[Mesa-dev] NIR issue with SPIRV ops that have operands with different bit-size
Samuel Iglesias Gonsálvez
siglesias at igalia.com
Wed Apr 25 10:06:14 UTC 2018
Thanks to all for the opinions.
I'm going to implement the conversion then.
Sam
On 24/04/18 15:45, Jason Ekstrand wrote:
> On Tue, Apr 24, 2018 at 6:42 AM, Ian Romanick <idr at freedesktop.org
> <mailto:idr at freedesktop.org>> wrote:
>
> On 04/24/2018 05:44 AM, Rob Clark wrote:
> > On Tue, Apr 24, 2018 at 4:24 AM, Samuel Iglesias Gonsálvez
> > <siglesias at igalia.com <mailto:siglesias at igalia.com>> wrote:
> >> Hello,
> >>
> >> Recently, we have found problems between some SPIRV opcodes and
> NIR.
> >>
> >> SPIR-V allows opcodes to mix different bit-sizes for their
> operands, such as for some bitfield operations and other ops like
> bitwise shifts.
> >>
> >> In NIR, when the ALU opcode doesn't have specified bitsizes for
> their operands, it is expected to have both the same bitsize (see
> the assert in nir_build_alu() at nir_builder.h). I suppose this
> assumption comes from the time that NIR were only fed with GLSL IR
> but now with SPIR-V that assert is wrong.
> >>
> >> Instead of adding new variants for the opcodes (such as
> nir_op_ishl16, or so) to workaround the issue, I think it is
> needed to fix this by removing this assumption from NIR and its
> validator. I send this email to ask for ideas and to find the best
> way to handle this.
> >>
> >
> > Karol hit the same thing (with for example, shift instructions) with
> > the work for spv compute/kernel support. I *think* the number of
> > special cases isn't too high, so probably vtn should just insert the
> > appropriate conversion instruction (ie. u2u32, etc) so that if
> the src
> > bitsize is incorrect it will be converted.
>
> That's what I was going to suggest. I guess it's possible that
> some HW
> might benefit from using the smaller bit-size, but the code generator
> should be able to see through the conversions to get the smaller data.
>
>
> Agreed. I think most of these case should have no difference between
> the theoretical new opcode and original + conversion.
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