[Mesa-dev] [PATCH 2/3] i965/fs: Make emit_uniformize a no-op for immediates

Francisco Jerez currojerez at riseup.net
Tue Oct 20 11:56:18 PDT 2015

Kristian Høgsberg <krh at bitplanet.net> writes:

> On Tue, Oct 20, 2015 at 3:16 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> Kristian Høgsberg <krh at bitplanet.net> writes:
>>> On Mon, Oct 19, 2015 at 4:19 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>>>> Neil Roberts <neil at linux.intel.com> writes:
>>>>> Just a thought, would it be better to move this check into the
>>>>> eliminate_find_live_channel optimisation? That way it could catch
>>>>> sources that become immediates through later optimisations. One problem
>>>>> with this that I've seen before is that eliminating the
>>>>> FIND_LIVE_CHANNEL doesn't cause the subsequent BROADCAST to be
>>>>> eliminated because the copy propagation doesn't work.
>>>> I believe in this particular case the BROADCAST instruction should
>>>> already be eliminated (by opt_algebraic), because its first source is an
>>>> immediate, so this optimization would in fact be redundant with
>>>> opt_algebraic+constant propagation if it weren't because the latter is
>>>> unable to handle scalar copies.
>>> Yes, is happens as you say: first the FIND_LIVE_CHANNEL gets
>>> eliminated because it's outside control flow
>>> (fs_visitor::eliminate_find_live_channel). Then the broadcast gets
>>> reduced to a MOV in opt_algebraic because src0 is uniform (immediate
>>> constant). The problem then is the dst of the MOV has stride 0 which
>>> can_propagate_from() then bails on.
>>>> Another possibility that would likely
>>>> be more general than this (because it would handle cases in which, as
>>>> Neil said, the argument becomes an immediate only after optimization,
>>>> and because it would also avoid the issue with liveness analysis
>>>> extending the live ranges of scalar values incorrectly [1]) would be to
>>>> extend the destination register of the BROADCAST instructions to a
>>>> vector [2] and then add a case to the switch statement of
>>>> try_constant_propagate() so that the immediate MOV resulting from
>>>> opt_algebraic is propagated into surface instructions (see attachment).
>>> I wrote exactly this code to make constant propagate work, plus adding
>>> the extra opcodes to the switch statement. It works and I could
>>> certainly send that out after this, but
>>> 1) This doesn't mean we shouldn't do the if (src.file == IMM)
>>> shortcut. It saves the compiler a bit of work in the very common case
>>> of
>>> non-indirect buffer access.
>>> 2) I'm not even sure it makes sense to extend copy-propagation to do
>>> this (which is why I went back to just the IMM test). Anything that
>>> would be an immediate at this point should be an immediate, if not
>>> we're missing something in nir.
>> Still this doesn't address the root of the problem, which is that
>> emit_uniformize() emits scalar code that the rest of the compiler is not
>> able to handle properly.
> This is not a case of papering over the root cause, it's about not
> creating the root cause in the first place. The output of
> emit_uniformize() always ends up as either a surface or a sampler
> index, which we only look at later in the generator. There are no
> other cases where the result of emit_uniformize() might be part of an
> expression that we can copy propagate or otherwise optimize. If the
> input to emit_uniformize() isn't an immediate where it could be, nir
> optimization needs fixing. So if we add these two lines to
> emit_uniformize() to pass immediates straight through, we avoid
> generating code that we have to extend the copy prop pass to handle.

Kristian, there are legitimate uses of emit_uniformize() in which the
argument is not an immediate but still can be optimized out later on --
E.g. for images it will frequently be a uniform register, or a
non-constant expression calculated within uniform control flow (in which
case the FIND_LIVE_CHANNEL instruction emitted here will be reduced to a
constant MOV by eliminate_find_live_channel()).  In such cases we still
want copy-propagation to kick in, but it wont because of the problem I
was talking about with scalar writes.  Even if the instructions emitted
by emit_uniformize() cannot be optimized out, liveness analysis will
overestimate the live ranges of the temporaries used by
emit_uniformize() for the same reason, potentially causing the register
allocator to spill or run out of registers.

