[Mesa-dev] [PATCH] glsl: break the gl_FragData array into separate gl_FragData[i] variables

[Paul Berry]

On 29 October 2013 11:20, Eric Anholt <eric at anholt.net> wrote:

> Paul Berry <stereotype441 at gmail.com> writes:
>
> > On 27 October 2013 12:58, Marek Olšák <maraeo at gmail.com> wrote:
> >
> >> On Sun, Oct 27, 2013 at 4:19 PM, Paul Berry <stereotype441 at gmail.com>
> >> wrote:
> >> > On 20 October 2013 09:20, Marek Olšák <maraeo at gmail.com> wrote:
> >> >>
> >> >> From: Marek Olšák <marek.olsak at amd.com>
> >> >>
> >> >> This avoids a defect in lower_output_reads.
> >> >>
> >> >> The problem is lower_output_reads treats the gl_FragData array as a
> >> single
> >> >> variable. It first redirects all output writes to a temporary
> variable
> >> >> (array)
> >> >> and then writes the whole temporary variable to the output,
> generating
> >> >> assignments to all elements of gl_FragData.
> >> >
> >> >
> >> > Can you go into more detail about why this is a problem?  At first
> >> glance it
> >> > seems like it should be fine, because failing to assign to an element
> of
> >> > gl_FragData is supposed to result in undefined data going to that
> >> fragment
> >> > output.  So lowering to a shader that does assign to that element of
> >> > gl_FragData seems like it should be harmless.  What am I missing here?
> >>
> >> Thanks for the review. The problem is drivers cannot eliminate useless
> >> writes to gl_FragData, and each enabled gl_FragData output decreases
> >> performance. The GLSL compiler cannot eliminate the writes either,
> >> because gl_FragData is an array.
> >>
> >> Maybe the GLSL compiler should resize arrays based on which elements
> >> are written, so that unused elements are not declared, but this is not
> >> enough for gl_FragData, where the i-th output can be written, but
> >> (i-1)-th output doesn't have to be.
> >>
> >
> > Ah, ok.  When I saw the word "defect", I misunderstood you to be fixing a
> > correctness-of-rendering bug.  As a performance optimization, I get it
> now.
> >
> > For driver back-ends that don't need lower_output_reads (such as i915 and
> > i965), this optimization isn't needed.  Would you mind adding a flag to
> > ShaderCompilerOptions so that tgsi-based drivers can opt in to this new
> > optimization?  I want to make sure that the code generation of i965 and
> > i915 isn't affected.
>
> Actually, if Marek has identified that applications not setting all
> their gl_FragData[] is a performance issue, I want to see this applied
> to i965, too.  This optimization would remove entire FB_WRITE opcodes
> for us, which could be pretty significant if applications hit this case.
>

Yes, but Marek's patch won't benefit i965 even in that case.  The only
effect of Marek's patch is to prevent lower_output_reads (which i965
doesn't use) from converting a shader that writes to only some elements of
gl_FragData into a shader that writes to all elements of gl_FragData.

To optimize i965 for shaders that don't write to all elements of
gl_FragData we need to do back-end work that's unrelated to Marek's patch.
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