[llvm] [DivRemPairs] Do not freeze poisons that can't be undef (PR #92627)

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 created https://github.com/llvm/llvm-project/pull/92627

Per comments in DivRemPairs, the rewrite from
```llvm
    %div = div %X, %Y
    %rem = rem %X, %Y
```
to
```llvm
    %div = div %X, %Y
    %.mul = mul %div, %Y
    %rem = sub %X, %mul
```
is unsound when %X or %Y are undef.

However, it is known to be sound if %X or %Y are poison but can't be undef, since both the pre- and post-rewrite %rem are `poison`.

A comment in the pass listed a TODO for changing a usage of isGuaranteedNotToBeUndefOrPoison() in the pass to something that only detects undef. Such a function has been implemented since the time that TODO was written, but has not been used.

Therefore, this commit updates DivRemPairs to use
isGuaranteedNotToBeUndef() instead.

>From aac0790375885623011cea30dda6d14f0a424e54 Mon Sep 17 00:00:00 2001
From: Krzysztof Drewniak <Krzysztof.Drewniak at amd.com>
Date: Sat, 18 May 2024 01:35:18 +0000
Subject: [PATCH] [DivRemPairs] Do not freeze poisons that can't be undef

Per comments in DivRemPairs, the rewrite from
    %div = div %X, %Y
    %rem = rem %X, %Y
to
    %div = div %X, %Y
    %.mul = mul %div, %Y
    %rem = sub %X, %mul
is unsound when %X or %Y are undef.

However, it is known to be sound if %X or %Y are poison but can't be
undef, since both the pre- and post-rewrite %rem are `poison`.

A comment in the pass listed a TODO for changing a usage of
isGuaranteedNotToBeUndefOrPoison() in the pass to something that only
detects undef. Such a function has been implemented since the time
that TODO was written, but has not been used.

Therefore, this commit updates DivRemPairs to use
isGuaranteedNotToBeUndef() instead.
---
 llvm/lib/Transforms/Scalar/DivRemPairs.cpp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/DivRemPairs.cpp b/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
index 45f36a36b5dd0..f7ada9fb8eb8a 100644
--- a/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
+++ b/llvm/lib/Transforms/Scalar/DivRemPairs.cpp
@@ -381,8 +381,7 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
       //   %mul = mul %div, 1   // %mul = undef
       //   %rem = sub %x, %mul  // %rem = undef - undef = undef
       // If X is not frozen, %rem becomes undef after transformation.
-      // TODO: We need a undef-specific checking function in ValueTracking
-      if (!isGuaranteedNotToBeUndefOrPoison(X, nullptr, DivInst, &DT)) {
+      if (!isGuaranteedNotToBeUndef(X, nullptr, DivInst, &DT)) {
         auto *FrX =
             new FreezeInst(X, X->getName() + ".frozen", DivInst->getIterator());
         DivInst->setOperand(0, FrX);
@@ -390,7 +389,7 @@ static bool optimizeDivRem(Function &F, const TargetTransformInfo &TTI,
       }
       // Same for Y. If X = 1 and Y = (undef | 1), %rem in src is either 1 or 0,
       // but %rem in tgt can be one of many integer values.
-      if (!isGuaranteedNotToBeUndefOrPoison(Y, nullptr, DivInst, &DT)) {
+      if (!isGuaranteedNotToBeUndef(Y, nullptr, DivInst, &DT)) {
         auto *FrY =
             new FreezeInst(Y, Y->getName() + ".frozen", DivInst->getIterator());
         DivInst->setOperand(1, FrY);