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

[Matt Turner]

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