[llvm] [ConstraintElim] Apply add with neg constant first during decomp. (PR #164791)

[Florian Hahn via llvm-commits]

https://github.com/fhahn created https://github.com/llvm/llvm-project/pull/164791

We have dedicated decomposition logic for (add %x, -C), but if we have (add nsw %x, -C)  we will first apply the generic logic for NSWAdd, which gives worse results for negative constants in practice.

Update the code to first apply the pattern with negative constants.

Helps to remove a number of runtime checks in practice: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2968

Alive2 proofs for the test changes: https://alive2.llvm.org/ce/z/JfR2Ma

>From 9be37263f19cfee124c2f3bba4e57cdafa06447b Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 22 Oct 2025 14:06:52 +0100
Subject: [PATCH] [ConstraintElim] Apply add with neg constant first during
 decomp.

We have dedicated decomposition logic for (add %x, -C), but if we have
(add nsw %x, -C)  we will first apply the generic logic for NSWAdd,
which gives worse results for negative constants in practice.

Update the code to first apply the pattern with negative constants.

Helps to remove a number of runtime checks in practice:
https://github.com/dtcxzyw/llvm-opt-benchmark/pull/2968

Alive2 proofs for the test changes: https://alive2.llvm.org/ce/z/JfR2Ma
---
 .../Scalar/ConstraintElimination.cpp          | 20 +++++++++----------
 .../ConstraintElimination/add-nsw.ll          |  6 ++----
 .../gep-arithmetic-add.ll                     |  6 ++----
 3 files changed, 14 insertions(+), 18 deletions(-)

diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 4acc3f2d84690..d347cedb42988 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -614,6 +614,16 @@ static Decomposition decompose(Value *V,
     return {V, IsKnownNonNegative};
   }
 
+  if (match(V, m_Add(m_Value(Op0), m_ConstantInt(CI))) && CI->isNegative() &&
+      canUseSExt(CI)) {
+    Preconditions.emplace_back(
+        CmpInst::ICMP_UGE, Op0,
+        ConstantInt::get(Op0->getType(), CI->getSExtValue() * -1));
+    if (auto Decomp = MergeResults(Op0, CI, true))
+      return *Decomp;
+    return {V, IsKnownNonNegative};
+  }
+
   if (match(V, m_NSWAdd(m_Value(Op0), m_Value(Op1)))) {
     if (!isKnownNonNegative(Op0, DL))
       Preconditions.emplace_back(CmpInst::ICMP_SGE, Op0,
@@ -627,16 +637,6 @@ static Decomposition decompose(Value *V,
     return {V, IsKnownNonNegative};
   }
 
-  if (match(V, m_Add(m_Value(Op0), m_ConstantInt(CI))) && CI->isNegative() &&
-      canUseSExt(CI)) {
-    Preconditions.emplace_back(
-        CmpInst::ICMP_UGE, Op0,
-        ConstantInt::get(Op0->getType(), CI->getSExtValue() * -1));
-    if (auto Decomp = MergeResults(Op0, CI, true))
-      return *Decomp;
-    return {V, IsKnownNonNegative};
-  }
-
   // Decompose or as an add if there are no common bits between the operands.
   if (match(V, m_DisjointOr(m_Value(Op0), m_ConstantInt(CI)))) {
     if (auto Decomp = MergeResults(Op0, CI, IsSigned))
diff --git a/llvm/test/Transforms/ConstraintElimination/add-nsw.ll b/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
index 5127e92a7c345..4b8ac09801ab2 100644
--- a/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
+++ b/llvm/test/Transforms/ConstraintElimination/add-nsw.ll
@@ -757,8 +757,7 @@ define i1 @add_neg_1_known_sge_ult_1(i32 %a) {
 ; CHECK-NEXT:    [[A_SGE:%.*]] = icmp sge i32 [[A:%.*]], 1
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[A_SGE]])
 ; CHECK-NEXT:    [[SUB:%.*]] = add nsw i32 [[A]], -1
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[SUB]], [[A]]
-; CHECK-NEXT:    ret i1 [[C]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %a.sge = icmp sge i32 %a, 1
@@ -823,8 +822,7 @@ define i1 @add_neg_3_known_sge_ult_1(i32 %a) {
 ; CHECK-NEXT:    [[A_SGE:%.*]] = icmp sge i32 [[A:%.*]], 3
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[A_SGE]])
 ; CHECK-NEXT:    [[SUB:%.*]] = add nsw i32 [[A]], -3
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i32 [[SUB]], [[A]]
-; CHECK-NEXT:    ret i1 [[C]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %a.sge = icmp sge i32 %a, 3
diff --git a/llvm/test/Transforms/ConstraintElimination/gep-arithmetic-add.ll b/llvm/test/Transforms/ConstraintElimination/gep-arithmetic-add.ll
index 52adc78b4e159..8dcac7832179a 100644
--- a/llvm/test/Transforms/ConstraintElimination/gep-arithmetic-add.ll
+++ b/llvm/test/Transforms/ConstraintElimination/gep-arithmetic-add.ll
@@ -389,8 +389,7 @@ define i1 @gep_count_add_1_sge_known_ult_1(i32 %count, ptr %p) {
 ; CHECK-NEXT:    [[SUB:%.*]] = add nsw i32 [[COUNT]], -1
 ; CHECK-NEXT:    [[SUB_EXT:%.*]] = zext i32 [[SUB]] to i64
 ; CHECK-NEXT:    [[GEP_SUB:%.*]] = getelementptr inbounds i32, ptr [[P]], i64 [[SUB_EXT]]
-; CHECK-NEXT:    [[C:%.*]] = icmp ult ptr [[GEP_SUB]], [[GEP_COUNT]]
-; CHECK-NEXT:    ret i1 [[C]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %sge = icmp sge i32 %count, 1
@@ -415,8 +414,7 @@ define i1 @gep_count_add_1_sge_known_uge_1(i32 %count, ptr %p) {
 ; CHECK-NEXT:    [[SUB:%.*]] = add nsw i32 [[COUNT]], -1
 ; CHECK-NEXT:    [[SUB_EXT:%.*]] = zext i32 [[SUB]] to i64
 ; CHECK-NEXT:    [[GEP_SUB:%.*]] = getelementptr inbounds i32, ptr [[P]], i64 [[SUB_EXT]]
-; CHECK-NEXT:    [[C:%.*]] = icmp uge ptr [[GEP_SUB]], [[P]]
-; CHECK-NEXT:    ret i1 [[C]]
+; CHECK-NEXT:    ret i1 true
 ;
 entry:
   %sge = icmp sge i32 %count, 1