Don't assume that external global variables are aligned at their preferred

alignment. Only the minimum alignment guaranteed by the ABI may be assumed.

llvm-svn: 78668

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index dbaadd824a1..af8649499b2 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -81,8 +81,16 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
   // The address of an aligned GlobalValue has trailing zeros.
   if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
     unsigned Align = GV->getAlignment();
-    if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) 
-      Align = TD->getPrefTypeAlignment(GV->getType()->getElementType());
+    if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) {
+      const Type *ObjectType = GV->getType()->getElementType();
+      // If the object is defined in the current Module, we'll be giving
+      // it the preferred alignment. Otherwise, we have to assume that it
+      // may only have the minimum ABI alignment.
+      if (!GV->isDeclaration() && !GV->mayBeOverridden())
+        Align = TD->getPrefTypeAlignment(ObjectType);
+      else
+        Align = TD->getABITypeAlignment(ObjectType);
+    }
     if (Align > 0)
       KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
                                               CountTrailingZeros_32(Align));