InstCombine: Don't shrink allocas when combining with a bitcast.

When considering folding a bitcast of an alloca into the alloca itself,
make sure we don't shrink the amount of memory being allocated, or
things rapidly go sideways.

rdar://13324424

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176547 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index a960ab2499..d162223a6f 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -104,6 +104,12 @@ Instruction *InstCombiner::PromoteCastOfAllocation(BitCastInst &CI,
   uint64_t CastElTySize = TD->getTypeAllocSize(CastElTy);
   if (CastElTySize == 0 || AllocElTySize == 0) return 0;
 
+  // If the allocation has multiple uses, only promote it if we're not
+  // shrinking the amount of memory being allocated.
+  uint64_t AllocElTyStoreSize = TD->getTypeStoreSize(AllocElTy);
+  uint64_t CastElTyStoreSize = TD->getTypeStoreSize(CastElTy);
+  if (!AI.hasOneUse() && CastElTyStoreSize < AllocElTyStoreSize) return 0;
+
   // See if we can satisfy the modulus by pulling a scale out of the array
   // size argument.
   unsigned ArraySizeScale;