[Mesa-dev] [PATCH 09/16] i965/fs: Calculate interference better in register_coalesce.

[Jordan Justen]

On Fri, Jan 10, 2014 at 5:59 PM, Matt Turner <mattst88 at gmail.com> wrote:
> On Fri, Jan 10, 2014 at 5:19 PM, Jordan Justen <jljusten at gmail.com> wrote:
>> On Thu, Dec 19, 2013 at 1:40 PM, Matt Turner <mattst88 at gmail.com> wrote:
>>> +         live_intervals->start[var_to] = MIN2(live_intervals->start[var_to],
>>> +                                              live_intervals->start[var_from]);
>>> +         live_intervals->end[var_to] = MAX2(live_intervals->end[var_to],
>>> +                                            live_intervals->end[var_from]);
>>> +         reg_from = -1;
>>
>> Do you think these are what lead to the lost programs?
>
> I don't think so.
>
>> I haven't been able to determine why they are needed starting with this patch...
>
> Consider
>
> add vgrf3:F, vgrf1:F, vgrf2:F
> mov vgrf4:F, vgrf3:F
> mul vgrf5:F, vgrf5:F, vgrf4:F
>
> register coalescing turns this into
>
> add vgrf4:F, vgrf1:F, vgrf2:F
> mul vgrf5:F, vgrf5:F, vgrf4:F
>
> and now our live intervals are wrong, and calculating live intervals
> is expensive. Instead of recalculating live intervals after each
> successful iteration, we just fix var_to's start and end to keep them
> valid.

Wasn't this also the case before this patch?

> To make sure that none of the other instructions are affected, we
> temporarily replace the MOV with a NOP and then after the pass is over
> remove all of the NOPs when it's safe to invalidate the live
> intervals.