[llvm] Add 3 way compare <=> integer intrinsics to Langref (PR #83227)

[Nikita Popov via llvm-commits]

================
@@ -14531,6 +14531,63 @@ 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_sthreecmp:
+
+'``llvm.sthreecmp.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``@llvm.sthreecmp`` on any
+integer bit width or any vector of integer elements.
+
+::
+
+      declare i32 @llvm.sthreecmp.i32(i32 %a, i32 %b)
+      declare <4 x i32> @llvm.sthreecmp.v4i32(<4 x i32> %a, <4 x i32> %b)
+
+Overview:
+"""""""""
+
+Return ``-1`` if ``%a`` is less than ``%b``, ``0`` if they are equal, and 
+``1`` if ``%a`` is greater than ``%b``. Vector intrinsics operate on a per-element basis. 
+
+Arguments:
+""""""""""
+
+The arguments (``%a`` and ``%b``) may be of any signed 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 uphold the ``-1`` return value.
+
+.. _int_uthreecmp:
+
+'``llvm.uthreecmp.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+
+This is an overloaded intrinsic. You can use ``@llvm.sthreecmp`` on any
+integer bit width or any vector of integer elements.
+
+::
+
+      declare i2 @llvm.uthreecmp.i32(i32 %a, i32 %b)
----------------
nikic wrote:

There wasn't much feedback regarding the return type on the discourse thread, but it seems like there is a mild preference to have the result type overload over a fixed type.

The way to implement would be like this in Intrinsics.td:
```
def int_scmp : DefaultAttrsIntrinsic<
    [llvm_anyint_ty], [llvm_anyint_ty, LLVMMatchType<1>],
    [IntrNoMem, IntrSpeculatable, IntrWillReturn]>;
```
and then add a check in Verifier.cpp that a) the return type and the first argument type have the same number of elements (if they are vectors) and b) the return scalar type has width at least 2.

https://github.com/llvm/llvm-project/pull/83227