On Thu, Aug 3, 2023 at 4:10 PM Jan Beulich via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Drop SSE5 leftovers from both its comment and its default calculation.
> A value of 2 simply cannot occur anymore. Instead extend the comment to
> mention the use of the attribute in "length_vex", clarifying why
> "prefix_extra" can actually be meaningful on VEX-encoded insns despite
> those not having any real prefixes except possibly segment overrides.
>
Ok.
> gcc/
>
> * config/i386/i386.md (prefix_extra): Correct comment. Fold
> cases yielding 2 into ones yielding 1.
> ---
> I question the 3DNow! aspect here: There's no extra prefix there. It's
> an immediate instead which "sub-divides" major opcode 0f0f.
>
> --- a/gcc/config/i386/i386.md
> +++ b/gcc/config/i386/i386.md
> @@ -620,13 +620,11 @@
> (const_int 0)))
>
> ;; There are also additional prefixes in 3DNOW, SSSE3.
> -;; ssemuladd,sse4arg default to 0f24/0f25 and DREX byte,
> -;; sseiadd1,ssecvt1 to 0f7a with no DREX byte.
> ;; 3DNOW has 0f0f prefix, SSSE3 and SSE4_{1,2} 0f38/0f3a.
> +;; While generally inapplicable to VEX/XOP/EVEX encodings, "length_vex" uses
> +;; the attribute evaluating to zero to know that VEX2 encoding may be usable.
> (define_attr "prefix_extra" ""
> - (cond [(eq_attr "type" "ssemuladd,sse4arg")
> - (const_int 2)
> - (eq_attr "type" "sseiadd1,ssecvt1")
> + (cond [(eq_attr "type" "ssemuladd,sse4arg,sseiadd1,ssecvt1")
> (const_int 1)
> ]
> (const_int 0)))
>
--
BR,
Hongtao
