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diff --git a/docs/GetElementPtr.html b/docs/GetElementPtr.html index 32c1bce..37dcd78 100644 --- a/docs/GetElementPtr.html +++ b/docs/GetElementPtr.html @@ -17,7 +17,7 @@ <ol> <li><a href="#intro">Introduction</a></li> - <li><a href="#questions">The Questions</a> + <li><a href="#addresses">Address Computation</a> <ol> <li><a href="#extra_index">Why is the extra 0 index required?</a></li> <li><a href="#deref">What is dereferenced by GEP?</a></li> @@ -25,6 +25,30 @@ subsequent ones?</a></li> <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li> <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li> + <li><a href="#vectors">Can GEP index into vector elements?</a> + <li><a href="#unions">Can GEP index into unions?</a> + <li><a href="#addrspace">What effect do address spaces have on GEPs?</a> + <li><a href="#int">How is GEP different from ptrtoint, arithmetic, and inttoptr?</a></li> + <li><a href="#be">I'm writing a backend for a target which needs custom lowering for GEP. How do I do this?</a> + <li><a href="#vla">How does VLA addressing work with GEPs?</a> + </ol></li> + <li><a href="#rules">Rules</a> + <ol> + <li><a href="#bounds">What happens if an array index is out of bounds?</a> + <li><a href="#negative">Can array indices be negative?</a> + <li><a href="#compare">Can I compare two values computed with GEPs?</a> + <li><a href="#types">Can I do GEP with a different pointer type than the type of the underlying object?</a> + <li><a href="#null">Can I cast an object's address to integer and add it to null?</a> + <li><a href="#ptrdiff">Can I compute the distance between two objects, and add that value to one address to compute the other address?</a> + <li><a href="#tbaa">Can I do type-based alias analysis on LLVM IR?</a> + <li><a href="#overflow">What happens if a GEP computation overflows?</a> + <li><a href="#check">How can I tell if my front-end is following the rules?</a> + </ol></li> + <li><a href="#rationale">Rationale</a> + <ol> + <li><a href="#goals">Why is GEP designed this way?</a></li> + <li><a href="#i32">Why do struct member indices always use i32?</a></li> + <li><a href="#uglygep">What's an uglygep?</a> </ol></li> <li><a href="#summary">Summary</a></li> </ol> @@ -37,9 +61,10 @@ <!-- *********************************************************************** --> <div class="doc_section"><a name="intro"><b>Introduction</b></a></div> <!-- *********************************************************************** --> + <div class="doc_text"> <p>This document seeks to dispel the mystery and confusion surrounding LLVM's - GetElementPtr (GEP) instruction. Questions about the wiley GEP instruction are + GetElementPtr (GEP) instruction. Questions about the wily GEP instruction are probably the most frequently occurring questions once a developer gets down to coding with LLVM. Here we lay out the sources of confusion and show that the GEP instruction is really quite simple. @@ -47,22 +72,14 @@ </div> <!-- *********************************************************************** --> -<div class="doc_section"><a name="questions"><b>The Questions</b></a></div> +<div class="doc_section"><a name="addresses"><b>Address Computation</b></a></div> <!-- *********************************************************************** --> <div class="doc_text"> <p>When people are first confronted with the GEP instruction, they tend to relate it to known concepts from other programming paradigms, most notably C - array indexing and field selection. However, GEP is a little different and - this leads to the following questions; all of which are answered in the - following sections.</p> - <ol> - <li><a href="#firstptr">What is the first index of the GEP instruction?</a> - </li> - <li><a href="#extra_index">Why is the extra 0 index required?</a></li> - <li><a href="#deref">What is dereferenced by GEP?</a></li> - <li><a href="#lead0">Why don't GEP x,0,0,1 and GEP x,1 alias? </a></li> - <li><a href="#trail0">Why do GEP x,1,0,0 and GEP x,1 alias? </a></li> - </ol> + array indexing and field selection. GEP closely resembles C array indexing + and field selection, however it's is a little different and this leads to + the following questions.</p> </div> <!-- *********************************************************************** --> @@ -85,7 +102,7 @@ X = &Foo->F; <p>it is natural to think that there is only one index, the selection of the field <tt>F</tt>. However, in this example, <tt>Foo</tt> is a pointer. That - pointer must be indexed explicitly in LLVM. C, on the other hand, indexs + pointer must be indexed explicitly in LLVM. C, on the other hand, indices through it transparently. To arrive at the same address location as the C code, you would provide the GEP instruction with two index operands. The first operand indexes through the pointer; the second operand indexes the @@ -155,7 +172,7 @@ entry: <div class="doc_code"> <pre> -%MyVar = unintialized global i32 +%MyVar = uninitialized global i32 ... %idx1 = getelementptr i32* %MyVar, i64 0 %idx2 = getelementptr