[Mesa-dev] [PATCH 2/7] nir/int64: Add some more lowering helpers

[Connor Abbott]

On Mon, Oct 15, 2018 at 8:41 PM Jason Ekstrand <jason at jlekstrand.net> wrote:
>
> On Mon, Oct 15, 2018 at 1:39 PM Ian Romanick <idr at freedesktop.org> wrote:
>>
>> On 10/14/2018 03:58 PM, Jason Ekstrand wrote:
>> > On October 14, 2018 17:12:34 Matt Turner <mattst88 at gmail.com> wrote:
>> >> +static nir_ssa_def *
>> >> +lower_iabs64(nir_builder *b, nir_ssa_def *x)
>> >> +{
>> >> +   nir_ssa_def *x_hi = nir_unpack_64_2x32_split_y(b, x);
>> >> +   nir_ssa_def *x_is_neg = nir_ilt(b, x_hi, nir_imm_int(b, 0));
>> >> +   return nir_bcsel(b, x_is_neg, lower_ineg64(b, x), x);
>> >
>> > lower_bcsel?  Or, since we're depending on this running multiple times,
>> > just nir_ineg?  I go back and forth on whether a pass like this should
>> > run in a loop or be smart enough to lower intermediate bits on the fly.
>> > We should probably pick one.
>>
>> In principle, I agree.  I've been bitten a couple times by lowering
>> passes that generate other things that need to be lowered on some
>> platforms (that I didn't test).  In this case, I think the loop is the
>> right answer since each operation is lowered by a separate flag.
>
>
> That's the easy answer, certainly.  The other option is to have every lowered thing builder check the flag and conditionally do the lowering.  That's annoying and hard to get right so a loop is probably best for now.

Couldn't you just have the builder be right after the instruction,
instead of before it, and make the outer loop use a non-safe iterator
so that it will immediately run over the instructions generated? Doing
another pass over the whole shader is usually a little expensive.

>
> --Jason
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