Anyway don't take me wrong, I'm not NAK-ing your patch or anything, I
just have the feeling that by fixing this more generally you could've
saved us [or most likely me ;)] work in the near future.

> Kristian
>> Re-implementing a special case of the
>> optimization already done by opt_algebraic() might have helped in this
>> specific case, but it won't help when the argument is of some other kind
>> of uniform value which are also frequently encountered in practice and
>> could also be copy-propagated, and it won't help avoid the liveness
>> analysis bug [1] in cases where the argument is not an immediate (the
>> bug is reported to break some thousands SSBO dEQP tests, although I
>> don't know what fraction of them actually use non-constant indexing).
>> The alternative solution I provided patches for (and you seem to have
>> implemented yourself independently) would address all these issues at
>> once.
>>> Kristian
>>>>> I made this patch a while ago but I never posted it anywhere because
>>>>> it's a of a kludge and it would probably be better to fix the copy
>>>>> propagation:
>>>>> https://github.com/bpeel/mesa/commit/e4c3286075f891f466fb8558106d2aaa
>>>> Heh, yeah, I'd rather fix copy propagation instead, which I believe will
>>>> become much easier with the use-def-chain analysis pass I'm working on.
>>>>> Either way though I don't think it would do any harm to have Kristian's
>>>>> patch as well even if we did improve elimintate_find_live_channel so it
>>>>> is:
>>>>> Reviewed-by: Neil Roberts <neil at linux.intel.com>
>>>>> - Neil
>>>>> Kristian Høgsberg Kristensen <krh at bitplanet.net> writes:
>>>>>> An immdiate is already uniform so just return it up front. Without this,
>>>>>> brw_fs_surface_builder ends up passing immediate surface indices through
>>>>>> SHADER_OPCODE_BROADCAST. This writes to a stride 0 dst, which we can't
>>>>>> constant propagate out of, and further, we don't constant propagate into
>>>>>> the typed/untype read/write opcodes at all.  The end result is that all
>>>>>> typed/untyped read/write/atomics end up as indirect sends.
>>>>>> Code generation should always produce either an immediate or an actual
>>>>>> indirect surface index, so we can fix this by just special casing
>>>>>> immediates in emit_uniformize.
>>>>>> Signed-off-by: Kristian Høgsberg Kristensen <krh at bitplanet.net>
>>>>>> ---
>>>>>>  src/mesa/drivers/dri/i965/brw_fs_builder.h | 3 +++
>>>>>>  1 file changed, 3 insertions(+)
>>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_builder.h b/src/mesa/drivers/dri/i965/brw_fs_builder.h
>>>>>> index df10a9d..98ce71e 100644
>>>>>> --- a/src/mesa/drivers/dri/i965/brw_fs_builder.h
>>>>>> +++ b/src/mesa/drivers/dri/i965/brw_fs_builder.h
>>>>>> @@ -394,6 +394,9 @@ namespace brw {
>>>>>>           const dst_reg chan_index = component(vgrf(BRW_REGISTER_TYPE_UD), 0);
>>>>>>           const dst_reg dst = component(vgrf(src.type), 0);
>>>>>> +         if (src.file == IMM)
>>>>>> +            return src;
>>>>>> +
>>>>>>           ubld.emit(SHADER_OPCODE_FIND_LIVE_CHANNEL, chan_index);
>>>>>>           ubld.emit(SHADER_OPCODE_BROADCAST, dst, src, chan_index);
>>>>>> --
>>>>>> 2.6.2
>>>>>> _______________________________________________
>>>>>> mesa-dev mailing list
>>>>>> mesa-dev at lists.freedesktop.org
>>>>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>>>> _______________________________________________
>>>>> mesa-dev mailing list
>>>>> mesa-dev at lists.freedesktop.org
>>>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
>>>> [1] http://lists.freedesktop.org/archives/mesa-dev/2015-October/097085.html
>>>> [2] http://lists.freedesktop.org/archives/mesa-dev/attachments/20151014/25dd38dc/attachment.patch
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