i32* %MyVar, i64 1 @@ -210,7 +227,7 @@ idx3 = (char*) &MyVar + 8 field of the structure <tt>%MyStruct</tt>. When people first look at it, they wonder why the <tt>i64 0</tt> index is needed. However, a closer inspection of how globals and GEPs work reveals the need. Becoming aware of the following - facts will dispell the confusion:</p> + facts will dispel the confusion:</p> <ol> <li>The type of <tt>%MyStruct</tt> is <i>not</i> <tt>{ float*, i32 }</tt> but rather <tt>{ float*, i32 }*</tt>. That is, <tt>%MyStruct</tt> is a @@ -297,8 +314,8 @@ idx3 = (char*) &MyVar + 8 <div class="doc_code"> <pre> %MyVar = global { [10 x i32 ] } -%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1 -%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1 +%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 0, i32 0, i64 1 +%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1 </pre> </div> @@ -326,8 +343,8 @@ idx3 = (char*) &MyVar + 8 <div class="doc_code"> <pre> %MyVar = global { [10 x i32 ] } -%idx1 = getlementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0 -%idx2 = getlementptr { [10 x i32 ] }* %MyVar, i64 1 +%idx1 = getelementptr { [10 x i32 ] }* %MyVar, i64 1, i32 0, i64 0 +%idx2 = getelementptr { [10 x i32 ] }* %MyVar, i64 1 </pre> </div> @@ -337,6 +354,352 @@ idx3 = (char*) &MyVar + 8 </div> <!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="vectors"><b>Can GEP index into vector elements?</b></a> +</div> +<div class="doc_text"> + <p>This hasn't always been forcefully disallowed, though it's not recommended. + It leads to awkward special cases in the optimizers, and fundamental + inconsistency in the IR. In the future, it will probably be outright + disallowed.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="unions"><b>Can GEP index into unions?</b></a> +</div> +<div class="doc_text"> + <p>Unknown.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="addrspace"><b>What effect do address spaces have on GEPs?</b></a> +</div> +<div class="doc_text"> + <p>None, except that the address space qualifier on the first operand pointer + type always matches the address space qualifier on the result type.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="int"><b>How is GEP different from ptrtoint, arithmetic, + and inttoptr?</b></a> +</div> +<div class="doc_text"> + <p>It's very similar; there are only subtle differences.</p> + + <p>With ptrtoint, you have to pick an integer type. One approach is to pick i64; + this is safe on everything LLVM supports (LLVM internally assumes pointers + are never wider than 64 bits in many places), and the optimizer will actually + narrow the i64 arithmetic down to the actual pointer size on targets which + don't support 64-bit arithmetic in most cases. However, there are some cases + where it doesn't do this. With GEP you can avoid this problem. + + <p>Also, GEP carries additional pointer aliasing rules. It's invalid to take a + GEP from one object, address into a different separately allocated + object, and dereference it. IR producers (front-ends) must follow this rule, + and consumers (optimizers, specifically alias analysis) benefit from being + able to rely on it. See the <a href="#rules">Rules</a> section for more + information.</p> + + <p>And, GEP is more concise in common cases.</p> + + <p>However, for the underlying integer computation implied, there + is no difference.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="be"><b>I'm writing a backend for a target which needs custom + lowering for GEP. How do I do this?</b></a> +</div> +<div class="doc_text"> + <p>You don't. The integer computation implied by a GEP is target-independent. + Typically what you'll need to do is make your backend pattern-match + expressions trees involving ADD, MUL, etc., which are what GEP is lowered + into. This has the advantage of letting your code work correctly in more + cases.</p> + + <p>GEP does use target-dependent parameters for the size and layout of data + types, which targets can customize.</p> + + <p>If you require support for addressing units which are not 8 bits, you'll + need to fix a lot of code in the backend, with GEP lowering being only a + small piece of the overall picture.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="vla"><b>How does VLA addressing work with GEPs?</b></a> +</div> +<div class="doc_text"> + <p>GEPs don't natively support VLAs. LLVM's type system is entirely static, + and GEP address computations are guided by an LLVM type.</p> + + <p>VLA indices can be implemented as linearized indices. For example, an + expression like X[a][b][c], must be effectively lowered into a form + like X[a*m+b*n+c], so that it appears to the GEP as a single-dimensional + array reference.</p> + + <p>This means if you want to write an analysis which understands array + indices and you want to support VLAs, your code will have to be + prepared to reverse-engineer the linearization. One way to solve this + problem is to use the ScalarEvolution library, which always presents + VLA and non-VLA indexing in the same manner.