[llvm-commits] [llvm] r108101 - /llvm/trunk/lib/VMCore/Instruction.cpp

[Nick Lewycky]

Author: nicholas
Date: Sun Jul 11 15:36:29 2010
New Revision: 108101

URL: http://llvm.org/viewvc/llvm-project?rev=108101&view=rev
Log:
If it's safe to speculatively execute load(alloca) the it's safe to execute
load(gep(alloca)) where the gep is all-zeros. There's more we could do here but
this is a common case.

Modified:
    llvm/trunk/lib/VMCore/Instruction.cpp

Modified: llvm/trunk/lib/VMCore/Instruction.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/VMCore/Instruction.cpp?rev=108101&r1=108100&r2=108101&view=diff
==============================================================================
--- llvm/trunk/lib/VMCore/Instruction.cpp (original)
+++ llvm/trunk/lib/VMCore/Instruction.cpp Sun Jul 11 15:36:29 2010
@@ -401,12 +401,20 @@
       return false;
     // Note that it is not safe to speculate into a malloc'd region because
     // malloc may return null.
-    if (isa<AllocaInst>(getOperand(0)))
+    // It's also not safe to follow a bitcast, for example:
+    //   bitcast i8* (alloca i8) to i32*
+    // would result in a 4-byte load from a 1-byte alloca.
+    Value *Op0 = getOperand(0);
+    if (GEPOperator *GEP = dyn_cast<GEPOperator>(Op0)) {
+      // TODO: it's safe to do this for any GEP with constant indices that
+      // compute inside the allocated type, but not for any inbounds gep.
+      if (GEP->hasAllZeroIndices())
+        Op0 = GEP->getPointerOperand();
+    }
+    if (isa<AllocaInst>(Op0))
       return true;
     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(getOperand(0)))
       return !GV->hasExternalWeakLinkage();
-    // FIXME: Handle cases involving GEPs.  We have to be careful because
-    // a load of a out-of-bounds GEP has undefined behavior.
     return false;
   }
   case Call: