Update to LLVM 3.5a.

Change-Id: Ifadecab779f128e62e430c2b4f6ddd84953ed617

1 files changed, 217 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/AddDiscriminators.cpp b/lib/Transforms/Utils/AddDiscriminators.cpp
new file mode 100644
index 0000000..f42635e
--- /dev/null
+++ b/lib/Transforms/Utils/AddDiscriminators.cpp
@@ -0,0 +1,217 @@
+//===- AddDiscriminators.cpp - Insert DWARF path discriminators -----------===//
+//
+//                      The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file adds DWARF discriminators to the IR. Path discriminators are
+// used to decide what CFG path was taken inside sub-graphs whose instructions
+// share the same line and column number information.
+//
+// The main user of this is the sample profiler. Instruction samples are
+// mapped to line number information. Since a single line may be spread
+// out over several basic blocks, discriminators add more precise location
+// for the samples.
+//
+// For example,
+//
+//   1  #define ASSERT(P)
+//   2      if (!(P))
+//   3        abort()
+//   ...
+//   100   while (true) {
+//   101     ASSERT (sum < 0);
+//   102     ...
+//   130   }
+//
+// when converted to IR, this snippet looks something like:
+//
+// while.body:                                       ; preds = %entry, %if.end
+//   %0 = load i32* %sum, align 4, !dbg !15
+//   %cmp = icmp slt i32 %0, 0, !dbg !15
+//   br i1 %cmp, label %if.end, label %if.then, !dbg !15
+//
+// if.then:                                          ; preds = %while.body
+//   call void @abort(), !dbg !15
+//   br label %if.end, !dbg !15
+//
+// Notice that all the instructions in blocks 'while.body' and 'if.then'
+// have exactly the same debug information. When this program is sampled
+// at runtime, the profiler will assume that all these instructions are
+// equally frequent. This, in turn, will consider the edge while.body->if.then
+// to be frequently taken (which is incorrect).
+//
+// By adding a discriminator value to the instructions in block 'if.then',
+// we can distinguish instructions at line 101 with discriminator 0 from
+// the instructions at line 101 with discriminator 1.
+//
+// For more details about DWARF discriminators, please visit
+// http://wiki.dwarfstd.org/index.php?title=Path_Discriminators
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "add-discriminators"
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DIBuilder.h"
+#include "llvm/IR/DebugInfo.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+namespace {
+  struct AddDiscriminators : public FunctionPass {
+    static char ID; // Pass identification, replacement for typeid
+    AddDiscriminators() : FunctionPass(ID) {
+      initializeAddDiscriminatorsPass(*PassRegistry::getPassRegistry());
+    }
+
+    bool runOnFunction(Function &F) override;
+  };
+}
+
+char AddDiscriminators::ID = 0;
+INITIALIZE_PASS_BEGIN(AddDiscriminators, "add-discriminators",
+                      "Add DWARF path discriminators", false, false)
+INITIALIZE_PASS_END(AddDiscriminators, "add-discriminators",
+                    "Add DWARF path discriminators", false, false)
+
+// Command line option to disable discriminator generation even in the
+// presence of debug information. This is only needed when debugging
+// debug info generation issues.
+static cl::opt<bool>
+NoDiscriminators("no-discriminators", cl::init(false),
+                 cl::desc("Disable generation of discriminator information."));
+
+FunctionPass *llvm::createAddDiscriminatorsPass() {
+  return new AddDiscriminators();
+}
+
+static bool hasDebugInfo(const Function &F) {
+  NamedMDNode *CUNodes = F.getParent()->getNamedMetadata("llvm.dbg.cu");
+  return CUNodes != 0;
+}
+
+/// \brief Assign DWARF discriminators.
+///
+/// To assign discriminators, we examine the boundaries of every
+/// basic block and its successors. Suppose there is a basic block B1
+/// with successor B2. The last instruction I1 in B1 and the first
+/// instruction I2 in B2 are located at the same file and line number.
+/// This situation is illustrated in the following code snippet:
+///
+///       if (i < 10) x = i;
+///
+///     entry:
+///       br i1 %cmp, label %if.then, label %if.end, !dbg !10
+///     if.then:
+///       %1 = load i32* %i.addr, align 4, !dbg !10
+///       store i32 %1, i32* %x, align 4, !dbg !10
+///       br label %if.end, !dbg !10
+///     if.end:
+///       ret void, !dbg !12
+///
+/// Notice how the branch instruction in block 'entry' and all the
+/// instructions in block 'if.then' have the exact same debug location
+/// information (!dbg !10).
