Fold compares irrespective of whether allocation can be elided

Summary
When a non-escaping pointer is compared to a global value, the
comparison can be folded even if the corresponding malloc/allocation
call cannot be elided.
We need to make sure the global value is not null, since comparisons to
null cannot be folded.

In future, we should also handle cases when the the comparison
instruction dominates the pointer escape.

Reviewers: sanjoy
Subscribers s.egerton, llvm-commits

Differential Revision: http://reviews.llvm.org/D19549

llvm-svn: 268390

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/CaptureTracking.cpp b/llvm/lib/Analysis/CaptureTracking.cpp
index 5125a63..5997975 100644
--- a/llvm/lib/Analysis/CaptureTracking.cpp
+++ b/llvm/lib/Analysis/CaptureTracking.cpp
@@ -300,7 +300,7 @@ void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
             Worklist.push_back(&UU);
       }
       break;
-    case Instruction::ICmp:
+    case Instruction::ICmp: {
       // Don't count comparisons of a no-alias return value against null as
       // captures. This allows us to ignore comparisons of malloc results
       // with null, for example.
@@ -309,11 +309,19 @@ void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
         if (CPN->getType()->getAddressSpace() == 0)
           if (isNoAliasCall(V->stripPointerCasts()))
             break;
+      // Comparison against value stored in global variable. Given the pointer
+      // does not escape, its value cannot be guessed and stored separately in a
+      // global variable.
+      unsigned OtherIndex = (I->getOperand(0) == V) ? 1 : 0;
+      auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIndex));
+      if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
+        break;
       // Otherwise, be conservative. There are crazy ways to capture pointers
       // using comparisons.
       if (Tracker->captured(U))
         return;
       break;
+    }
     default:
       // Something else - be conservative and say it is captured.
       if (Tracker->captured(U))