - return ParsePrimary(); - - // If this is a unary operator, read it. - int Opc = CurTok; - getNextToken(); - if (ExprAST *Operand = ParseUnary()) - return new UnaryExprAST(Opc, Operand); - return 0; -} - -/// binoprhs -/// ::= ('+' unary)* -static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) { - // If this is a binop, find its precedence. - while (1) { - int TokPrec = GetTokPrecedence(); - - // If this is a binop that binds at least as tightly as the current binop, - // consume it, otherwise we are done. - if (TokPrec < ExprPrec) - return LHS; - - // Okay, we know this is a binop. - int BinOp = CurTok; - getNextToken(); // eat binop - - // Parse the unary expression after the binary operator. - ExprAST *RHS = ParseUnary(); - if (!RHS) return 0; - - // If BinOp binds less tightly with RHS than the operator after RHS, let - // the pending operator take RHS as its LHS. - int NextPrec = GetTokPrecedence(); - if (TokPrec < NextPrec) { - RHS = ParseBinOpRHS(TokPrec+1, RHS); - if (RHS == 0) return 0; - } - - // Merge LHS/RHS. - LHS = new BinaryExprAST(BinOp, LHS, RHS); - } -} - -/// expression -/// ::= unary binoprhs -/// -static ExprAST *ParseExpression() { - ExprAST *LHS = ParseUnary(); - if (!LHS) return 0; - - return ParseBinOpRHS(0, LHS); -} - -/// prototype -/// ::= id '(' id* ')' -/// ::= binary LETTER number? (id, id) -/// ::= unary LETTER (id) -static PrototypeAST *ParsePrototype() { - std::string FnName; - - unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary. - unsigned BinaryPrecedence = 30; - - switch (CurTok) { - default: - return ErrorP("Expected function name in prototype"); - case tok_identifier: - FnName = IdentifierStr; - Kind = 0; - getNextToken(); - break; - case tok_unary: - getNextToken(); - if (!isascii(CurTok)) - return ErrorP("Expected unary operator"); - FnName = "unary"; - FnName += (char)CurTok; - Kind = 1; - getNextToken(); - break; - case tok_binary: - getNextToken(); - if (!isascii(CurTok)) - return ErrorP("Expected binary operator"); - FnName = "binary"; - FnName += (char)CurTok; - Kind = 2; - getNextToken(); - - // Read the precedence if present. - if (CurTok == tok_number) { - if (NumVal < 1 || NumVal > 100) - return ErrorP("Invalid precedecnce: must be 1..100"); - BinaryPrecedence = (unsigned)NumVal; - getNextToken(); - } - break; - } - - if (CurTok != '(') - return ErrorP("Expected '(' in prototype"); - - std::vector<std::string> ArgNames; - while (getNextToken() == tok_identifier) - ArgNames.push_back(IdentifierStr); - if (CurTok != ')') - return ErrorP("Expected ')' in prototype"); - - // success. - getNextToken(); // eat ')'. - - // Verify right number of names for operator. - if (Kind && ArgNames.size() != Kind) - return ErrorP("Invalid number of operands for operator"); - - return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence); -} - -/// definition ::= 'def' prototype expression -static FunctionAST *ParseDefinition() { - getNextToken(); // eat def. - PrototypeAST *Proto = ParsePrototype(); - if (Proto == 0) return 0; - - if (ExprAST *E = ParseExpression()) - return new FunctionAST(Proto, E); - return 0; -} - -/// toplevelexpr ::= expression -static FunctionAST *ParseTopLevelExpr() { - if (ExprAST *E = ParseExpression()) { - // Make an anonymous proto. - PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>()); - return new FunctionAST(Proto, E); - } - return 0; -} - -/// external ::= 'extern' prototype -static PrototypeAST *ParseExtern() { - getNextToken(); // eat extern. - return ParsePrototype(); -} - -//===----------------------------------------------------------------------===// -// Code Generation -//===----------------------------------------------------------------------===// - -static Module *TheModule; -static IRBuilder<> Builder(getGlobalContext()); -static std::map<std::string, AllocaInst*> NamedValues; -static FunctionPassManager *TheFPM; - -Value *ErrorV(const char *Str) { Error(Str); return 0; } - -/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of -/// the function. This is used for mutable variables etc. -static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction, - const std::string &VarName) { - IRBuilder<> TmpB(&TheFunction->getEntryBlock(), - TheFunction->getEntryBlock().begin()); - return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0, - VarName.c_str()); -} - -Value *NumberExprAST::Codegen() { - return ConstantFP::get(getGlobalContext(), APFloat(Val)); -} - -Value *VariableExprAST::Codegen() { - // Look this variable up in the function. - Value *V = NamedValues[Name]; - if (V == 0) return ErrorV("Unknown variable name"); - - // Load the value. - return Builder.CreateLoad(V, Name.c_str()); -} - -Value *UnaryExprAST::Codegen() { - Value *OperandV = Operand->Codegen(); - if (OperandV == 0) return 0; - - Function *F = TheModule->getFunction(std::string("unary")+Opcode); - if (F == 0) - return ErrorV("Unknown unary operator"); - - return Builder.CreateCall(F, OperandV, "unop"); -} - -Value *BinaryExprAST::Codegen() { - // Special case '=' because we don't want to emit the LHS as an expression. - if (Op == '=') { - // Assignment requires the LHS to be an identifier. - VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS); - if (!LHSE) - return ErrorV("destination of '=' must be a variable"); - // Codegen the RHS. - Value *Val = RHS->Codegen(); - if (Val == 0) return 0; - - // Look up the name. - Value *Variable = NamedValues[LHSE->getName()]; - if (Variable == 0) return ErrorV("Unknown variable name"); - - Builder.CreateStore(Val, Variable); - return Val; - } - - Value *L = LHS->Codegen(); - Value *R = RHS->Codegen(); - if (L == 0 || R == 0) return 0; - - switch (Op) { - case '+': return Builder.CreateAdd(L, R, "addtmp"); - case '-': return Builder.CreateSub(L, R, "subtmp"); - case '*': return Builder.CreateMul(L, R, "multmp"); - case '<': - L = Builder.CreateFCmpULT(L, R, "cmptmp"); - // Convert bool 0/1 to double 0.0 or 1.0 - return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()), - "booltmp"); - default: break; - } - - // If it wasn't a builtin binary operator, it must be a user defined one. Emit - // a call to it. - Function *F = TheModule->getFunction(std::string("binary")+Op); - assert(F && "binary operator not found!"); - - Value *Ops[] = { L, R }; - return Builder.CreateCall(F, Ops, Ops+2, "binop"); -} - -Value *CallExprAST::Codegen() { - // Look up the name in the global module table. - Function *CalleeF = TheModule->getFunction(Callee); - if (CalleeF == 0) - return ErrorV("Unknown function referenced"); - - // If argument mismatch error. - if (CalleeF->arg_size() != Args.size()) - return ErrorV("Incorrect # arguments passed"); - - std::vector<Value*> ArgsV; - for (unsigned i = 0, e = Args.size(); i != e; ++i) { - ArgsV.push_back(Args[i]->Codegen()); - if (ArgsV.back() == 0) return 0; - } - - return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp"); -} - -Value *IfExprAST::Codegen() { - Value *CondV = Cond->Codegen(); - if (CondV == 0) return 0; - - // Convert condition to a bool by comparing equal to 0.0. - CondV = Builder.CreateFCmpONE(CondV, - ConstantFP::get(getGlobalContext(), APFloat(0.0)), - "ifcond"); - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Create blocks for the then and else cases. Insert the 'then' block at the - // end of the function. - BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction); - BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else"); - BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont"); - - Builder.CreateCondBr(CondV, ThenBB, ElseBB); - - // Emit then value. - Builder.SetInsertPoint(ThenBB); - - Value *ThenV = Then->Codegen(); - if (ThenV == 0) return 0; - - Builder.CreateBr(MergeBB); - // Codegen of 'Then' can change the current block, update ThenBB for the PHI. - ThenBB = Builder.GetInsertBlock(); - - // Emit else block. - TheFunction->getBasicBlockList().push_back(ElseBB); - Builder.SetInsertPoint(ElseBB); - - Value *ElseV = Else->Codegen(); - if (ElseV == 0) return 0; - - Builder.CreateBr(MergeBB); - // Codegen of 'Else' can change the current block, update ElseBB for the PHI. - ElseBB = Builder.GetInsertBlock(); - - // Emit merge block. - TheFunction->getBasicBlockList().push_back(MergeBB); - Builder.SetInsertPoint(MergeBB); - PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), - "iftmp"); - - PN->addIncoming(ThenV, ThenBB); - PN->addIncoming(ElseV, ElseBB); - return PN; -} - -Value *ForExprAST::Codegen() { - // Output this as: - // var = alloca double - // ... - // start = startexpr - // store start -> var - // goto loop - // loop: - // ... - // bodyexpr - // ... - // loopend: - // step = stepexpr - // endcond = endexpr - // - // curvar = load var - // nextvar = curvar + step - // store nextvar -> var - // br endcond, loop, endloop - // outloop: - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Create an alloca for the variable in the entry block. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->Codegen(); - if (StartVal == 0) return 0; - - // Store the value into the alloca. - Builder.CreateStore(StartVal, Alloca); - - // Make the new basic block for the loop header, inserting after current - // block. - BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction); - - // Insert an explicit fall through from the current block to the LoopBB. - Builder.CreateBr(LoopBB); - - // Start insertion in LoopBB. - Builder.SetInsertPoint(LoopBB); - - // Within the loop, the variable is defined equal to the PHI node. If it - // shadows an existing variable, we have to restore it, so save it now. - AllocaInst *OldVal = NamedValues[VarName]; - NamedValues[VarName] = Alloca; - - // Emit the body of the loop. This, like any other expr, can change the - // current BB. Note that we ignore the value computed by the body, but don't - // allow an error. - if (Body->Codegen() == 0) - return 0; - - // Emit the step value. - Value *StepVal; - if (Step) { - StepVal = Step->Codegen(); - if (StepVal == 0) return 