+///
+/// To distinguish instructions in block 'entry' from instructions in
+/// block 'if.then', we generate a new lexical block for all the
+/// instruction in block 'if.then' that share the same file and line
+/// location with the last instruction of block 'entry'.
+///
+/// This new lexical block will have the same location information as
+/// the previous one, but with a new DWARF discriminator value.
+///
+/// One of the main uses of this discriminator value is in runtime
+/// sample profilers. It allows the profiler to distinguish instructions
+/// at location !dbg !10 that execute on different basic blocks. This is
+/// important because while the predicate 'if (x < 10)' may have been
+/// executed millions of times, the assignment 'x = i' may have only
+/// executed a handful of times (meaning that the entry->if.then edge is
+/// seldom taken).
+///
+/// If we did not have discriminator information, the profiler would
+/// assign the same weight to both blocks 'entry' and 'if.then', which
+/// in turn will make it conclude that the entry->if.then edge is very
+/// hot.
+///
+/// To decide where to create new discriminator values, this function
+/// traverses the CFG and examines instruction at basic block boundaries.
+/// If the last instruction I1 of a block B1 is at the same file and line
+/// location as instruction I2 of successor B2, then it creates a new
+/// lexical block for I2 and all the instruction in B2 that share the same
+/// file and line location as I2. This new lexical block will have a
+/// different discriminator number than I1.
+bool AddDiscriminators::runOnFunction(Function &F) {
+  // No need to do anything if there is no debug info for this function.
+  // If the function has debug information, but the user has disabled
+  // discriminators, do nothing.
+  if (!hasDebugInfo(F) || NoDiscriminators) return false;
+
+  bool Changed = false;
+  Module *M = F.getParent();
+  LLVMContext &Ctx = M->getContext();
+  DIBuilder Builder(*M);
+
+  // Traverse all the blocks looking for instructions in different
+  // blocks that are at the same file:line location.
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
+    BasicBlock *B = I;
+    TerminatorInst *Last = B->getTerminator();
+    DebugLoc LastLoc = Last->getDebugLoc();
+    if (LastLoc.isUnknown()) continue;
+    DILocation LastDIL(LastLoc.getAsMDNode(Ctx));
+
+    for (unsigned I = 0; I < Last->getNumSuccessors(); ++I) {
+      BasicBlock *Succ = Last->getSuccessor(I);
+      Instruction *First = Succ->getFirstNonPHIOrDbgOrLifetime();
+      DebugLoc FirstLoc = First->getDebugLoc();
+      if (FirstLoc.isUnknown()) continue;
+      DILocation FirstDIL(FirstLoc.getAsMDNode(Ctx));
+
+      // If the first instruction (First) of Succ is at the same file
+      // location as B's last instruction (Last), add a new
+      // discriminator for First's location and all the instructions
+      // in Succ that share the same location with First.
+      if (FirstDIL.atSameLineAs(LastDIL)) {
+        // Create a new lexical scope and compute a new discriminator
+        // number for it.
+        StringRef Filename = FirstDIL.getFilename();
+        unsigned LineNumber = FirstDIL.getLineNumber();
+        unsigned ColumnNumber = FirstDIL.getColumnNumber();
+        DIScope Scope = FirstDIL.getScope();
+        DIFile File = Builder.createFile(Filename, Scope.getDirectory());
+        unsigned Discriminator = FirstDIL.computeNewDiscriminator(Ctx);
+        DILexicalBlock NewScope = Builder.createLexicalBlock(
+            Scope, File, LineNumber, ColumnNumber, Discriminator);
+        DILocation NewDIL = FirstDIL.copyWithNewScope(Ctx, NewScope);
+        DebugLoc newDebugLoc = DebugLoc::getFromDILocation(NewDIL);
+
+        // Attach this new debug location to First and every
+        // instruction following First that shares the same location.
+        for (BasicBlock::iterator I1(*First), E1 = Succ->end(); I1 != E1;
+             ++I1) {
+          if (I1->getDebugLoc() != FirstLoc) break;
+          I1->setDebugLoc(newDebugLoc);
+          DEBUG(dbgs() << NewDIL.getFilename() << ":" << NewDIL.getLineNumber()
+                       << ":" << NewDIL.getColumnNumber() << ":"
+                       << NewDIL.getDiscriminator() << *I1 << "\n");
+        }
+        DEBUG(dbgs() << "\n");
+        Changed = true;
+      }
+    }
+  }
+  return Changed;
+}