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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231755 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 0 deletions

diff --git a/test/Transforms/InstCombine/constant-fold-hang.ll b/test/Transforms/InstCombine/constant-fold-hang.ll
new file mode 100644
index 00000000000..2ca6b86ccc2
--- /dev/null
+++ b/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 }