On Tue, Oct 20, 2015 at 11:56 AM, Francisco Jerez <[email protected]> wrote: > Kristian Høgsberg <[email protected]> writes: > >> On Tue, Oct 20, 2015 at 3:16 AM, Francisco Jerez <[email protected]> >> wrote: >>> Kristian Høgsberg <[email protected]> writes: >>> >>>> On Mon, Oct 19, 2015 at 4:19 AM, Francisco Jerez <[email protected]> >>>> wrote: >>>>> Neil Roberts <[email protected]> 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 ;-) 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 <[email protected]> >>>>>> >>>>>> - Neil >>>>>> >>>>>> Kristian Høgsberg Kristensen <[email protected]> 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 <[email protected]> >>>>>>> --- >>>>>>> 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 >>>>>>> [email protected] >>>>>>> http://lists.freedesktop.org/mailman/listinfo/mesa-dev >>>>>> _______________________________________________ >>>>>> mesa-dev mailing list >>>>>> [email protected] >>>>>> 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 >>>>> _______________________________________________ mesa-dev mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/mesa-dev
