[llvm] [ConstraintElim] Add facts for llvm.abs >= 0 (PR #79070)

[Alexander Shaposhnikov via llvm-commits]

https://github.com/alexander-shaposhnikov updated https://github.com/llvm/llvm-project/pull/79070

>From da728ca30a5c6e888f2f886786c27895a4a18814 Mon Sep 17 00:00:00 2001
From: Alexander Shaposhnikov <ashaposhnikov at google.com>
Date: Mon, 22 Jan 2024 21:49:21 +0000
Subject: [PATCH] [ConstraintElim] Add facts about nonnegativity of llvm.abs

---
 llvm/lib/Transforms/Scalar/ConstraintElimination.cpp | 5 ++++-
 llvm/test/Transforms/ConstraintElimination/abs.ll    | 6 ++----
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index ae8e2292519cb..587662cd7959f 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -1730,7 +1730,10 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT, LoopInfo &LI,
     if (!CB.isConditionFact()) {
       Value *X;
       if (match(CB.Inst, m_Intrinsic<Intrinsic::abs>(m_Value(X)))) {
-        // TODO: Add CB.Inst >= 0 fact.
+        // If is_int_min_poison is true then we may assume llvm.abs >= 0.
+        if (cast<ConstantInt>(CB.Inst->getOperand(1))->isOne())
+          AddFact(CmpInst::ICMP_SGE, CB.Inst,
+                  ConstantInt::get(CB.Inst->getType(), 0));
         AddFact(CmpInst::ICMP_SGE, CB.Inst, X);
         continue;
       }
diff --git a/llvm/test/Transforms/ConstraintElimination/abs.ll b/llvm/test/Transforms/ConstraintElimination/abs.ll
index a7b20997fa4cd..9fc68b0e72663 100644
--- a/llvm/test/Transforms/ConstraintElimination/abs.ll
+++ b/llvm/test/Transforms/ConstraintElimination/abs.ll
@@ -142,8 +142,7 @@ define i1 @abs_is_nonnegative_int_min_is_poison(i32 %arg) {
 ; CHECK-LABEL: define i1 @abs_is_nonnegative_int_min_is_poison(
 ; CHECK-SAME: i32 [[ARG:%.*]]) {
 ; CHECK-NEXT:    [[ABS:%.*]] = tail call i32 @llvm.abs.i32(i32 [[ARG]], i1 true)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sge i32 [[ABS]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %abs = tail call i32 @llvm.abs.i32(i32 %arg, i1 true)
   %cmp = icmp sge i32 %abs, 0
@@ -153,8 +152,7 @@ define i1 @abs_is_nonnegative_int_min_is_poison(i32 %arg) {
 define i1 @abs_is_nonnegative_constant_arg() {
 ; CHECK-LABEL: define i1 @abs_is_nonnegative_constant_arg() {
 ; CHECK-NEXT:    [[ABS:%.*]] = tail call i32 @llvm.abs.i32(i32 -3, i1 true)
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sge i32 [[ABS]], 0
-; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK-NEXT:    ret i1 true
 ;
   %abs = tail call i32 @llvm.abs.i32(i32 -3, i1 true)
   %cmp = icmp sge i32 %abs, 0