On Tue, Jun 27, 2023 at 8:40 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch fixes the FAIL of gcc.target/i386/pr78794.c on ia32, which
> is caused by minor STV rtx_cost differences with -march=silvermont.
> It turns out that generic tuning results in pandn, but the lack of
> accurate parameterization for COMPARE in compute_convert_gain combined
> with small differences in scalar<->SSE costs on silvermont results in
> this DImode chain not being converted.
>
> The solution is to provide more accurate costs/gains for converting
> (DImode and SImode) comparisons.
>
> I'd been holding off of doing this as I'd thought it would be possible
> to turn pandn;ptestz into ptestc (for an even bigger scalar-to-vector
> win) but I've recently realized that these optimizations (as I've
> implemented them) occur in the wrong order (stv2 occurs after
> combine), so it isn't easy for STV to convert CCZmode into CCCmode.
> Doh! Perhaps something can be done in peephole2...
>
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures. Ok for mainline?
>
>
> 2023-06-27 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> PR target/78794
> * config/i386/i386-features.cc (compute_convert_gain): Provide
> more accurate gains for conversion of scalar comparisons to
> PTEST.
LGTM.
Thanks,
Uros.
>
> Thanks for your patience.
> Roger
> --
>
