[InstCombine] allow for constant-folding in GEP transform

This would crash the reduced test or as described in
https://llvm.org/PR51485
...because we can't mark a constant (-expression) with 'inbounds'.

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 434659c..1a80374 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2118,10 +2118,12 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
               // -- have to recreate %src & %gep
               // put NewSrc at same location as %src
               Builder.SetInsertPoint(cast<Instruction>(PtrOp));
-              auto *NewSrc = cast<GetElementPtrInst>(
-                  Builder.CreateGEP(GEPEltType, SO0, GO1, Src->getName()));
-              NewSrc->setIsInBounds(Src->isInBounds());
-              auto *NewGEP =
+              Value *NewSrc =
+                  Builder.CreateGEP(GEPEltType, SO0, GO1, Src->getName());
+              // Propagate 'inbounds' if the new source was not constant-folded.
+              if (auto *NewSrcGEPI = dyn_cast<GetElementPtrInst>(NewSrc))
+                NewSrcGEPI->setIsInBounds(Src->isInBounds());
+              GetElementPtrInst *NewGEP =
                   GetElementPtrInst::Create(GEPEltType, NewSrc, {SO1});
               NewGEP->setIsInBounds(GEP.isInBounds());
               return NewGEP;

diff --git a/llvm/test/Transforms/InstCombine/gep-combine-loop-invariant.ll b/llvm/test/Transforms/InstCombine/gep-combine-loop-invariant.ll
index faaaff8..58d16e1 100644 (file)
--- a/llvm/test/Transforms/InstCombine/gep-combine-loop-invariant.ll
+++ b/llvm/test/Transforms/InstCombine/gep-combine-loop-invariant.ll
@@ -186,3 +186,26 @@ loop:
   call void @blackhole(<2 x i8*> %e6)
   br label %loop
 }
+
+; This would crash because we did not expect to be able to constant fold a GEP.
+
+define void @PR51485(<2 x i64> %v) {
+; CHECK-LABEL: @PR51485(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[SL1:%.*]] = shl nuw nsw <2 x i64> [[V:%.*]], <i64 7, i64 7>
+; CHECK-NEXT:    [[E6:%.*]] = getelementptr inbounds i8, i8* getelementptr (i8, i8* bitcast (void (<2 x i64>)* @PR51485 to i8*), i64 80), <2 x i64> [[SL1]]
+; CHECK-NEXT:    call void @blackhole(<2 x i8*> [[E6]])
+; CHECK-NEXT:    br label [[LOOP]]
+;
+entry:
+  br label %loop
+
+loop:
+  %sl1 = shl nuw nsw <2 x i64> %v, <i64 7, i64 7>
+  %e5 = getelementptr inbounds i8, i8* bitcast (void (<2 x i64>)* @PR51485 to i8*), <2 x i64> %sl1
+  %e6 = getelementptr inbounds i8, <2 x i8*> %e5, i64 80
+  call void @blackhole(<2 x i8*> %e6)
+  br label %loop
+}