[llvm] [InstCombine] Improve range computation in `foldICmpAddConstant` (PR #155096)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/155096

Address comment https://github.com/llvm/llvm-project/pull/110511#discussion_r1788946221.

>From 61c7f29df90616a5a522874ba6a42806dc1a2480 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sat, 23 Aug 2025 20:37:59 +0800
Subject: [PATCH] [InstCombine] Improve range computation in
 `foldICmpAddConstant`

---
 .../InstCombine/InstCombineCompares.cpp       | 26 +++++++++----------
 llvm/test/Transforms/InstCombine/icmp-add.ll  | 16 ++++--------
 2 files changed, 17 insertions(+), 25 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 2386e7ad47fb7..3a8e043038153 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3320,20 +3320,18 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
     Type *NewCmpTy = V->getType();
     unsigned NewCmpBW = NewCmpTy->getScalarSizeInBits();
     if (shouldChangeType(Ty, NewCmpTy)) {
-      if (CR.getActiveBits() <= NewCmpBW) {
-        ConstantRange SrcCR = CR.truncate(NewCmpBW);
-        CmpInst::Predicate EquivPred;
-        APInt EquivInt;
-        APInt EquivOffset;
-
-        SrcCR.getEquivalentICmp(EquivPred, EquivInt, EquivOffset);
-        return new ICmpInst(
-            EquivPred,
-            EquivOffset.isZero()
-                ? V
-                : Builder.CreateAdd(V, ConstantInt::get(NewCmpTy, EquivOffset)),
-            ConstantInt::get(NewCmpTy, EquivInt));
-      }
+      ConstantRange SrcCR = CR.truncate(NewCmpBW, TruncInst::NoUnsignedWrap);
+      CmpInst::Predicate EquivPred;
+      APInt EquivInt;
+      APInt EquivOffset;
+
+      SrcCR.getEquivalentICmp(EquivPred, EquivInt, EquivOffset);
+      return new ICmpInst(
+          EquivPred,
+          EquivOffset.isZero()
+              ? V
+              : Builder.CreateAdd(V, ConstantInt::get(NewCmpTy, EquivOffset)),
+          ConstantInt::get(NewCmpTy, EquivInt));
     }
   }
 
diff --git a/llvm/test/Transforms/InstCombine/icmp-add.ll b/llvm/test/Transforms/InstCombine/icmp-add.ll
index cb428097f2ae1..2cfde5a2801fc 100644
--- a/llvm/test/Transforms/InstCombine/icmp-add.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-add.ll
@@ -3209,13 +3209,11 @@ entry:
   ret i1 %cmp
 }
 
-; TODO: should be canonicalized to (x - 4) u> 2
 define i1 @zext_range_check_ugt(i8 %x) {
 ; CHECK-LABEL: @zext_range_check_ugt(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[TMP0:%.*]] = add nsw i32 [[CONV]], -7
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[TMP0]], -3
+; CHECK-NEXT:    [[TMP0:%.*]] = add i8 [[X:%.*]], -7
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[TMP0]], -3
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
 entry:
@@ -3225,13 +3223,11 @@ entry:
   ret i1 %cmp
 }
 
-; TODO: should be canonicalized to (x - 4) u> 2
 define i1 @zext_range_check_ult_alter(i8 %x) {
 ; CHECK-LABEL: @zext_range_check_ult_alter(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[CONV]], -7
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[ADD]], -3
+; CHECK-NEXT:    [[TMP0:%.*]] = add i8 [[X:%.*]], -7
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i8 [[TMP0]], -3
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
 entry:
@@ -3289,9 +3285,7 @@ entry:
 define i1 @zext_range_check_ult_range_check_failure(i8 %x) {
 ; CHECK-LABEL: @zext_range_check_ult_range_check_failure(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[CONV]], -4
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[ADD]], 253
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i8 [[X:%.*]], 3
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
 entry: