[llvm] [ConstraintElim] Check of other OP is guaranteed to not be undef/poison. (PR #76182)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/76182

Address the TODO from  #75750 by checking if the second operand may be poison. We can simplify the first operand if it is implied by the other operand, if the other operand is guaranteed to not be undef/poison. If it may be poison, it would mean the select would be unconditionally poison after the transform.

Note that the original TODO incorrectly said that the first operand would need checking, but it is the other one that needs checking.

>From c97d8166990836de4a3686e65552913ef17cb593 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 20 Dec 2023 12:12:56 +0000
Subject: [PATCH] [ConstraintElim] Check of other OP is guaranteed to not be
 undef/poison.

Address the TODO from  #75750 by checking if the second operand may be
poison. We can simplify the first operand if it is implied by the other
operand, if the other operand is guaranteed to not be undef/poison.
If it may be poison, it would mean the select would be unconditionally
poison after the transform.

Note that the original TODO incorrectly said that the first operand
would need checking, but it is the other one that needs checking.
---
 llvm/lib/Transforms/Scalar/ConstraintElimination.cpp     | 9 +++++----
 .../ConstraintElimination/and-implied-by-operands.ll     | 2 +-
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 98cfadddee8efb..1780e23af081f2 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -1378,10 +1378,11 @@ static bool checkOrAndOpImpliedByOther(
   CmpInst *CmpToCheck = cast<CmpInst>(CB.getInstructionToSimplify());
   unsigned OtherOpIdx = JoinOp->getOperand(0) == CmpToCheck ? 1 : 0;
 
-  // Don't try to simplify the first condition of a select by the second, as
-  // this may make the select more poisonous than the original one.
-  // TODO: check if the first operand may be poison.
-  if (OtherOpIdx != 0 && isa<SelectInst>(JoinOp))
+  // Don't try to simplify the first condition of a select using the other
+  // condition, if this makes the select more poisonous than the original one.
+  if (OtherOpIdx != 0 && isa<SelectInst>(JoinOp) &&
+      !isGuaranteedNotToBeUndefOrPoison(JoinOp->getOperand(OtherOpIdx), nullptr,
+                                        JoinOp))
     return false;
 
   if (!match(JoinOp->getOperand(OtherOpIdx),
diff --git a/llvm/test/Transforms/ConstraintElimination/and-implied-by-operands.ll b/llvm/test/Transforms/ConstraintElimination/and-implied-by-operands.ll
index 5c49ca0e96f309..329bd94beac852 100644
--- a/llvm/test/Transforms/ConstraintElimination/and-implied-by-operands.ll
+++ b/llvm/test/Transforms/ConstraintElimination/and-implied-by-operands.ll
@@ -486,7 +486,7 @@ define i1 @and_select_second_implies_first_guaranteed_not_poison(ptr noundef %A,
 ; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds ptr, ptr [[B]], i64 -1
 ; CHECK-NEXT:    [[C_2:%.*]] = icmp ugt ptr [[GEP]], [[A]]
 ; CHECK-NEXT:    call void @no_noundef(i1 [[C_2]])
-; CHECK-NEXT:    [[AND:%.*]] = select i1 [[C_1]], i1 [[C_2]], i1 false
+; CHECK-NEXT:    [[AND:%.*]] = select i1 true, i1 [[C_2]], i1 false
 ; CHECK-NEXT:    ret i1 [[AND]]
 ;
 entry: