[IndVarSimplify] Don't replace IV user with unsafe loop-invariant (PR45360)

Summary:
As [[ https://bugs.llvm.org/show_bug.cgi?id=45360 | PR45360 ]] reports,
with new cost-model we can sometimes end up being able to expand `udiv`/`urem` instructions.
And that exposes at least one instance of when we do that
regardless of whether or not it is safe to do.
In this particular case, it's `SimplifyIndvar::replaceIVUserWithLoopInvariant()`.

It seems to me, we simply need to check with `isSafeToExpandAt()` first.

The test isn't great. I'm not sure how to make it only run `-indvars`.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=45360 | PR45360 ]].

Reviewers: mkazantsev, reames, helloqirun

Reviewed By: mkazantsev

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82108

1 files changed, 8 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index 05632ca..d3d0c33 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -657,7 +657,7 @@ static Instruction *GetLoopInvariantInsertPosition(Loop *L, Instruction *Hint) {
   return Hint;
 }
 
-/// Replace the UseInst with a constant if possible.
+/// Replace the UseInst with a loop invariant expression if it is safe.
 bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
   if (!SE->isSCEVable(I->getType()))
     return false;
@@ -673,6 +673,13 @@ bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
     return false;
 
   auto *IP = GetLoopInvariantInsertPosition(L, I);
+
+  if (!isSafeToExpandAt(S, IP, *SE)) {
+    LLVM_DEBUG(dbgs() << "INDVARS: Can not replace IV user: " << *I
+                      << " with non-speculable loop invariant: " << *S << '\n');
+    return false;
+  }
+
   auto *Invariant = Rewriter.expandCodeFor(S, I->getType(), IP);
 
   I->replaceAllUsesWith(Invariant);