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_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 <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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