[llvm] d94958b - [InstCombine] Fold `icmp samesign u{gt/lt} (X +nsw C2), C` -> `icmp s{gt/lt} X, (C - C2)` (#169960)

[via llvm-commits]

Author: Tirthankar Mazumder
Date: 2025-12-08T13:05:37+01:00
New Revision: d94958b2f27affe00c42c1338f99674d2f6271c8

URL: https://github.com/llvm/llvm-project/commit/d94958b2f27affe00c42c1338f99674d2f6271c8
DIFF: https://github.com/llvm/llvm-project/commit/d94958b2f27affe00c42c1338f99674d2f6271c8.diff

LOG: [InstCombine] Fold `icmp samesign u{gt/lt} (X +nsw C2), C` -> `icmp s{gt/lt} X, (C - C2)` (#169960)

Fixes #166973

Partially addresses #134028

Alive2 proof: https://alive2.llvm.org/ce/z/BqHQNN

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
    llvm/test/Transforms/InstCombine/icmp-add.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 33eee8e059486..abf4381ebd794 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3132,7 +3132,7 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
 
   Value *Op0, *Op1;
   Instruction *Ext0, *Ext1;
-  const CmpInst::Predicate Pred = Cmp.getPredicate();
+  const CmpPredicate Pred = Cmp.getCmpPredicate();
   if (match(Add,
             m_Add(m_CombineAnd(m_Instruction(Ext0), m_ZExtOrSExt(m_Value(Op0))),
                   m_CombineAnd(m_Instruction(Ext1),
@@ -3167,22 +3167,29 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
 
   // If the add does not wrap, we can always adjust the compare by subtracting
   // the constants. Equality comparisons are handled elsewhere. SGE/SLE/UGE/ULE
-  // are canonicalized to SGT/SLT/UGT/ULT.
-  if ((Add->hasNoSignedWrap() &&
-       (Pred == ICmpInst::ICMP_SGT || Pred == ICmpInst::ICMP_SLT)) ||
-      (Add->hasNoUnsignedWrap() &&
-       (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULT))) {
+  // have been canonicalized to SGT/SLT/UGT/ULT.
+  if (Add->hasNoUnsignedWrap() &&
+      (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULT)) {
     bool Overflow;
-    APInt NewC =
-        Cmp.isSigned() ? C.ssub_ov(*C2, Overflow) : C.usub_ov(*C2, Overflow);
+    APInt NewC = C.usub_ov(*C2, Overflow);
     // If there is overflow, the result must be true or false.
-    // TODO: Can we assert there is no overflow because InstSimplify always
-    // handles those cases?
     if (!Overflow)
       // icmp Pred (add nsw X, C2), C --> icmp Pred X, (C - C2)
       return new ICmpInst(Pred, X, ConstantInt::get(Ty, NewC));
   }
 
+  CmpInst::Predicate ChosenPred = Pred.getPreferredSignedPredicate();
+
+  if (Add->hasNoSignedWrap() &&
+      (ChosenPred == ICmpInst::ICMP_SGT || ChosenPred == ICmpInst::ICMP_SLT)) {
+    bool Overflow;
+    APInt NewC = C.ssub_ov(*C2, Overflow);
+    if (!Overflow)
+      // icmp samesign ugt/ult (add nsw X, C2), C
+      //   -> icmp sgt/slt X, (C - C2)
+      return new ICmpInst(ChosenPred, X, ConstantInt::get(Ty, NewC));
+  }
+
   if (ICmpInst::isUnsigned(Pred) && Add->hasNoSignedWrap() &&
       C.isNonNegative() && (C - *C2).isNonNegative() &&
       computeConstantRange(X, /*ForSigned=*/true).add(*C2).isAllNonNegative())

diff  --git a/llvm/test/Transforms/InstCombine/icmp-add.ll b/llvm/test/Transforms/InstCombine/icmp-add.ll
index 8449c7c5ea935..2806feec9ce29 100644
--- a/llvm/test/Transforms/InstCombine/icmp-add.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-add.ll
@@ -3440,3 +3440,79 @@ define i1 @val_is_aligend_pred_mismatch(i32 %num) {
   %_0 = icmp sge i32 %num.masked, %num
   ret i1 %_0
 }
+
+define i1 @icmp_samesign_with_nsw_add(i32 %arg0) {
+; CHECK-LABEL: @icmp_samesign_with_nsw_add(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[V1:%.*]] = icmp sgt i32 [[ARG0:%.*]], 25
+; CHECK-NEXT:    ret i1 [[V1]]
+;
+entry:
+  %v0 = add nsw i32 %arg0, -18
+  %v1 = icmp samesign ugt i32 %v0, 7
+  ret i1 %v1
+}
+
+; Negative test; Fold shouldn't fire since -124 - 12 causes signed overflow
+define i1 @icmp_samesign_with_nsw_add_neg(i8 %arg0) {
+; CHECK-LABEL: @icmp_samesign_with_nsw_add_neg(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[TMP0:%.*]] = add i8 [[ARG0:%.*]], -121
+; CHECK-NEXT:    [[V1:%.*]] = icmp ult i8 [[TMP0]], 123
+; CHECK-NEXT:    ret i1 [[V1]]
+;
+entry:
+  %v0 = add nsw i8 %arg0, 12
+  %v1 = icmp samesign ugt i8 %v0, -124
+  ret i1 %v1
+}
+
+define i1 @icmp_with_nuw_add(i32 %arg0) {
+; CHECK-LABEL: @icmp_with_nuw_add(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[V1:%.*]] = icmp ult i32 [[ARG0:%.*]], 11
+; CHECK-NEXT:    ret i1 [[V1]]
+;
+entry:
+  %v0 = add nuw i32 %arg0, 7
+  %v1 = icmp ult i32 %v0, 18
+  ret i1 %v1
+}
+
+define i1 @icmp_partial_negative_samesign_ult_to_slt(i8 range(i8 -1, 5) %x) {
+; CHECK-LABEL: @icmp_partial_negative_samesign_ult_to_slt(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i8 [[X:%.*]], 2
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %add = add nsw i8 %x, -5
+  %cmp = icmp samesign ult i8 %add, -3
+  ret i1 %cmp
+}
+
+define i1 @icmp_pos_samesign_slt_to_ult(i8 range(i8 1, 5) %x) {
+; CHECK-LABEL: @icmp_pos_samesign_slt_to_ult(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp samesign ult i8 [[X:%.*]], 2
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %add = add nsw i8 %x, 1
+  %cmp = icmp samesign slt i8 %add, 3
+  ret i1 %cmp
+}
+
+; Since higher priority is given to unsigned predicates, the predicate should
+; not change
+define i1 @icmp_nuw_nsw_samesign(i32 %arg0) {
+; CHECK-LABEL: @icmp_nuw_nsw_samesign(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[ARG0:%.*]], 9
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %v0 = add nuw nsw i32 %arg0, 1
+  %cmp = icmp samesign ult i32 %v0, 10
+  ret i1 %cmp
+}