Re: Support for AVX512 ternary logic instruction

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, Jan 19, 2019 at 11:23 AM Wojciech Muła
<wojciech_mula@poczta.onet.pl> wrote:
>
> Hi all!
>
> One thing I miss in GCC is lack of support for the AVX512 ternary logic
> instruction VPTERNLOG. This instruction evaluates any three-argument logic
> function; this means that an expression like "a | b & ~c" can be folded into
> single instruction; my experiments showed that up to six SIMD instructions
> can be replaced this way[1]. Seems it's something worth considering.
>
> However, instead of reporting a vague bug "it'd be nice to have..." :),
> I was thinking about adding support for this instruction.
>
> I have no experience in GCC at all, but figured out that support for this
> instruction can be added with new define_insn in `gcc/config/i386/sse.md`.
> My very first try was to compile just "a | b | c", so created this
> rule (not really working, but is properly matched by the compiler):
>
> (define_insn "ors_combined_into_vpternlog"
>   [(set
>     (match_operand:V8DI 0 "register_operand" "=r")
>     (ior:V8DI
>         (ior:V8DI
>             (match_operand:V8DI 1 "register_operand" "r")
>             (match_operand:V8DI 2 "register_operand" "r")
>         )
>         (match_operand:V8DI 3 "register_operand" "r")
>     )
>    )]
>   "TARGET_AVX512F"
>   "vpternlog<ssemodesuffix>\t{0xfe, %3, %2, %0%|%0%, %2, %3, 0xfe}"
>   [(set_attr "type" "sselog")
>    (set_attr "prefix" "evex")])
>
> The main concern is if it's a proper approach? Seems that to match
> other logic functions, like "a & b | c", a separate pattern is required.
> Since an argument can be either negated or not, and we can use three
> logic ops (or, and, xor) there would be 72 patterns. So maybe a new
> optimization pass would be easier to create and maintain? (Just a silly
> guess.)
>
> I'd be grateful for any comments and advice.
>
> best regards
> Wojciech
>
> [1] https://github.com/WojciechMula/ternary-logic

Please open a gcc bug:

https://gcc.gnu.org/bugzilla/

and CC me.   We will look into it.

Thanks.

-- 
H.J.