[Mesa-dev] [PATCH] gallivm: use fallback code for mul_hi with llvm >= 7.0

[Roland Scheidegger]

Am 29.08.19 um 15:05 schrieb Jose Fonseca:
> This change is 
> 
>   Reviewed-by: Jose Fonseca <jfonseca at vmware.com>
> 
> Regarding follow up change, do you think the LLVM pattern is sane/doable?
Yes, should be doable and not too bad (I did not verify that what we're
doing doesn't actually get recognized, since it's theoretically possible
some other lowering could produce the pattern, although it seems unlikely).
I think though this code isn't hit a lot - it was once used by draw,
which is why I noticed the suboptimal code generated and added the
optimized version, but nowadays it's just for mulhi, so should be fairly
rare in practice.


> 
> If not we should try ask them to reconsider relying strictly upon
> pattern matching.  I get the feeling upstream LLVM is throwing the baby
> with the water with these changes.  I do understand the advantages of
> moving away from vendor specific intrinsics, but I think that for things
> which have no natural representation on LLVM base IR, they should add a
> vendor-agnostic intrinsic, for example a new "/llvm.mulhi.*"  set of
> instrinsics/, as narrow pattern matching is bound to produce performance
> cliffs nobody will notice.
They did add new generic intrinsics for some things, but not this one
indeed.
I'm not exactly a big fan of this pattern matching in favor of
intrinsics neither, at least if the patterns are non-trivial...

Roland



> /
> /
> /Jose/
> 
> ------------------------------------------------------------------------
> *From:* sroland at vmware.com <sroland at vmware.com>
> *Sent:* Wednesday, August 28, 2019 20:37
> *To:* mesa-dev at lists.freedesktop.org <mesa-dev at lists.freedesktop.org>;
> Jose Fonseca <jfonseca at vmware.com>; airlied at freedesktop.org
> <airlied at freedesktop.org>
> *Cc:* Roland Scheidegger <sroland at vmware.com>
> *Subject:* [PATCH] gallivm: use fallback code for mul_hi with llvm >= 7.0
>  
> From: Roland Scheidegger <sroland at vmware.com>
> 
> LLVM 7.0 ditched the pmulu intrinsics.
> This is only a trivial patch to use the fallback code instead.
> It'll likely produce atrocious code since the pattern doesn't match what
> llvm itself uses in its autoupgrade paths, hence the pattern won't be
> recognized.
> 
> Should fix https://bugs.freedesktop.org/show_bug.cgi?id=111496
> ---
>  src/gallium/auxiliary/gallivm/lp_bld_arit.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> index c4931c0b230..f1866c6625f 100644
> --- a/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> +++ b/src/gallium/auxiliary/gallivm/lp_bld_arit.c
> @@ -1169,8 +1169,13 @@ lp_build_mul_32_lohi_cpu(struct lp_build_context
> *bld,
>      * https://llvm.org/bugs/show_bug.cgi?id=30845
>      * So, whip up our own code, albeit only for length 4 and 8 (which
>      * should be good enough)...
> +    * FIXME: For llvm >= 7.0 we should match the autoupgrade pattern
> +    * (bitcast/and/mul/shuffle for unsigned, bitcast/shl/ashr/mul/shuffle
> +    * for signed), which the fallback code does not, without this llvm
> +    * will likely still produce atrocious code.
>      */
> -   if ((bld->type.length == 4 || bld->type.length == 8) &&
> +   if (HAVE_LLVM < 0x0700 &&
> +       (bld->type.length == 4 || bld->type.length == 8) &&
>         ((util_cpu_caps.has_sse2 && (bld->type.sign == 0)) ||
>          util_cpu_caps.has_sse4_1)) {
>        const char *intrinsic = NULL;
> -- 
> 2.17.1
>