[RFC] Add min/max operations.
I propose to add MinOp, MaxOp operations to Standard or Math dialects. There were plans to move arithmetics from Standard dialect into some new dialects, if that is still the case, then I volunteer to do the split.
Background
Legalization of higher-level dialects that operate on tensors/memrefs, e.g. HLO, TF, results in a loop nest or some operation like linalg.generic with scalar operations in its region. For most operations there is a corresponding op in Standard or Math dialects. Sum of two tensors mhlo.add can be expressed with std.addf, hyperbolic tangent is mapped to math.tanh.
Unfortunately, mhlo.maximum is rewritten as 2 ops when we deal with integers
%0 = cmpi sgt, %lhs, %rhs : i32
%1 = select %0, %lhs, %rhs : i32
and 5 ops when we have an op on floats that correctly propagates NANs
%NAN = constant 0x7FC00000 : f32
%0 = cmpf ogt, %lhs, %rhs : f32
%1 = select %0, %lhs, %rhs : f32
%2 = cmpf uno, %lhs, %rhs : f32
%3 = select %2, %NAN, %37 : f32
There is nothing wrong with this approach if the compiler does not require to perform vectorization or conversion to atomic operations and it just lowers the IR gradually to LLVM. The only downside that I see here is that MLIR users have to reinvent the wheel and write conversion for maximum/minimum which leads to duplicate code.
It becomes unnecessarily more complex when one needs to pattern match cmp and select to convert to std.atomic_rmw or to a corresponding vector operation. Moreover, LLVM has llvm.maximum, llvm.umax and llvm.smax intrinsics and I think it would make sense to add std.max or math.max to MLIR.