Fix an infinite loop in InstCombine when an instruction with no users and side effects can be constant folded.

ReplaceInstUsesWith needs to return nullptr when the input has no users,
because in that case it does not mutate the program.  Otherwise, we can
get stuck in an infinite loop of repeatedly attempting to constant fold
and instruction with no users.

llvm-svn: 231755

2 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 8c784ab1e68..fb2321df2e0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -372,6 +372,10 @@ public:
   /// I to the worklist, replace all uses of I with the new value, then return
   /// I, so that the inst combiner will know that I was modified.
   Instruction *ReplaceInstUsesWith(Instruction &I, Value *V) {
+    // If there are no uses to replace, then we return nullptr to indicate that
+    // no changes were made to the program.
+    if (I.use_empty()) return nullptr;
+
     Worklist.AddUsersToWorkList(I); // Add all modified instrs to worklist.
 
     // If we are replacing the instruction with itself, this must be in a
diff --git a/llvm/test/Transforms/InstCombine/constant-fold-hang.ll b/llvm/test/Transforms/InstCombine/constant-fold-hang.ll
new file mode 100644
index 00000000000..2ca6b86ccc2
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/constant-fold-hang.ll
@@ -0,0 +1,14 @@
+; RUN: opt -instcombine < %s
+
+; Function Attrs: nounwind readnone ssp
+define void @mulByZero(<4 x i16> %x) #0 {
+entry:
+  %a = tail call <4 x i32> @llvm.arm.neon.vmulls.v4i32(<4 x i16> %x, <4 x i16> zeroinitializer) #2
+  ret void
+}
+
+; Function Attrs: nounwind readnone
+declare <4 x i32> @llvm.arm.neon.vmulls.v4i32(<4 x i16>, <4 x i16>) #1
+
+attributes #0 = { nounwind readnone ssp }
+attributes #1 = { nounwind readnone }