[Mesa-dev] [PATCH 2/3] i965/fs: Make emit_uniformize a no-op for immediates
Francisco Jerez
currojerez at riseup.net
Fri Oct 23 05:38:31 PDT 2015
Kristian Høgsberg <krh at bitplanet.net> writes:
> On Tue, Oct 20, 2015 at 11:56 AM, Francisco Jerez <currojerez at riseup.net> wrote:
>> 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_chaso nnel()). 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.
>
> Yeah, fair point. I went and took a look at non-constant surface index
> and sent out a new series that make that work better as well as ssbo
> write optimizations. I took out the imm shortcut and put in the
> generic optimizations. It makes non-const surface index a little
> better, but as I wrote in the cover letter, it still doesn't work that
> well as we don't propagate the surface index into the send
> instructions.
>
>> 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.
>
> I may be saving you some time, but I've been working on this for
> longer than I expected ;-)
>
Thanks, and sorry for the trouble. :)
> Kristian
>
>>> 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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