[Mesa-dev] [PATCH 1/2] glsl: add driconf to zero-init unintialized vars

[Kenneth Graunke]

On Monday, June 27, 2016 11:43:28 AM PDT Matt Turner wrote:
> On Mon, Jun 27, 2016 at 4:44 AM, Rob Clark <robdclark at gmail.com> wrote:
> > On Mon, Jun 27, 2016 at 7:13 AM, Alan Swanson <reiver at improbability.net> wrote:
> >> On 2016-06-25 13:37, Rob Clark wrote:
> >>>
> >>> Some games are sloppy.. perhaps because it is defined behavior for DX or
> >>> perhaps because nv blob driver defaults things to zero.
> >>>
> >>> So add driconf param to force uninitialized variables to default to zero.
> >>>
> >>> This issue was observed with rust, from steam store.  But has surfaced
> >>> elsewhere in the past.
> >>>
> >>> Signed-off-by: Rob Clark <robclark at freedesktop.org>
> >>> ---
> >>> Note that I left out the drirc bit, since not entirely sure how to
> >>> identify this game.  (I don't actually have the game, just working off
> >>> of an apitrace)
> >>>
> >>> Possibly worth mentioning that for the shaders using uninitialized vars
> >>> having zero-initializers lets constant-propagation get rid of a whole
> >>> lot of instructions.  One shader I saw dropped to less than half of
> >>> it's original instruction count.
> >>
> >>
> >> If the default for uninitialised variables is undefined, then with the
> >> reported shader optimisations why bother with the (DRI) option when
> >> zeroing could still essentially be classed as undefined?
> >>
> >> Cuts the patch down to just the src/compiler/glsl/ast_to_hir.cpp change.
> >
> > I did suggest that on #dri-devel, but Jason had a theoretical example
> > where it would hurt.. iirc something like:
> >
> >   float maybe_undef;
> >   for (int i = 0; i < some_uniform_at_least_one; i++)
> >      maybe_undef = ...
> >
> > also, he didn't want to hide shader bugs that app should fix.
> >
> > It would be interesting to rush shaderdb w/ glsl_zero_init=true and
> > see what happens, but I didn't get around to that yet.
> 
> Here's what I get on i965. It's not a clear win.
> 
> total instructions in shared programs: 5249030 -> 5249002 (-0.00%)
> instructions in affected programs: 28936 -> 28908 (-0.10%)
> helped: 66
> HURT: 132
> 
> total cycles in shared programs: 57966694 -> 57956306 (-0.02%)
> cycles in affected programs: 1136118 -> 1125730 (-0.91%)
> helped: 78
> HURT: 106

I suspect most of the help is because we're missing undef optimizations,
such as CSE...while zero could be CSE'd.  (I have a patch, but it hurts
things too...)
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