On Thu, Jan 12, 2017 at 6:04 PM, Nicolai Hähnle <nhaeh...@gmail.com> wrote: > On 12.01.2017 23:46, Ilia Mirkin wrote: >> >> On Thu, Jan 12, 2017 at 4:03 PM, Matteo Bruni <matteo.myst...@gmail.com> >> wrote: >>> >>> So, what would be really nice to have is a GLSL extension for some >>> kind of switch to select the requested behavior WRT NaN. For example a >>> three-way option with "don't generate NaN in arithmetic operations", >>> "do generate NaN" and "don't care". It could also be a GL state if >>> that's easier to implement with the existing hardware, since an >>> individual application isn't supposed to require different behavior >>> from one shader to the next. >>> >>> Is anyone interested in / favorable to something like this? It would >>> solve the issue with defining NaN behavior in GLSL while making things >>> a bit more compatible with "other API a lot of games are ported from >>> which happens to be supported by all the desktop GPUs". >> >> >> Not that I'm biased, but on the NVIDIA Tesla series (G80-GT21x), this >> enable is handled via a global flag, not in the shader binary, so this >> is all-or-nothing for a whole pipeline. On GF100+, I believe there is >> also an enable via a global flag, but there are also a FMUL.FMZ (and >> FFMA.FMZ) flag, which I *think* has the same effect. So for GF100+ hw, >> this could be done at the instruction level. > > > Well, I would also have advocated for what is effectively a > per-program/pipeline flag anyway, even though GCN hardware can theoretically > do it per-instruction. Tracking a per-instruction bit in the compiler > quickly becomes fragile (e.g. there's no good way for us to model this > information per-instruction in LLVM IR). Per-shader isn't any better than > per-instruction due to linking, and per-shader-stage is awkward if we ever > want to do fancier cross-stage optimizations. > > It's really quite simple. Introduce an extension with a name like > MESA_shader_float_dx9. The behavior I'd suggest is: > > Enabling/requiring the extension in a shader causes various semantics > changes to bring floating point behavior in line with DX9 in that shader's > code: > > - 0*x = 0
Yes. But only for fp32, not for fp64. > - sqrt/rsqrt are guaranteed to take the absolute value of their argument Is that necessary? If the software knows about the ext, it also knows to stick the abs() in. > - anything else? I'd say MESA_shader_float_zero_wins or something, if we don't stick the sqrt stuff into it. Here is a software model of the Tesla-era shader execution created by Marcin (mwk): https://github.com/envytools/envytools/blob/master/nvhw/fp.c#L168 The bit in question is "zero_wins", so just look at what that modifies. So if you have a*b and a || b == 0 (i.e. they are +0 or -0) then +0 is returned. > > It is a link error to link a program in which some shaders have the > extension and others don't. > > There's funny interactions like having compute shaders with dx9 float > semantics, but that's pretty much it :) > > Somebody just needs to write up a draft, but it only makes sense if we can > at least get all the Mesa drivers and Wine behind this. If there's a simple > global flag on NVidia hardware, then it should be easy to provide an initial > implementation for nouveau *hint* *hint* ;) > > For radeonsi, more work is required (like support in LLVM). All sounds good to me. Should be relatively straightforward for nouveau. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
