[llvm] [InstCombine] Fold an unsigned icmp of ucmp/scmp with a constant to an icmp of the original arguments (PR #104471)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Volodymyr Vasylkun (Poseydon42)

<details>
<summary>Changes</summary>

Proofs: https://alive2.llvm.org/ce/z/9mv8HU

The `ult` case in particular would resolve a regression caused by #<!-- -->101049 

---
Full diff: https://github.com/llvm/llvm-project/pull/104471.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+10) 
- (modified) llvm/test/Transforms/InstCombine/scmp.ll (+25) 
- (modified) llvm/test/Transforms/InstCombine/ucmp.ll (+24) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 10488ecb747a48..9ef3a456871665 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -4018,6 +4018,16 @@ foldICmpOfCmpIntrinsicWithConstant(ICmpInst::Predicate Pred, IntrinsicInst *I,
       NewPredicate = ICmpInst::ICMP_ULE;
     break;
 
+  case ICmpInst::ICMP_ULT:
+    if (C.ugt(1) && !C.isAllOnes())
+      NewPredicate = ICmpInst::ICMP_UGE;
+    break;
+
+  case ICmpInst::ICMP_UGT:
+    if (!C.isZero() && !C.isAllOnes())
+      NewPredicate = ICmpInst::ICMP_ULT;
+    break;
+
   default:
     break;
   }
diff --git a/llvm/test/Transforms/InstCombine/scmp.ll b/llvm/test/Transforms/InstCombine/scmp.ll
index 2523872562cad9..460ea96b16fcfa 100644
--- a/llvm/test/Transforms/InstCombine/scmp.ll
+++ b/llvm/test/Transforms/InstCombine/scmp.ll
@@ -157,6 +157,31 @@ define i1 @scmp_sle_neg_1(i32 %x, i32 %y) {
   ret i1 %2
 }
 
+; scmp(x, y) u< C => x s>= y when C u> 1 and C != -1
+define i1 @scmp_ult_positive_const_gt_than_1_lt_than_umax(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @scmp_ult_positive_const_gt_than_1_lt_than_umax(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp sge i32 [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[TMP1]]
+;
+  %1 = call i8 @llvm.scmp(i32 %x, i32 %y)
+  %2 = icmp ult i8 %1, 4
+  ret i1 %2
+}
+
+; scmp(x, y) s> C => x s< y when C != 0 and C != -1
+define i1 @ucmp_ugt_const_not_0_or_neg1(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @ucmp_ugt_const_not_0_or_neg1(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp slt i32 [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[TMP2]]
+;
+  %1 = call i8 @llvm.scmp(i32 %x, i32 %y)
+  %2 = icmp ugt i8 %1, 12
+  ret i1 %2
+}
+
+
 ; ========== Fold -scmp(x, y) => scmp(y, x) ==========
 define i8 @scmp_negated(i32 %x, i32 %y) {
 ; CHECK-LABEL: define i8 @scmp_negated(
diff --git a/llvm/test/Transforms/InstCombine/ucmp.ll b/llvm/test/Transforms/InstCombine/ucmp.ll
index 7210455094baac..8690e23ea2e0e5 100644
--- a/llvm/test/Transforms/InstCombine/ucmp.ll
+++ b/llvm/test/Transforms/InstCombine/ucmp.ll
@@ -157,6 +157,30 @@ define i1 @ucmp_sle_neg_1(i32 %x, i32 %y) {
   ret i1 %2
 }
 
+; ucmp(x, y) u< C => x u>= y when C u> 1 and C != -1
+define i1 @ucmp_ult_positive_const_gt_than_1_lt_than_umax(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @ucmp_ult_positive_const_gt_than_1_lt_than_umax(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp uge i32 [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[TMP1]]
+;
+  %1 = call i8 @llvm.ucmp(i32 %x, i32 %y)
+  %2 = icmp ult i8 %1, 4
+  ret i1 %2
+}
+
+; ucmp(x, y) u> C => x u< y when C != 0 and C != -1
+define i1 @ucmp_ugt_const_not_0_or_neg1(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @ucmp_ugt_const_not_0_or_neg1(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i32 [[X]], [[Y]]
+; CHECK-NEXT:    ret i1 [[TMP2]]
+;
+  %1 = call i8 @llvm.ucmp(i32 %x, i32 %y)
+  %2 = icmp ugt i8 %1, 12
+  ret i1 %2
+}
+
 ; ========== Fold -ucmp(x, y) => ucmp(y, x) ==========
 define i8 @ucmp_negated(i32 %x, i32 %y) {
 ; CHECK-LABEL: define i8 @ucmp_negated(

``````````

</details>


https://github.com/llvm/llvm-project/pull/104471