[llvm-commits] [llvm] r47006 - /llvm/trunk/lib/Transforms/Scalar/GVN.cpp

[Eli Friedman]

Author: efriedma
Date: Tue Feb 12 06:08:14 2008
New Revision: 47006

URL: http://llvm.org/viewvc/llvm-project?rev=47006&view=rev
Log:
Fix for bug 1996: optimize out loads of undef.  This code basically just 
checks for a malloc/alloca immediately followed by a load.

Modified:
    llvm/trunk/lib/Transforms/Scalar/GVN.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/GVN.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/GVN.cpp?rev=47006&r1=47005&r2=47006&view=diff

==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/GVN.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/GVN.cpp Tue Feb 12 06:08:14 2008
@@ -1010,7 +1010,34 @@
       dep = MD.getDependency(L, dep);
     }
   }
-  
+
+  if (dep != MemoryDependenceAnalysis::None &&
+      dep != MemoryDependenceAnalysis::NonLocal &&
+      isa<AllocationInst>(dep)) {
+    // Check that this load is actually from the
+    // allocation we found
+    Value* v = L->getOperand(0);
+    while (true) {
+      if (BitCastInst *BC = dyn_cast<BitCastInst>(v))
+        v = BC->getOperand(0);
+      else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(v))
+        v = GEP->getOperand(0);
+      else
+        break;
+    }
+    if (v == dep) {
+      // If this load depends directly on an allocation, there isn't
+      // anything stored there; therefore, we can optimize this load
+      // to undef.
+      MD.removeInstruction(L);
+
+      L->replaceAllUsesWith(UndefValue::get(L->getType()));
+      toErase.push_back(L);
+      deletedLoad = true;
+      NumGVNLoad++;
+    }
+  }
+
   if (!deletedLoad)
     last = L;