[llvm] [InstCombine] Fold `icmp spred (X *nsw Z), (Y *nsw Z) -> icmp pred Z, 0` if `scmp(X, Y)` is known (PR #118726)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

```
icmp spred (X *nsw Z), (Y *nsw Z) -> icmp swap(spred) Z, 0 if X s< Y
icmp spred (X *nsw Z), (Y *nsw Z) -> icmp spred       Z, 0 if X s> Y
```
Alive2: https://alive2.llvm.org/ce/z/F2D0GE


---
Full diff: https://github.com/llvm/llvm-project/pull/118726.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+9) 
- (modified) llvm/test/Transforms/InstCombine/icmp-mul.ll (+104) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index fed21db393ed22..e31af8104e147c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -5337,6 +5337,15 @@ Instruction *InstCombinerImpl::foldICmpBinOp(ICmpInst &I,
             return new ICmpInst(Pred, X, Y);
           if (ZKnown.isNegative())
             return new ICmpInst(ICmpInst::getSwappedPredicate(Pred), X, Y);
+          Value *LessThan = simplifyICmpInst(ICmpInst::ICMP_SLT, X, Y,
+                                             SQ.getWithInstruction(&I));
+          if (LessThan && match(LessThan, m_One()))
+            return new ICmpInst(ICmpInst::getSwappedPredicate(Pred), Z,
+                                Constant::getNullValue(Z->getType()));
+          Value *GreaterThan = simplifyICmpInst(ICmpInst::ICMP_SGT, X, Y,
+                                                SQ.getWithInstruction(&I));
+          if (GreaterThan && match(GreaterThan, m_One()))
+            return new ICmpInst(Pred, Z, Constant::getNullValue(Z->getType()));
         }
       } else {
         bool NonZero;
diff --git a/llvm/test/Transforms/InstCombine/icmp-mul.ll b/llvm/test/Transforms/InstCombine/icmp-mul.ll
index c9f9b6d809e8a5..49e1e11fe6c36f 100644
--- a/llvm/test/Transforms/InstCombine/icmp-mul.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-mul.ll
@@ -1464,3 +1464,107 @@ entry:
   %cmp = icmp slt i8 %mul1, %mul2
   ret i1 %cmp
 }
+
+define i1 @test_icmp_slt_mul_known_sgt(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_slt_mul_known_sgt(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[Z:%.*]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul nsw i64 %x, %z
+  %mul2 = mul nsw i64 %y, %z
+  %cmp = icmp slt i64 %mul1, %mul2
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_sle_mul_known_sgt(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_sle_mul_known_sgt(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[Z:%.*]], -1
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul nsw i64 %x, %z
+  %mul2 = mul nsw i64 %y, %z
+  %cmp = icmp sle i64 %mul1, %mul2
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_mul_known_slt(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_mul_known_slt(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[Z:%.*]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul nsw i64 %x, %z
+  %mul2 = mul nsw i64 %y, %z
+  %cmp = icmp slt i64 %mul2, %mul1
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_slt_mul_known_sgt_commuted1(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_slt_mul_known_sgt_commuted1(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[Z:%.*]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul nsw i64 %z, %x
+  %mul2 = mul nsw i64 %y, %z
+  %cmp = icmp slt i64 %mul1, %mul2
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_slt_mul_known_sgt_commuted2(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_slt_mul_known_sgt_commuted2(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt i64 [[Z:%.*]], 0
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul nsw i64 %x, %z
+  %mul2 = mul nsw i64 %z, %y
+  %cmp = icmp slt i64 %mul1, %mul2
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_slt_mul_unknown(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_slt_mul_unknown(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[Y:%.*]] = add i64 [[X:%.*]], 1
+; CHECK-NEXT:    [[MUL1:%.*]] = mul nsw i64 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[MUL2:%.*]] = mul nsw i64 [[Z]], [[Y]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[MUL1]], [[MUL2]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add i64 %x, 1
+  %mul1 = mul nsw i64 %x, %z
+  %mul2 = mul nsw i64 %z, %y
+  %cmp = icmp slt i64 %mul1, %mul2
+  ret i1 %cmp
+}
+
+define i1 @test_icmp_slt_mul_no_nsw(i64 %x, i64 %z) {
+; CHECK-LABEL: @test_icmp_slt_mul_no_nsw(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[Y:%.*]] = add nsw i64 [[X:%.*]], 1
+; CHECK-NEXT:    [[MUL1:%.*]] = mul i64 [[X]], [[Z:%.*]]
+; CHECK-NEXT:    [[MUL2:%.*]] = mul nsw i64 [[Z]], [[Y]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i64 [[MUL1]], [[MUL2]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+entry:
+  %y = add nsw i64 %x, 1
+  %mul1 = mul i64 %x, %z
+  %mul2 = mul nsw i64 %z, %y
+  %cmp = icmp slt i64 %mul1, %mul2
+  ret i1 %cmp
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/118726