[LangRef][IR] Add 3-way compare intrinsics llvm.scmp/llvm.ucmp (#83227)

This PR adds the `[us]cmp` intrinsics to the LangRef, `Intrinsics.td`
and some tests to the IRVerifier.

RFC: https://discourse.llvm.org/t/rfc-add-3-way-comparison-intrinsics/76685

llvm/docs/LangRef.rst

@@ -14574,6 +14574,63 @@ The arguments (``%a`` and ``%b``) may be of any integer type or a vector with
 integer element type. The argument types must match each other, and the return
 type must match the argument type.
 
+.. _int_scmp:
+
+'``llvm.scmp.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``@llvm.scmp`` on any
+integer bit width or any vector of integer elements.
+
+::
+
+      declare i2 @llvm.scmp.i2.i32(i32 %a, i32 %b)
+      declare <4 x i32> @llvm.scmp.v4i32.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview:
+"""""""""
+
+Return ``-1`` if ``%a`` is signed less than ``%b``, ``0`` if they are equal, and 
+``1`` if ``%a`` is signed greater than ``%b``. Vector intrinsics operate on a per-element basis. 
+
+Arguments:
+""""""""""
+
+The arguments (``%a`` and ``%b``) may be of any integer type or a vector with
+integer element type. The argument types must match each other, and the return
+type must be at least as wide as ``i2``, to hold the three possible return values.
+
+.. _int_ucmp:
+
+'``llvm.ucmp.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``@llvm.ucmp`` on any
+integer bit width or any vector of integer elements.
+
+::
+
+      declare i2 @llvm.ucmp.i2.i32(i32 %a, i32 %b)
+      declare <4 x i32> @llvm.ucmp.v4i32.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview:
+"""""""""
+
+Return ``-1`` if ``%a`` is unsigned less than ``%b``, ``0`` if they are equal, and 
+``1`` if ``%a`` is unsigned greater than ``%b``. Vector intrinsics operate on a per-element basis. 
+
+Arguments:
+""""""""""
+
+The arguments (``%a`` and ``%b``) may be of any integer type or a vector with
+integer element type. The argument types must match each other, and the return
+type must be at least as wide as ``i2``, to hold the three possible return values.
 
 .. _int_memcpy:
 

llvm/include/llvm/IR/Intrinsics.td

@@ -1528,6 +1528,12 @@ def int_umax : DefaultAttrsIntrinsic<
 def int_umin : DefaultAttrsIntrinsic<
     [llvm_anyint_ty], [LLVMMatchType<0>, LLVMMatchType<0>],
     [IntrNoMem, IntrSpeculatable, IntrWillReturn]>;
+def int_scmp : DefaultAttrsIntrinsic<
+    [llvm_anyint_ty], [llvm_anyint_ty, LLVMMatchType<1>],
+    [IntrNoMem, IntrSpeculatable, IntrWillReturn]>;
+def int_ucmp : DefaultAttrsIntrinsic<
+    [llvm_anyint_ty], [llvm_anyint_ty, LLVMMatchType<1>],
+    [IntrNoMem, IntrSpeculatable, IntrWillReturn]>;
 
 //===------------------------- Memory Use Markers -------------------------===//
 //

llvm/lib/IR/Verifier.cpp

@@ -5265,6 +5265,29 @@ void Verifier::visitIntrinsicCall(Intrinsic::ID ID, CallBase &Call) {
     }
     break;
   }
+  case Intrinsic::ucmp:
+  case Intrinsic::scmp: {
+    Type *SrcTy = Call.getOperand(0)->getType();
+    Type *DestTy = Call.getType();
+
+    Check(DestTy->getScalarSizeInBits() >= 2,
+          "result type must be at least 2 bits wide", Call);
+
+    bool IsDestTypeVector = DestTy->isVectorTy();
+    Check(SrcTy->isVectorTy() == IsDestTypeVector,
+          "ucmp/scmp argument and result types must both be either vector or "
+          "scalar types",
+          Call);
+    if (IsDestTypeVector) {
+      auto SrcVecLen = cast<VectorType>(SrcTy)->getElementCount();
+      auto DestVecLen = cast<VectorType>(DestTy)->getElementCount();
+      Check(SrcVecLen == DestVecLen,
+            "return type and arguments must have the same number of "
+            "elements",
+            Call);
+    }
+    break;
+  }
   case Intrinsic::coro_id: {
     auto *InfoArg = Call.getArgOperand(3)->stripPointerCasts();
     if (isa<ConstantPointerNull>(InfoArg))

llvm/test/Verifier/intrinsic-cmp.ll

@@ -0,0 +1,22 @@
+; RUN: not opt -S -passes=verify 2>&1 < %s | FileCheck %s
+
+define void @matching_vector_lens(<4 x i32> %arg1, <4 x i32> %arg2) {
+  ; CHECK: return type and arguments must have the same number of elements
+  %res = call <8 x i32> @llvm.scmp.v8i32.v4i32(<4 x i32> %arg1, <4 x i32> %arg2)
+  ret void
+}
+
+define void @result_len_is_at_least_2bits_wide(i32 %arg1, i32 %arg2) {
+  ; CHECK: result type must be at least 2 bits wide
+  %res2 = call i1 @llvm.scmp.i1.i32(i32 %arg1, i32 %arg2)
+  ret void
+}
+
+define void @both_args_are_vecs_or_neither(<4 x i32> %arg1, i32 %arg2) {
+  ; CHECK: ucmp/scmp argument and result types must both be either vector or scalar types
+  %res3 = call i2 @llvm.scmp.i2.v4i32(<4 x i32> %arg1, <4 x i32> %arg1)
+  ; CHECK: ucmp/scmp argument and result types must both be either vector or scalar types
+  %res4 = call <4 x i32> @llvm.scmp.v4i32.i32(i32 %arg2, i32 %arg2)
+  ret void
+}
+