Re: [PATCH] [x86] reenable dword MOVE_MAX for better memmove inlining

[Richard Biener <richard.guenther@gmail.com>]

On Thu, May 25, 2023 at 1:10 PM Alexandre Oliva <oliva@adacore.com> wrote:
>
> On May 25, 2023, Richard Biener <richard.guenther@gmail.com> wrote:
>
> > I mean we could do what RTL expansion would do later and do
> > by-pieces, thus emit multiple loads/stores but not n loads and then
> > n stores but interleaved.
>
> That wouldn't help e.g. gcc.dg/memcpy-6.c's fold_move_8, because
> MOVE_MAX and MOVE_MAX_PIECES currently limits inline expansion to 4
> bytes on x86 without SSE, both in gimple and RTL, and interleaved loads
> and stores wouldn't help with memmove.  We can't fix that by changing
> code that uses MOVE_MAX and/or MOVE_MAX_PIECES, when these limits are
> set too low.
>
> I'm also concerned that doing more such expansion in gimple folding
> would be reversed in later gimple passes.  That's good in that it would
> enable efficient rtl movmem/cpymem instruction selection, but it's not
> clear to me that there would generally be benefits to such early
> open-coding in gimple.

Btw, there was a short period where the MOVE_MAX limit was restricted
but that had fallout and we've reverted since then.

Richard.

> --
> Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
>    Free Software Activist                       GNU Toolchain Engineer
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