`~a & ~b & ~c` should not compile to `~(a | b | c)`

[Validark]

Zig Godbolt
C Godbolt

Currently compiles to:

foo:
        mov     rax, rdi
        or      rax, rsi
        or      rax, rdx
        not     rax
        ret

Should be:

foo:
        not     rdx
        or      rdi, rsi
        andn    rax, rdi, rdx
        ret

The difference is that the first snippet is going to take 3 cycles to execute because there is a serial dependency chain between all 3 operations. The second snippet can take 2 cycles to execute because the not and the or can run in the same cycle.

[RKSimon]

Instcombine canonicalizes to:

define i64 @foo(i64 %a, i64 %b, i64 %c) {
  %and.demorgan = or i64 %b, %a
  %and3.demorgan = or i64 %and.demorgan, %c
  %and3 = xor i64 %and3.demorgan, -1
  ret i64 %and3
}

But we should consider a reassociation in the backend for cases where we have an ANDNOT instruction

[RKSimon]

@kper do you want to be assigned to this?

[kper]

Yes please!