[Mesa-dev] [PATCH 2/2] i965/fs: Only apply Gen4 work-arounds if regs_written > 1.

[Matt Turner]

On Sat, Jan 17, 2015 at 5:17 AM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Tuesday, January 13, 2015 03:40:32 PM Matt Turner wrote:
>> On Tue, Jan 13, 2015 at 3:35 PM, Matt Turner <mattst88 at gmail.com> wrote:
>> > Otherwise, we would have necessarily read the results or eliminated the
>> > dead SEND. In either case, no work around is necessary.
>> >
>> > Noticed when debugging the problem the previous patch fixed that any
>> > time we hit a math instruction, we'd walk through subsequent
>> > instructions, and of course each time discover that its result was in
>> > fact used.
>> > ---
>>
>> I was thinking through the pre-send dependency work around:
>>
>> /**
>>  * Implements this workaround for the original 965:
>>  *
>>  *     "[DevBW, DevCL] Implementation Restrictions: As the hardware does not
>>  *      check for post destination dependencies on this instruction, software
>>  *      must ensure that there is no destination hazard for the case of ‘write
>>  *      followed by a posted write’ shown in the following example.
>>  *
>>  *      1. mov r3 0
>>  *      2. send r3.xy <rest of send instruction>
>>  *      3. mov r2 r3
>>  *
>>  *      Due to no post-destination dependency check on the ‘send’, the above
>>  *      code sequence could have two instructions (1 and 2) in flight at the
>>  *      same time that both consider ‘r3’ as the target of their final writes.
>>  */
>>
>> While this is a hardware problem or something, isn't it impossible for
>> us to hit? If the first MOV's results weren't read, we would have dead
>> code eliminated it. If they were read (necessarily between it and the
>> SEND), we would never have both instructions in flight at once.
>
> It's definitely pretty rare, though I'm not certain I can say it never
> happens.  If you care to look into it further, I found the bug report
> which spawned this code:
>
> https://bugs.freedesktop.org/show_bug.cgi?id=58960
>
> The attachment contains a sample application which I managed to compile via:
> $ for file in *.h; do moc-qt4 $file > moc-$(basename $file .h).cpp; done
> $ g++ -Wall -g $(pkg-config --libs --cflags QtCore QtGui QtOpenGL gl) *.cpp
>
> It would be great if we could make a Piglit test.

No regressions from this patch as well, and no changes to the
generated code in that program.