0; - } else { - // If not specified, use 1.0. - StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0)); - } - - // Compute the end condition. - Value *EndCond = End->Codegen(); - if (EndCond == 0) return EndCond; - - // Reload, increment, and restore the alloca. This handles the case where - // the body of the loop mutates the variable. - Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str()); - Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar"); - Builder.CreateStore(NextVar, Alloca); - - // Convert condition to a bool by comparing equal to 0.0. - EndCond = Builder.CreateFCmpONE(EndCond, - ConstantFP::get(getGlobalContext(), APFloat(0.0)), - "loopcond"); - - // Create the "after loop" block and insert it. - BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction); - - // Insert the conditional branch into the end of LoopEndBB. - Builder.CreateCondBr(EndCond, LoopBB, AfterBB); - - // Any new code will be inserted in AfterBB. - Builder.SetInsertPoint(AfterBB); - - // Restore the unshadowed variable. - if (OldVal) - NamedValues[VarName] = OldVal; - else - NamedValues.erase(VarName); - - - // for expr always returns 0.0. - return Constant::getNullValue(Type::getDoubleTy(getGlobalContext())); -} - -Value *VarExprAST::Codegen() { - std::vector<AllocaInst *> OldBindings; - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Register all variables and emit their initializer. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) { - const std::string &VarName = VarNames[i].first; - ExprAST *Init = VarNames[i].second; - - // Emit the initializer before adding the variable to scope, this prevents - // the initializer from referencing the variable itself, and permits stuff - // like this: - // var a = 1 in - // var a = a in ... # refers to outer 'a'. - Value *InitVal; - if (Init) { - InitVal = Init->Codegen(); - if (InitVal == 0) return 0; - } else { // If not specified, use 0.0. - InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0)); - } - - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - Builder.CreateStore(InitVal, Alloca); - - // Remember the old variable binding so that we can restore the binding when - // we unrecurse. - OldBindings.push_back(NamedValues[VarName]); - - // Remember this binding. - NamedValues[VarName] = Alloca; - } - - // Codegen the body, now that all vars are in scope. - Value *BodyVal = Body->Codegen(); - if (BodyVal == 0) return 0; - - // Pop all our variables from scope. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) - NamedValues[VarNames[i].first] = OldBindings[i]; - - // Return the body computation. - return BodyVal; -} - -Function *PrototypeAST::Codegen() { - // Make the function type: double(double,double) etc. - std::vector<const Type*> Doubles(Args.size(), - Type::getDoubleTy(getGlobalContext())); - FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()), - Doubles, false); - - Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule); - - // If F conflicted, there was already something named 'Name'. If it has a - // body, don't allow redefinition or reextern. - if (F->getName() != Name) { - // Delete the one we just made and get the existing one. - F->eraseFromParent(); - F = TheModule->getFunction(Name); - - // If F already has a body, reject this. - if (!F->empty()) { - ErrorF("redefinition of function"); - return 0; - } - - // If F took a different number of args, reject. - if (F->arg_size() != Args.size()) { - ErrorF("redefinition of function with different # args"); - return 0; - } - } - - // Set names for all arguments. - unsigned Idx = 0; - for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size(); - ++AI, ++Idx) - AI->setName(Args[Idx]); - - return F; -} - -/// CreateArgumentAllocas - Create an alloca for each argument and register the -/// argument in the symbol table so that references to it will succeed. -void PrototypeAST::CreateArgumentAllocas(Function *F) { - Function::arg_iterator AI = F->arg_begin(); - for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) { - // Create an alloca for this variable. - AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]); - - // Store the initial value into the alloca. - Builder.CreateStore(AI, Alloca); - - // Add arguments to variable symbol table. - NamedValues[Args[Idx]] = Alloca; - } -} - -Function *FunctionAST::Codegen() { - NamedValues.clear(); - - Function *TheFunction = Proto->Codegen(); - if (TheFunction == 0) - return 0; - - // If this is an operator, install it. - if (Proto->isBinaryOp()) - BinopPrecedence[Proto->getOperatorName()] = Proto->getBinaryPrecedence(); - - // Create a new basic block to start insertion into. - BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction); - Builder.SetInsertPoint(BB); - - // Add all arguments to the symbol table and create their allocas. - Proto->CreateArgumentAllocas(TheFunction); - - if (Value *RetVal = Body->Codegen()) { - // Finish off the function. - Builder.CreateRet(RetVal); - - // Validate the generated code, checking