Kristian Høgsberg <k...@bitplanet.net> writes: > On Tue, Oct 20, 2015 at 11:56 AM, Francisco Jerez <curroje...@riseup.net> > wrote: >> Kristian Høgsberg <k...@bitplanet.net> writes: >> >>> On Tue, Oct 20, 2015 at 3:16 AM, Francisco Jerez <curroje...@riseup.net> >>> wrote: >>>> Kristian Høgsberg <k...@bitplanet.net> writes: >>>> >>>>> On Mon, Oct 19, 2015 at 4:19 AM, Francisco Jerez <curroje...@riseup.net> >>>>> wrote: >>>>>> Neil Roberts <n...@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 <n...@linux.intel.com> >>>>>>> >>>>>>> - Neil >>>>>>> >>>>>>> Kristian Høgsberg Kristensen <k...@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 <k...@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@lists.freedesktop.org >>>>>>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev >>>>>>> _______________________________________________ >>>>>>> mesa-dev mailing list >>>>>>> mesa-dev@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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