[InstCombine] don't assume 'inbounds' for bitcast pointer to GEP transform (PR43501)

https://bugs.llvm.org/show_bug.cgi?id=43501
We can't declare a GEP 'inbounds' in general. But we may salvage that information if
we have known dereferenceable bytes on the source pointer.

Differential Revision: https://reviews.llvm.org/D68244

llvm-svn: 373847

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 74c69808f15..c58e63d08e3 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -2338,8 +2338,15 @@ Instruction *InstCombiner::visitBitCast(BitCastInst &CI) {
     // If we found a path from the src to dest, create the getelementptr now.
     if (SrcElTy == DstElTy) {
       SmallVector<Value *, 8> Idxs(NumZeros + 1, Builder.getInt32(0));
-      return GetElementPtrInst::CreateInBounds(SrcPTy->getElementType(), Src,
-                                               Idxs);
+      GetElementPtrInst *GEP =
+          GetElementPtrInst::Create(SrcPTy->getElementType(), Src, Idxs);
+
+      // If the source pointer is dereferenceable, then assume it points to an
+      // allocated object and apply "inbounds" to the GEP.
+      bool CanBeNull;
+      if (Src->getPointerDereferenceableBytes(DL, CanBeNull))
+        GEP->setIsInBounds();
+      return GEP;
     }
   }