[llvm] [SCCP] Simplify general icmp instructions (PR #165976)

[Kunqiu Chen via llvm-commits]

https://github.com/Camsyn created https://github.com/llvm/llvm-project/pull/165976

If we know X ∈ R1, the range check x ∈ R2 can be relaxed into X ∈ R1 ∩ R2 or  X ∈ R2 ∪ Inverse(R1). 

The relaxed one may be more efficient if we can represent it with one icmp eq, which could motivate more optimization like SimplifyCFG.


This optimization is a more generic version of InstCombine's `foldICmpWithDominatingICmp`, which only works on those CmpBB with ONE DomBB, i.e., cannot handle the following case:
```cpp
if (x == 0) return false;
if (x == 1) return false;
// Could be simplified as x == 2;
return x <u 3);
```


Alive2 Proof : https://alive2.llvm.org/ce/z/HMgkuu



>From b72103e73d0544e018fea890385b7d957759fa26 Mon Sep 17 00:00:00 2001
From: Camsyn <camsyn at foxmail.com>
Date: Fri, 31 Oct 2025 22:34:36 +0800
Subject: [PATCH] [SCCP] Simplify general icmp instructions

---
 llvm/lib/Transforms/Utils/SCCPSolver.cpp | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index 4947d03a2dc66..21cd21f214d67 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -299,6 +299,7 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
       Value *LHS = ICmp->getOperand(0);
       ICmpInst::Predicate Pred = ICmp->getPredicate();
       const APInt *Offset;
+      // Match icmp X + Offset, C
       if (match(LHS, m_OneUse(m_AddLike(m_Value(X), m_APInt(Offset)))))
         return ConstantRange::makeExactICmpRegion(Pred, *RHSC).sub(*Offset);
       // Match icmp eq/ne X & NegPow2, C
@@ -310,7 +311,10 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
           return Pred == ICmpInst::ICMP_EQ ? CR : CR.inverse();
         }
       }
-      return std::nullopt;
+      // Fallback: Match icmp X, C
+      // E.g., if X ∈ [0, 4], this can simplify icmp uge X, 4 to icmp eq X, 4
+      X = LHS;
+      return ConstantRange::makeExactICmpRegion(Pred, *RHSC);
     };
 
     if (auto CR = MatchTwoInstructionExactRangeCheck()) {