[llvm] [InstSimplify] Add constant folding support for `ucmp`/`scmp` intrinsics (PR #93730)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (Poseydon42)

<details>
<summary>Changes</summary>

This PR adds support for folding calls to `ucmp`/`scmp` intrinsics with constant arguments.

---
Full diff: https://github.com/llvm/llvm-project/pull/93730.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/ConstantFolding.cpp (+28) 
- (added) llvm/test/Transforms/InstSimplify/uscmp.ll (+41) 


``````````diff
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index 046a769453808..c11d3032020f4 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1506,6 +1506,8 @@ bool llvm::canConstantFoldCallTo(const CallBase *Call, const Function *F) {
   case Intrinsic::smin:
   case Intrinsic::umax:
   case Intrinsic::umin:
+  case Intrinsic::scmp:
+  case Intrinsic::ucmp:
   case Intrinsic::sadd_with_overflow:
   case Intrinsic::uadd_with_overflow:
   case Intrinsic::ssub_with_overflow:
@@ -2764,6 +2766,32 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
                   ? *C0
                   : *C1);
 
+    case Intrinsic::scmp:
+    case Intrinsic::ucmp:
+      if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
+        return PoisonValue::get(Ty);
+
+      if (!C0 || !C1)
+        return UndefValue::get(Ty);
+
+      if (ICmpInst::compare(*C0, *C1,
+                            IntrinsicID == Intrinsic::scmp
+                                ? ICmpInst::ICMP_SLT
+                                : ICmpInst::ICMP_ULT)) {
+        return ConstantInt::get(Ty, -1, true);
+      }
+      if (ICmpInst::compare(*C0, *C1, ICmpInst::ICMP_EQ)) {
+        return ConstantInt::get(Ty, 0);
+      }
+      if (ICmpInst::compare(*C0, *C1,
+                            IntrinsicID == Intrinsic::scmp
+                                ? ICmpInst::ICMP_SGT
+                                : ICmpInst::ICMP_UGT)) {
+        return ConstantInt::get(Ty, 1);
+      }
+
+      assert(false && "Integer values must compare as equal, or one must be less than the other");
+
     case Intrinsic::usub_with_overflow:
     case Intrinsic::ssub_with_overflow:
       // X - undef -> { 0, false }
diff --git a/llvm/test/Transforms/InstSimplify/uscmp.ll b/llvm/test/Transforms/InstSimplify/uscmp.ll
new file mode 100644
index 0000000000000..c4e9f7bd54fd7
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/uscmp.ll
@@ -0,0 +1,41 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=instsimplify -S | FileCheck %s
+define i8 @scmp_lt() {
+; CHECK-LABEL: define i8 @scmp_lt() {
+; CHECK-NEXT:    ret i8 -1
+;
+  %1 = call i8 @llvm.scmp(i32 -7, i32 3)
+  ret i8 %1
+}
+
+define i2 @ucmp_eq() {
+; CHECK-LABEL: define i2 @ucmp_eq() {
+; CHECK-NEXT:    ret i2 0
+;
+  %1 = call i2 @llvm.ucmp(i32 12, i32 12)
+  ret i2 %1
+}
+
+define i100 @ucmp_gt() {
+; CHECK-LABEL: define i100 @ucmp_gt() {
+; CHECK-NEXT:    ret i100 1
+;
+  %1 = call i100 @llvm.ucmp(i32 7, i32 3)
+  ret i100 %1
+}
+
+define <4 x i8> @ucmp_lt_splat() {
+; CHECK-LABEL: define <4 x i8> @ucmp_lt_splat() {
+; CHECK-NEXT:    ret <4 x i8> <i8 -1, i8 -1, i8 -1, i8 -1>
+;
+  %1 = call <4 x i8> @llvm.ucmp(<4 x i32> splat(i32 1), <4 x i32> splat(i32 3))
+  ret <4 x i8> %1
+}
+
+define <4 x i8> @scmp_nonsplat() {
+; CHECK-LABEL: define <4 x i8> @scmp_nonsplat() {
+; CHECK-NEXT:    ret <4 x i8> <i8 1, i8 0, i8 1, i8 -1>
+;
+  %1 = call <4 x i8> @llvm.scmp(<4 x i32> <i32 0, i32 1, i32 2, i32 3>, <4 x i32> <i32 -1, i32 1, i32 -2, i32 4>)
+  ret <4 x i8> %1
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/93730