for consistency. - verifyFunction(*TheFunction); - - // Optimize the function. - TheFPM->run(*TheFunction); - - return TheFunction; - } - - // Error reading body, remove function. - TheFunction->eraseFromParent(); - - if (Proto->isBinaryOp()) - BinopPrecedence.erase(Proto->getOperatorName()); - return 0; -} - -//===----------------------------------------------------------------------===// -// Top-Level parsing and JIT Driver -//===----------------------------------------------------------------------===// - -static ExecutionEngine *TheExecutionEngine; - -static void HandleDefinition() { - if (FunctionAST *F = ParseDefinition()) { - if (Function *LF = F->Codegen()) { - fprintf(stderr, "Read function definition:"); - LF->dump(); - } - } else { - // Skip token for error recovery. - getNextToken(); - } -} - -static void HandleExtern() { - if (PrototypeAST *P = ParseExtern()) { - if (Function *F = P->Codegen()) { - fprintf(stderr, "Read extern: "); - F->dump(); - } - } else { - // Skip token for error recovery. - getNextToken(); - } -} - -static void HandleTopLevelExpression() { - // Evaluate a top-level expression into an anonymous function. - if (FunctionAST *F = ParseTopLevelExpr()) { - if (Function *LF = F->Codegen()) { - // JIT the function, returning a function pointer. - void *FPtr = TheExecutionEngine->getPointerToFunction(LF); - - // Cast it to the right type (takes no arguments, returns a double) so we - // can call it as a native function. - double (*FP)() = (double (*)())(intptr_t)FPtr; - fprintf(stderr, "Evaluated to %f\n", FP()); - } - } else { - // Skip token for error recovery. - getNextToken(); - } -} - -/// top ::= definition | external | expression | ';' -static void MainLoop() { - while (1) { - fprintf(stderr, "ready> "); - switch (CurTok) { - case tok_eof: return; - case ';': getNextToken(); break; // ignore top-level semicolons. - case tok_def: HandleDefinition(); break; - case tok_extern: HandleExtern(); break; - default: HandleTopLevelExpression(); break; - } - } -} - -//===----------------------------------------------------------------------===// -// "Library" functions that can be "extern'd" from user code. -//===----------------------------------------------------------------------===// - -/// putchard - putchar that takes a double and returns 0. -extern "C" -double putchard(double X) { - putchar((char)X); - return 0; -} - -/// printd - printf that takes a double prints it as "%f\n", returning 0. -extern "C" -double printd(double X) { - printf("%f\n", X); - return 0; -} - -//===----------------------------------------------------------------------===// -// Main driver code. -//===----------------------------------------------------------------------===// - -int main() { - InitializeNativeTarget(); - LLVMContext &Context = getGlobalContext(); - - // Install standard binary operators. - // 1 is lowest precedence. - BinopPrecedence['='] = 2; - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; - BinopPrecedence['*'] = 40; // highest. - - // Prime the first token. - fprintf(stderr, "ready> "); - getNextToken(); - - // Make the module, which holds all the code. - TheModule = new Module("my cool jit", Context); - - // Create the JIT. This takes ownership of the module. - std::string ErrStr; - TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create(); - if (!TheExecutionEngine) { - fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str()); - exit(1); - } - - FunctionPassManager OurFPM(TheModule); - - // Set up the optimizer pipeline. Start with registering info about how the - // target lays out data structures. - OurFPM.add(new TargetData(*TheExecutionEngine->getTargetData())); - // Promote allocas to registers. - OurFPM.add(createPromoteMemoryToRegisterPass()); - // Do simple "peephole" optimizations and bit-twiddling optzns. - OurFPM.add(createInstructionCombiningPass()); - // Reassociate expressions. - OurFPM.add(createReassociatePass()); - // Eliminate Common SubExpressions. - OurFPM.add(createGVNPass()); - // Simplify the control flow graph (deleting unreachable blocks, etc). - OurFPM.add(createCFGSimplificationPass()); - - OurFPM.doInitialization(); - - // Set the global so the code gen can use this. - TheFPM = &OurFPM; - - // Run the main "interpreter loop" now. - MainLoop(); - - TheFPM = 0; - - // Print out all of the generated code. - TheModule->dump(); - - return 0; -} -</pre> -</div> - -<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a> -</div> - -<!-- *********************************************************************** --> -<hr> -<address> - <a href="http://jigsaw.w3.org/css-validator/check/referer"><img - src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a> - <a href="http://validator.w3.org/check/referer"><img - src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a> - - <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> - <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br> - Last modified: $Date$ -</address> -</body> -</html> |