[comb] Missing Optimization to Build Reduction Ops

[seldridge]

We are missing an optimization to build reduction ops when possible. Consider the following which is doing b[0] | b[1]:

module {
  hw.module @Foo(%a: i1, %b: i2) -> (c: i1) {
    %0 = comb.extract %b from 1 : (i2) -> i1
    %1 = comb.extract %b from 0 : (i2) -> i1
    %2 = comb.or %0, %1 {sv.namehint = "_b"} : i1
    hw.output %2 : i1
  }
}

This produces:

module Foo(
  input        a,
  input  [1:0] b,
  output       c);

  assign c = b[1] | b[0];
endmodule

It would be better to produce (comb.icmp ne 0):

assign c = |b;

This optimization should be possible for any of comb.or, comb.and, and comb.xor.

[hovind]

On 13ea7a51d79dd2d3dc26b1f2474978148727956f I get

// Generated by CIRCT unknown git version
module Foo(	// comb.mlir:2:3
  input        a,	// comb.mlir:2:21
  input  [1:0] b,	// comb.mlir:2:32
  output       c	// comb.mlir:2:44
);

  assign c = |b;	// comb.mlir:5:10, :6:5
endmodule

module {
  hw.module @Foo(in %a : i1, in %b : i2, out c : i1) {
    %c0_i2 = hw.constant 0 : i2
    %0 = comb.icmp ne %b, %c0_i2 {sv.namehint = "_b"} : i2
    hw.output %0 : i1
  }
}

It seems to me that @Schottkyc137 solved this in ecd49163a5f593b46df8e11ce80dd296e1723767 as a part of #3394.