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

Add facts for llvm.abs >= 0.

https://alive2.llvm.org/ce/z/GXnMHu

Author: alexander-shaposhnikov

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -1755,7 +1755,10 @@ 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;
       }

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