[InstSimplify] Implement constant folding for UCMP/SCMP intrinsics

Author: Poseydon42

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 +2766,33 @@ 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 }

llvm/test/Transforms/InstSimplify/uscmp.ll

@@ -2,44 +2,39 @@
 ; RUN: opt < %s -passes=instsimplify -S | FileCheck %s
 define i8 @scmp_lt() {
 ; CHECK-LABEL: define i8 @scmp_lt() {
-; CHECK-NEXT:    [[TMP1:%.*]] = call i8 @llvm.scmp.i8.i32(i32 -7, i32 3)
-; CHECK-NEXT:    ret i8 [[TMP1]]
+; 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:    [[TMP1:%.*]] = call i2 @llvm.ucmp.i2.i32(i32 12, i32 12)
-; CHECK-NEXT:    ret i2 [[TMP1]]
+; 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:    [[TMP1:%.*]] = call i100 @llvm.ucmp.i100.i32(i32 7, i32 3)
-; CHECK-NEXT:    ret i100 [[TMP1]]
+; 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:    [[TMP1:%.*]] = call <4 x i8> @llvm.ucmp.v4i8.v4i32(<4 x i32> <i32 1, i32 1, i32 1, i32 1>, <4 x i32> <i32 3, i32 3, i32 3, i32 3>)
-; CHECK-NEXT:    ret <4 x i8> [[TMP1]]
+; 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:    [[TMP1:%.*]] = call <4 x i8> @llvm.scmp.v4i8.v4i32(<4 x i32> <i32 0, i32 1, i32 2, i32 3>, <4 x i32> <i32 -1, i32 1, i32 -2, i32 4>)
-; CHECK-NEXT:    ret <4 x i8> [[TMP1]]
+; 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