[GVN] Avoid casting a vector of size less than 8 bits to i8

Summary:
A reprise of D25849.

This crash was found through fuzzing some time ago and was documented in PR28879.

No check for load size has been added due to the following tests:
  - Transforms/GVN/invariant.group.ll
  - Transforms/GVN/pr10820.ll

These tests expect load sizes that are not a multiple of eight.

Thanks to @davide for the original patch.

Reviewers: nlopes, davide, RKSimon, reames, efriedma

Reviewed By: efriedma

Subscribers: davide, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D48330

llvm-svn: 335294

1 files changed, 7 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/VNCoercion.cpp b/llvm/lib/Transforms/Utils/VNCoercion.cpp
index 5f71bdcf597..948d9bd5baa 100644
--- a/llvm/lib/Transforms/Utils/VNCoercion.cpp
+++ b/llvm/lib/Transforms/Utils/VNCoercion.cpp
@@ -20,8 +20,14 @@ bool canCoerceMustAliasedValueToLoad(Value *StoredVal, Type *LoadTy,
       StoredVal->getType()->isStructTy() || StoredVal->getType()->isArrayTy())
     return false;
 
+  uint64_t StoreSize = DL.getTypeSizeInBits(StoredVal->getType());
+
+  // The store size must be byte-aligned to support future type casts.
+  if (llvm::alignTo(StoreSize, 8) != StoreSize)
+    return false;
+
   // The store has to be at least as big as the load.
-  if (DL.getTypeSizeInBits(StoredVal->getType()) < DL.getTypeSizeInBits(LoadTy))
+  if (StoreSize < DL.getTypeSizeInBits(LoadTy))
     return false;
 
   // Don't coerce non-integral pointers to integers or vice versa.