On 29 October 2013 11:20, Eric Anholt <e...@anholt.net> wrote: > Paul Berry <stereotype...@gmail.com> writes: > > > On 27 October 2013 12:58, Marek Olšák <mar...@gmail.com> wrote: > > > >> On Sun, Oct 27, 2013 at 4:19 PM, Paul Berry <stereotype...@gmail.com> > >> wrote: > >> > On 20 October 2013 09:20, Marek Olšák <mar...@gmail.com> wrote: > >> >> > >> >> From: Marek Olšák <marek.ol...@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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