On Monday, June 27, 2016 11:43:28 AM PDT Matt Turner wrote: > On Mon, Jun 27, 2016 at 4:44 AM, Rob Clark <robdcl...@gmail.com> wrote: > > On Mon, Jun 27, 2016 at 7:13 AM, Alan Swanson <rei...@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 <robcl...@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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