[llvm] [InstSimplify] Provide information about the range of possible values that `ucmp`/`scmp` can return (PR #96410)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: None (Poseydon42)

<details>
<summary>Changes</summary>

This makes it possible to fold dumb comparisons like `ucmp(x, y) == 7`.

---
Full diff: https://github.com/llvm/llvm-project/pull/96410.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ValueTracking.cpp (+3) 
- (modified) llvm/test/Transforms/InstSimplify/uscmp.ll (+20) 


``````````diff
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 0df061923f625..6631973f96209 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -9393,6 +9393,9 @@ static ConstantRange getRangeForIntrinsic(const IntrinsicInst &II) {
     if (!II.getParent() || !II.getFunction())
       break;
     return getVScaleRange(II.getFunction(), Width);
+  case Intrinsic::scmp:
+  case Intrinsic::ucmp:
+    return ConstantRange::getNonEmpty(APInt::getAllOnes(Width), APInt::getOneBitSet(Width, 0));
   default:
     break;
   }
diff --git a/llvm/test/Transforms/InstSimplify/uscmp.ll b/llvm/test/Transforms/InstSimplify/uscmp.ll
index db96aa39192d6..47720060acb52 100644
--- a/llvm/test/Transforms/InstSimplify/uscmp.ll
+++ b/llvm/test/Transforms/InstSimplify/uscmp.ll
@@ -229,6 +229,26 @@ define <4 x i8> @ucmp_with_addition_vec(<4 x i32> %x) {
   ret <4 x i8> %2
 }
 
+define i1 @scmp_eq_4(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @scmp_eq_4(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    ret i1 false
+;
+  %1 = call i8 @llvm.scmp(i32 %x, i32 %y)
+  %2 = icmp eq i8 %1, 4
+  ret i1 %2
+}
+
+define i1 @ucmp_ne_negative_2(i32 %x, i32 %y) {
+; CHECK-LABEL: define i1 @ucmp_ne_negative_2(
+; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
+; CHECK-NEXT:    ret i1 true
+;
+  %1 = call i8 @llvm.ucmp(i32 %x, i32 %y)
+  %2 = icmp ne i8 %1, -2
+  ret i1 %2
+}
+
 ; Negative case: mismatched signedness of predicates
 define i8 @scmp_known_ugt(i32 %x, i32 %y) {
 ; CHECK-LABEL: define i8 @scmp_known_ugt(

``````````

</details>


https://github.com/llvm/llvm-project/pull/96410