Re: [Mesa-dev] The ARB_shader_subroutine extension
On 11/16/2014 08:31 AM, Gustaw Smolarczyk wrote: Hello once again, This time, I would like to ask about the shader subroutine extension. I believe this extension is not very popular, but is still needed for GL4 compliance. What is the reason for its unpopularity? Is it because one needs to reset subroutine uniform values after any glUse*? Or it just didn't provide enough value? Wouldn't it be good to implement uber-shaders (especially using subroutine uniform array)? It has a very strong negative affect on register allocation. Basically, the compiler has to, at the very least, allocate registers for the worst subroutine. This means that subroutines that use fewer registers will often perform worse (on some GPUs much, much worse) than they should. On Intel GPUs this means that you might just get SIMD8 for a shader that could have had SIMD16. I've been told that on AMD GPUs it means you have much fewer vertices or pixels executing in parallel leading to dramatic performance hits. Is anybody actively working on it? I guess, there are changes needed in the GLSL compiler. It could be easily lowered to a switch-on-uniform-int thing, or something like that, if only compliance to the GL4 matters and not the performance. Regards, Gustaw ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] The ARB_shader_subroutine extension
Yeah, I understand this concern. But this could be said about any uber-shader approach. For cases when one cannot afford to change shader program in between draw calls (using features like multi draw indirect), it should be better to use shader subroutine arrays instead of a switch. If the difference in register pressure between the simplest and the most complex subroutine is not that large, the problem you described wouldn't exist. One can always batch draw calls with very simple subroutines separately. It would be great if there was a possibility to change *the whole* program for a given draw, so that renderings with simple shaders wouldn't be penalized by the artificial register pressure caused by the dynamically unused paths. But I guess, the hardware is unable to implement such a feature. 2014-11-18 19:51 GMT+01:00 Ian Romanick i...@freedesktop.org: On 11/16/2014 08:31 AM, Gustaw Smolarczyk wrote: Hello once again, This time, I would like to ask about the shader subroutine extension. I believe this extension is not very popular, but is still needed for GL4 compliance. What is the reason for its unpopularity? Is it because one needs to reset subroutine uniform values after any glUse*? Or it just didn't provide enough value? Wouldn't it be good to implement uber-shaders (especially using subroutine uniform array)? It has a very strong negative affect on register allocation. Basically, the compiler has to, at the very least, allocate registers for the worst subroutine. This means that subroutines that use fewer registers will often perform worse (on some GPUs much, much worse) than they should. On Intel GPUs this means that you might just get SIMD8 for a shader that could have had SIMD16. I've been told that on AMD GPUs it means you have much fewer vertices or pixels executing in parallel leading to dramatic performance hits. Is anybody actively working on it? I guess, there are changes needed in the GLSL compiler. It could be easily lowered to a switch-on-uniform-int thing, or something like that, if only compliance to the GL4 matters and not the performance. Regards, Gustaw ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
Re: [Mesa-dev] The ARB_shader_subroutine extension
Hi Gustaw, My understanding is that it's a combination of the awkward API, and dubious value. It's certainly not something that everyone is screaming for. As far as mesa's implementation goes, I started on the parser changes and tests a while ago; I believe Ian was going to pick it up. I'm not sure whether he's had time to do much on it though. -- Chris On Mon, Nov 17, 2014 at 5:31 AM, Gustaw Smolarczyk wielkie...@gmail.com wrote: Hello once again, This time, I would like to ask about the shader subroutine extension. I believe this extension is not very popular, but is still needed for GL4 compliance. What is the reason for its unpopularity? Is it because one needs to reset subroutine uniform values after any glUse*? Or it just didn't provide enough value? Wouldn't it be good to implement uber-shaders (especially using subroutine uniform array)? Is anybody actively working on it? I guess, there are changes needed in the GLSL compiler. It could be easily lowered to a switch-on-uniform-int thing, or something like that, if only compliance to the GL4 matters and not the performance. Regards, Gustaw ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev ___ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
