[Mesa-dev] [Mesa-stable] [PATCH 3/4] intel/cfg: Always add both successors to a break
Francisco Jerez
currojerez at riseup.net
Tue Oct 10 19:52:29 UTC 2017
Jason Ekstrand <jason at jlekstrand.net> writes:
> On Wed, Oct 4, 2017 at 5:29 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>
>> This won't completely solve the problem. For example, what if you
>> hoist the assignment to color2 outside the loop?
>>
>> vec4 color2;
>> while (1) {
>> vec4 color = texture();
>> color2 = color * 2;
>> if (...) {
>> break;
>> }
>> }
>> gl_FragColor = color2;
>>
>>
>> Now the definition still dominates the use, even with the modified
>> control-flow graph, and you have the same problem
>
>
> Curro had me convinced that some detail of the liveness analysis pass saved
> us here but now I can't remember what. :-(
>
I think it's a shame that my name didn't come up on this discussion
until it was time to deflect Connor's blame away from you. Not that I
generally care a lot about other people putting their names on the
result of my research while I'm out on vacation, but at least face the
fact that the patch you put your name on is incomplete, on your own.
As it turns out there's a minimal solution for the problem Connor
brought up that doesn't involve ad-hoc post-dataflow-analysis fix-ups,
nor disabling dataflow analysis for non-writemask-all assignments, but
why on earth would I get involved in this discussion now?
>
>> The real problem is
>> that the assignment to color2 is really a conditional assignment: if
>> we're going channel-by-channel, it's not, but if you consider the
>> *whole* register at the same time, it is. To really fix the problem,
>> you need to model exactly what the machine actually does: you need to
>> insert "fake" edges like these, that model the jumps that the machine
>> can take, and you need to make every assignment a conditional
>> assignment (i.e. it doesn't kill the register). It's probably not as
>> bad with Curro's patch on top, though. Also, once you do this you can
>> make register allocation more accurate by generating interferences
>> from the liveness information directly instead of from the intervals.
>>
>> One thing I've thought about is, in addition to maintaining this
>> "whole-vector" view of things, is to maintain a "per-channel" liveness
>> that doesn't use the extra edges, partial definitions etc. and then
>> use the "per-channel view" to calculate interference when the channels
>> always line up.
>>
>
> Yes, we've considered that and it's a good idea. However, I'm trying to
> fix bugs right now, not write the world's best liveness analysis pass. :-)
>
>
>> On Wed, Oct 4, 2017 at 7:58 PM, Jason Ekstrand <jason at jlekstrand.net>
>> wrote:
>> > Shader-db results on Sky Lake:
>> >
>> > total instructions in shared programs: 12955125 -> 12953698 (-0.01%)
>> > instructions in affected programs: 55956 -> 54529 (-2.55%)
>> > helped: 6
>> > HURT: 38
>> >
>> > All of the hurt programs were hurt by exactly one instruction because
>> > this patch affects copy propagation. Most of the helped instructions
>> > came from a single orbital explorer shader that was helped by 14.26%
>> >
>> > Cc: mesa-stable at lists.freedesktop.org
>> > ---
>> > src/intel/compiler/brw_cfg.cpp | 37 ++++++++++++++++++++++++++++++
>> +++++--
>> > 1 file changed, 35 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/src/intel/compiler/brw_cfg.cpp
>> b/src/intel/compiler/brw_cfg.cpp
>> > index fad12ee..d8bf725 100644
>> > --- a/src/intel/compiler/brw_cfg.cpp
>> > +++ b/src/intel/compiler/brw_cfg.cpp
>> > @@ -289,9 +289,42 @@ cfg_t::cfg_t(exec_list *instructions)
>> > assert(cur_while != NULL);
>> > cur->add_successor(mem_ctx, cur_while);
>> >
>> > + /* We also add the next block as a successor of the break. If
>> the
>> > + * break is predicated, we need to do this because the break
>> may not
>> > + * be taken. If the break is not predicated, we add it anyway
>> so
>> > + * that our live intervals computations will operate as if the
>> break
>> > + * may or may not be taken. Consider the following example:
>> > + *
>> > + * vec4 color2;
>> > + * while (1) {
>> > + * vec4 color = texture();
>> > + * if (...) {
>> > + * color2 = color * 2;
>> > + * break;
>> > + * }
>> > + * }
>> > + * gl_FragColor = color2;
>> > + *
>> > + * In this case, the definition of color2 dominates the use
>> because
>> > + * the loop only has the one exit. This means that the live
>> range
>> > + * interval for color2 goes from the statement in the if to
>> it's use
>> > + * below the loop. Now suppose that the texture operation has
>> a
>> > + * header register that gets assigned one of the registers
>> used for
>> > + * color2. If the loop condition is non-uniform and some of
>> the
>> > + * threads will take the break and others will continue. In
>> this
>> > + * case, the next pass through the loop, the WE_all setup of
>> the
>> > + * header register will stomp the disabled channels of color2
>> and
>> > + * corrupt the value.
>> > + *
>> > + * This same problem can occur if you have a mix of 64, 32, and
>> > + * 16-bit registers because the channels do not line up or if
>> you
>> > + * have a SIMD16 program and the first half of one value
>> overlaps the
>> > + * second half of the other. To solve it, we simply treat the
>> break
>> > + * as if it may also continue on because some of the threads
>> may
>> > + * continue on.
>> > + */
>> > next = new_block();
>> > - if (inst->predicate)
>> > - cur->add_successor(mem_ctx, next);
>> > + cur->add_successor(mem_ctx, next);
>> >
>> > set_next_block(&cur, next, ip);
>> > break;
>> > --
>> > 2.5.0.400.gff86faf
>> >
>> > _______________________________________________
>> > mesa-dev mailing list
>> > mesa-dev at lists.freedesktop.org
>> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>
> _______________________________________________
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> mesa-stable at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-stable
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