Treat lifetime begin/end markers as allocations/frees respectively for the

purposes for GVN/DSE.

llvm-svn: 85383

1 files changed, 21 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 32d027aa36d..dd8859b5e84 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -1248,6 +1248,15 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
                                              UndefValue::get(LI->getType())));
       continue;
     }
+    
+    // Loading immediately after lifetime begin or end -> undef.
+    if (IntrinsicInst* II = dyn_cast<IntrinsicInst>(DepInst)) {
+      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
+          II->getIntrinsicID() == Intrinsic::lifetime_end) {
+        ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB,
+                                             UndefValue::get(LI->getType())));
+      }
+    }
 
     if (StoreInst *S = dyn_cast<StoreInst>(DepInst)) {
       // Reject loads and stores that are to the same address but are of
@@ -1591,6 +1600,18 @@ bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
     NumGVNLoad++;
     return true;
   }
+  
+  // If this load occurs either right after a lifetime begin or a lifetime end,
+  // then the loaded value is undefined.
+  if (IntrinsicInst* II = dyn_cast<IntrinsicInst>(DepInst)) {
+    if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
+        II->getIntrinsicID() == Intrinsic::lifetime_end) {
+      L->replaceAllUsesWith(UndefValue::get(L->getType()));
+      toErase.push_back(L);
+      NumGVNLoad++;
+      return true;
+    }
+  }
 
   return false;
 }