On 13.01.2017 00:20, Ilia Mirkin wrote:
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.
Well, I don't know the intricacies of DX9. I agree that apps can do the
abs() themselves, so if the 0*x behavior is really the only other
difference, then zero_wins is a fine name as well.
Cheers,
Nicolai
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.
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