[InstSimplify] guard against unreachable code (PR43218)

This would crash:
https://bugs.llvm.org/show_bug.cgi?id=43218

llvm-svn: 370911

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/InstSimplifyPass.cpp b/llvm/lib/Transforms/Scalar/InstSimplifyPass.cpp
index 920b12b8a6a..e3edfe51458 100644
--- a/llvm/lib/Transforms/Scalar/InstSimplifyPass.cpp
+++ b/llvm/lib/Transforms/Scalar/InstSimplifyPass.cpp
@@ -34,6 +34,11 @@ static bool runImpl(Function &F, const SimplifyQuery &SQ,
 
   do {
     for (BasicBlock &BB : F) {
+      // Unreachable code can take on strange forms that we are not prepared to
+      // handle. For example, an instruction may have itself as an operand.
+      if (!SQ.DT->isReachableFromEntry(&BB))
+        continue;
+
       SmallVector<Instruction *, 8> DeadInstsInBB;
       for (Instruction &I : BB) {
         // The first time through the loop, ToSimplify is empty and we try to
@@ -87,7 +92,7 @@ struct InstSimplifyLegacyPass : public FunctionPass {
     AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
   }
 
-  /// runOnFunction - Remove instructions that simplify.
+  /// Remove instructions that simplify.
   bool runOnFunction(Function &F) override {
     if (skipFunction(F))
       return false;