Folding compares with unescaped allocations

Summary:
If we know that the pointer allocated within a function does not escape,
we can fold away comparisons that are done with global pointers

Patch by Anna Thomas!

Reviewers: reames, majnemer, sanjoy

Subscribers: mgrang, mcrosier, majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D19276

llvm-svn: 267035

1 files changed, 14 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 8a9d2821b44..586699d5889 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1863,6 +1863,14 @@ Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   return nullptr;
 }
 
+static bool isNeverEqualToUnescapedAlloc(Value *V) {
+  if (isa<ConstantPointerNull>(V))
+    return true;
+  if (auto *LI = dyn_cast<LoadInst>(V))
+    return isa<GlobalVariable>(LI->getPointerOperand());
+  return false;
+}
+
 static bool
 isAllocSiteRemovable(Instruction *AI, SmallVectorImpl<WeakVH> &Users,
                      const TargetLibraryInfo *TLI) {
@@ -1887,7 +1895,12 @@ isAllocSiteRemovable(Instruction *AI, SmallVectorImpl<WeakVH> &Users,
       case Instruction::ICmp: {
         ICmpInst *ICI = cast<ICmpInst>(I);
         // We can fold eq/ne comparisons with null to false/true, respectively.
-        if (!ICI->isEquality() || !isa<ConstantPointerNull>(ICI->getOperand(1)))
+        // We fold comparisons in some conditions provided the alloc has not
+        // escaped.
+        if (!ICI->isEquality())
+          return false;
+        unsigned OtherIndex = (ICI->getOperand(0) == PI) ? 1 : 0;
+        if (!isNeverEqualToUnescapedAlloc(ICI->getOperand(OtherIndex)))
           return false;
         Users.emplace_back(I);
         continue;