</p> +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="rules"><b>Rules</b></a></div> +<!-- *********************************************************************** --> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="bounds"><b>What happens if an array index is out of bounds?</b></a> +</div> +<div class="doc_text"> + <p>There are two senses in which an array index can be out of bounds.</p> + + <p>First, there's the array type which comes from the (static) type of + the first operand to the GEP. Indices greater than the number of elements + in the corresponding static array type are valid. There is no problem with + out of bounds indices in this sense. Indexing into an array only depends + on the size of the array element, not the number of elements.</p> + + <p>A common example of how this is used is arrays where the size is not known. + It's common to use array types with zero length to represent these. The + fact that the static type says there are zero elements is irrelevant; it's + perfectly valid to compute arbitrary element indices, as the computation + only depends on the size of the array element, not the number of + elements. Note that zero-sized arrays are not a special case here.</p> + + <p>This sense is unconnected with <tt>inbounds</tt> keyword. The + <tt>inbounds</tt> keyword is designed to describe low-level pointer + arithmetic overflow conditions, rather than high-level array + indexing rules. + + <p>Analysis passes which wish to understand array indexing should not + assume that the static array type bounds are respected.</p> + + <p>The second sense of being out of bounds is computing an address that's + beyond the actual underlying allocated object.</p> + + <p>With the <tt>inbounds</tt> keyword, the result value of the GEP is + undefined if the address is outside the actual underlying allocated + object and not the address one-past-the-end.</p> + + <p>Without the <tt>inbounds</tt> keyword, there are no restrictions + on computing out-of-bounds addresses. Obviously, performing a load or + a store requires an address of allocated and sufficiently aligned + memory. But the GEP itself is only concerned with computing addresses.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="negative"><b>Can array indices be negative?</b></a> +</div> +<div class="doc_text"> + <p>Yes. This is basically a special case of array indices being out + of bounds.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="compare"><b>Can I compare two values computed with GEPs?</b></a> +</div> +<div class="doc_text"> + <p>Yes. If both addresses are within the same allocated object, or + one-past-the-end, you'll get the comparison result you expect. If either + is outside of it, integer arithmetic wrapping may occur, so the + comparison may not be meaningful.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="types"><b>Can I do GEP with a different pointer type than the type of + the underlying object?</b></a> +</div> +<div class="doc_text"> + <p>Yes. There are no restrictions on bitcasting a pointer value to an arbitrary + pointer type. The types in a GEP serve only to define the parameters for the + underlying integer computation. They need not correspond with the actual + type of the underlying object.</p> + + <p>Furthermore, loads and stores don't have to use the same types as the type + of the underlying object. Types in this context serve only to specify + memory size and alignment. Beyond that there are merely a hint to the + optimizer indicating how the value will likely be used.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="null"><b>Can I cast an object's address to integer and add it + to null?</b></a> +</div> +<div class="doc_text"> + <p>You can compute an address that way, but if you use GEP to do the add, + you can't use that pointer to actually access the object, unless the + object is managed outside of LLVM.</p> + + <p>The underlying integer computation is sufficiently defined; null has a + defined value -- zero -- and you can add whatever value you want to it.</p> + + <p>However, it's invalid to access (load from or store to) an LLVM-aware + object with such a pointer. This includes GlobalVariables, Allocas, and + objects pointed to by noalias pointers.</p> + + <p>If you really need this functionality, you can do the arithmetic with + explicit integer instructions, and use inttoptr to convert the result to + an address. Most of GEP's special aliasing rules do not apply to pointers + computed from ptrtoint, arithmetic, and inttoptr sequences.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="ptrdiff"><b>Can I compute the distance between two objects, and add + that value to one address to compute the other address?</b></a> +</div> +<div class="doc_text"> + <p>As with arithmetic on null, You can use GEP to compute an address that + way, but you can't use that pointer to actually access the object if you + do, unless the object is managed outside of LLVM.</p> + + <p>Also as above, ptrtoint and inttoptr provide an alternative way to do this + which do not have this restriction.