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

[Rob Clark]

On Mon, Jun 27, 2016 at 2:43 PM, Matt Turner <mattst88 at gmail.com> 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

interesting.. so I guess there is at least some shader in there w/
some uninitialized vars..

BR,
-R