On Mon, Oct 17, 2016 at 4:10 PM, Ilia Mirkin <imir...@alum.mit.edu> wrote: > On Mon, Oct 17, 2016 at 9:59 AM, Marek Olšák <mar...@gmail.com> wrote: >> On Mon, Oct 17, 2016 at 3:48 PM, Ilia Mirkin <imir...@alum.mit.edu> wrote: >>> On Mon, Oct 17, 2016 at 9:46 AM, Marek Olšák <mar...@gmail.com> wrote: >>>> On Mon, Oct 17, 2016 at 3:44 PM, Ilia Mirkin <imir...@alum.mit.edu> wrote: >>>>> nouveau supports PIPE_SHADER_CAP_SUBROUTINES and properly details with >>>>> RET opcodes. The alternative is that the st lowers the whole thing >>>>> into a loop which adds IMHO unnecessary complexity to the resulting >>>>> code. Any reason not to leave that in place? >>>> >>>> It had already been disabled and probably broken too. If you have no >>>> interest in fixing and using it within a reasonable time frame, there >>>> is no reason for the code to be there. >>> >>> What's broken about it? To the best of my knowledge, it's working >>> fine. I'm specifically talking about RET (from the "MAIN" function), >>> not subroutines in general. >> >> OK. I thought you were talking about subroutines. >> >> The RET opcode support can stay there if somebody wants it very much. >> >> However, have you seen any apps using "return" in "main"? > > Yes. Among other things, a lot of compute shaders do if (foo) return; > somewhere at the top. See this example from F1 2015: > http://hastebin.com/vacevigoyi.go - I've seen it in the middle too > though. > >> Is there any serious performance concern with lowering "return" to a >> conditional branch, such that it's beneficial for drivers to implement >> RET? > > Depends on the driver. Nouveau's control flow analysis isn't extremely > sophisticated to undo the lowering that the GLSL compiler does. Which > means extra instructions, and extra RA complexity since the whole main > function ends up inside a loop. > >> If so, shouldn't it be fixed in the GLSL compiler instead? > > Well, from what I recall, the GLSL compiler's lowering goes something like > > void main() { > if (foo) return; > ... > } > > to > > void main() { > while (true) { > if (foo) > break; > ... > break; > } > } > > With additional complexity if the return is inside a for/while loop itself. > >> Is it worth having a rarely-used codepath in glsl_to_tgsi that most >> drivers skip anyway? > > It's 2 lines of code... one to stop the GLSL compiler from doing the > lowering, and another to emit the RET opcode when you see the > ir_op_return or whatever it's called. IMHO not such a great burden. > > All the other subroutines stuff can go as far as I'm concerned, I > doubt anyone will care to implement glsl subroutines that way. > > -ilia
OK, sounds good. Marek _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
