On Mon, Oct 15, 2018 at 1:39 PM Ian Romanick <i...@freedesktop.org> wrote:
> On 10/14/2018 03:58 PM, Jason Ekstrand wrote:
> > On October 14, 2018 17:12:34 Matt Turner <matts...@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.
--Jason
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