[llvm] [SCEV] Better preserve wrapping info in SimplifyICmpOperands for UGE. (PR #144404)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/144404

Update SimplifyICmpOperands to only try subtracting 1 from RHS first, if RHS is an op we can fold the subtract directly into. Otherwise try adding to LHS first, as we can preserve NUW flags.

This improves results in a few cases, including the modified test case from berkeley-abc and new code to be added in
https://github.com/llvm/llvm-project/pull/128061.

Note that there are more cases where the results can be improved by better ordering here which I'll try to investigate as follow-up.

>From cfcc8f32cab1816d989281b9e9b6ef67c915c94c Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 16 Jun 2025 16:32:37 +0100
Subject: [PATCH] [SCEV] Better preserve wrapping info in SimplifyICmpOperands
 for UGE.

Update SimplifyICmpOperands to only try subtracting 1 from RHS first, if
RHS is an op we can fold the subtract directly into. Otherwise try
adding to LHS first, as we can preserve NUW flags.

This improves results in a few cases, including the modified test case
from berkeley-abc and new code to be added in
https://github.com/llvm/llvm-project/pull/128061.

Note that there are more cases where the results can be improved by
better ordering here which I'll try to investigate as follow-up.
---
 llvm/lib/Analysis/ScalarEvolution.cpp                  | 10 +++++++++-
 .../IndVarSimplify/simplify-icmp-operands-order.ll     |  9 +++------
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2dfe625eb0dcc..ea7c7ece5632f 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10892,7 +10892,11 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
     }
     break;
   case ICmpInst::ICMP_UGE:
-    if (!getUnsignedRangeMin(RHS).isMinValue()) {
+    // If RHS is an op we can fold the -1, try that first.
+    if ((isa<SCEVConstant>(RHS) ||
+         match(RHS,
+               m_scev_AffineAddRec(m_SCEVConstant(), m_SCEV(), m_Loop()))) &&
+        !getUnsignedRangeMin(RHS).isMinValue()) {
       RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
       Pred = ICmpInst::ICMP_UGT;
       Changed = true;
@@ -10901,6 +10905,10 @@ bool ScalarEvolution::SimplifyICmpOperands(CmpPredicate &Pred, const SCEV *&LHS,
                        SCEV::FlagNUW);
       Pred = ICmpInst::ICMP_UGT;
       Changed = true;
+    } else if (!getUnsignedRangeMin(RHS).isMinValue()) {
+      RHS = getAddExpr(getConstant(RHS->getType(), (uint64_t)-1, true), RHS);
+      Pred = ICmpInst::ICMP_UGT;
+      Changed = true;
     }
     break;
   default:
diff --git a/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll b/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
index b0dbbd5eaedf4..fb2fdb116f904 100644
--- a/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
+++ b/llvm/test/Transforms/IndVarSimplify/simplify-icmp-operands-order.ll
@@ -53,15 +53,12 @@ loop.latch:
 
 define void @test_simplifycompare_rhs_not_constant1() {
 ; CHECK-LABEL: define void @test_simplifycompare_rhs_not_constant1() {
-; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    [[P:%.*]] = alloca i64, align 8
 ; CHECK-NEXT:    br label %[[LOOP:.*]]
 ; CHECK:       [[LOOP]]:
-; CHECK-NEXT:    [[PTR_IV:%.*]] = phi ptr [ [[P]], %[[ENTRY]] ], [ [[PTR_IV_NEXT:%.*]], %[[LOOP]] ]
-; CHECK-NEXT:    [[PTR_IV_NEXT]] = getelementptr i8, ptr [[PTR_IV]], i64 -8
-; CHECK-NEXT:    call void @use(ptr [[PTR_IV]])
-; CHECK-NEXT:    [[EC:%.*]] = icmp ult ptr [[PTR_IV_NEXT]], [[P]]
-; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT:.*]], label %[[LOOP]]
+; CHECK-NEXT:    call void @use(ptr [[P]])
+; CHECK-NEXT:    br i1 true, label %[[EXIT:.*]], label %[[LOOP]]
 ; CHECK:       [[EXIT]]:
 ; CHECK-NEXT:    ret void
 ;