[IR] Consider non-willreturn as side effect (PR50511)

This adjusts mayHaveSideEffect() to return true for !willReturn()
instructions. Just like other side-effects, non-willreturn calls
(aka "divergence") cannot be removed and cannot be reordered relative
to other side effects. This fixes a number of bugs where
non-willreturn calls are either incorrectly dropped or moved. In
particular, it also fixes the last open problem in
https://bugs.llvm.org/show_bug.cgi?id=50511.

I performed a cursory review of all current mayHaveSideEffect()
uses, which convinced me that these are indeed the desired default
semantics. Places that do not want to consider non-willreturn as a
sideeffect generally do not want mayHaveSideEffect() semantics at
all. I identified two such cases, which are addressed by D106591
and D106742. Finally, there is a use in SCEV for which we don't
really have an appropriate API right now -- what it wants is
basically "would this be considered forward progress". I've just
spelled out the previous semantics there.

Differential Revision: https://reviews.llvm.org/D106749

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/DemandedBits.cpp b/llvm/lib/Analysis/DemandedBits.cpp
index 467dea9..ca6d58f 100644
--- a/llvm/lib/Analysis/DemandedBits.cpp
+++ b/llvm/lib/Analysis/DemandedBits.cpp
@@ -80,7 +80,7 @@ void DemandedBitsWrapperPass::print(raw_ostream &OS, const Module *M) const {
 
 static bool isAlwaysLive(Instruction *I) {
   return I->isTerminator() || isa<DbgInfoIntrinsic>(I) || I->isEHPad() ||
-         I->mayHaveSideEffects() || !I->willReturn();
+         I->mayHaveSideEffects();
 }
 
 void DemandedBits::determineLiveOperandBits(