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="tbaa"><b>Can I do type-based alias analysis on LLVM IR?</b></a> +</div> +<div class="doc_text"> + <p>You can't do type-based alias analysis using LLVM's built-in type system, + because LLVM has no restrictions on mixing types in addressing, loads or + stores.</p> + + <p>It would be possible to add special annotations to the IR, probably using + metadata, to describe a different type system (such as the C type system), + and do type-based aliasing on top of that. This is a much bigger + undertaking though.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="overflow"><b>What happens if a GEP computation overflows?</b></a> +</div> +<div class="doc_text"> + <p>If the GEP has the <tt>inbounds</tt> keyword, the result value is + undefined.</p> + + <p>Otherwise, the result value is the result from evaluating the implied + two's complement integer computation. However, since there's no + guarantee of where an object will be allocated in the address space, + such values have limited meaning.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="check"><b>How can I tell if my front-end is following the + rules?</b></a> +</div> +<div class="doc_text"> + <p>There is currently no checker for the getelementptr rules. Currently, + the only way to do this is to manually check each place in your front-end + where GetElementPtr operators are created.</p> + + <p>It's not possible to write a checker which could find all rule + violations statically. It would be possible to write a checker which + works by instrumenting the code with dynamic checks though. Alternatively, + it would be possible to write a static checker which catches a subset of + possible problems. However, no such checker exists today.</p> + +</div> + +<!-- *********************************************************************** --> +<div class="doc_section"><a name="rationale"><b>Rationale</b></a></div> +<!-- *********************************************************************** --> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="goals"><b>Why is GEP designed this way?</b></a> +</div> +<div class="doc_text"> + <p>The design of GEP has the following goals, in rough unofficial + order of priority:</p> + <ul> + <li>Support C, C-like languages, and languages which can be + conceptually lowered into C (this covers a lot).</li> + <li>Support optimizations such as those that are common in + C compilers.</li> + <li>Provide a consistent method for computing addresses so that + address computations don't need to be a part of load and + store instructions in the IR.</li> + <li>Support non-C-like languages, to the extent that it doesn't + interfere with other goals.</li> + <li>Minimize target-specific information in the IR.</li> + </ul> +</div> + +<!-- *********************************************************************** --> +<div class="doc_subsection"> + <a name="i32"><b>Why do struct member indices always use i32?</b></a> +</div> +<div class="doc_text"> + <p>The specific type i32 is probably just a historical artifact, however it's + wide enough for all practical purposes, so there's been no need to change it. + It doesn't necessarily imply i32 address arithmetic; it's just an identifier + which identifies a field in a struct. Requiring that all struct indices be + the same reduces the range of possibilities for cases where two GEPs are + effectively the same but have distinct operand types.</p> + +</div> + +<!-- *********************************************************************** --> + +<div class="doc_subsection"> + <a name="uglygep"><b>What's an uglygep?</b></a> +</div> +<div class="doc_text"> + <p>Some LLVM optimizers operate on GEPs by internally lowering them into + more primitive integer expressions, which allows them to be combined + with other integer expressions and/or split into multiple separate + integer expressions. If they've made non-trivial changes, translating + back into LLVM IR can involve reverse-engineering the structure of + the addressing in order to fit it into the static type of the original + first operand. It isn't always possibly to fully reconstruct this + structure; sometimes the underlying addressing doesn't correspond with + the static type at all. In such cases the optimizer instead will emit + a GEP with the base pointer casted to a simple address-unit pointer, + using the name "uglygep". This isn't pretty, but it's just as + valid, and it's sufficient to preserve the pointer aliasing guarantees + that GEP provides.</p> + +</div> + +<!-- *********************************************************************** --> <div class="doc_section"><a name="summary"><b>Summary</b></a></div> <!-- *********************************************************************** --> @@ -365,7 +728,7 @@ idx3 = (char*) &MyVar + 8 <a href="http://validator.w3.org/check/referer"><img src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br/> - Last modified: $Date: 2009-10-12 07:46:08 -0700 (Mon, 12 Oct 2009) $ + Last modified: $Date: 2010-02-26 02:16:03 +0800 (五, 26 2月 2010) $ </address> </body> </html> |