On Fri, Nov 10, 2017 at 9:58 PM, Nicolai Hähnle <nhaeh...@gmail.com> wrote: > On 10.11.2017 19:24, Connor Abbott wrote: >> >> On Fri, Nov 10, 2017 at 1:19 PM, Marek Olšák <mar...@gmail.com> wrote: >>> >>> On Fri, Nov 10, 2017 at 6:55 PM, Nicolai Hähnle <nhaeh...@gmail.com> >>> wrote: >>>> >>>> On 10.11.2017 18:43, Marek Olšák wrote: >>>>> >>>>> >>>>> On Fri, Nov 10, 2017 at 2:09 AM, Connor Abbott <cwabbo...@gmail.com> >>>>> wrote: >>>>>> >>>>>> >>>>>> On Thu, Nov 9, 2017 at 7:17 PM, Marek Olšák <mar...@gmail.com> wrote: >>>>>>> >>>>>>> >>>>>>> On Fri, Nov 10, 2017 at 12:40 AM, Matt Arsenault <arse...@gmail.com> >>>>>>> wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> On Nov 10, 2017, at 07:41, Marek Olšák <mar...@gmail.com> wrote: >>>>>>>>> >>>>>>>>> Hi, >>>>>>>>> >>>>>>>>> This fixes the TCS gl_ClipDistance piglit failure that was >>>>>>>>> uncovered >>>>>>>>> by a recent LLVM change. The solution is to set volatile on loads >>>>>>>>> and stores to enforce proper ordering. >>>>>>>>> >>>>>>>>> Please review. >>>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> Every LDS access certainly should not be volatile. This kills all >>>>>>>> optimizations, like formation of ds_read2_b32. What ordering issue >>>>>>>> are you >>>>>>>> having? >>>>>>> >>>>>>> >>>>>>> >>>>>>> It might be caused by inttoptr(NULL) that we do to declare LDS. There >>>>>>> is simply no ordering enforced, which is weird. >>>>>> >>>>>> >>>>>> >>>>>> As soon as you do inttoptr(NULL), you've generated a poison value (in >>>>>> LLVM legalese), so LLVM will assume that you never dereference it and >>>>>> optimize accordingly. I think a GEP instruction without the inbounds >>>>>> parameter set will get rid of the poison value, although I'm not sure >>>>>> about the case where the offset is known to be zero. At least, that's >>>>>> my reading of the langref text for the GEP instruction >>>>>> (https://llvm.org/docs/LangRef.html#id215). If zero is a valid address >>>>>> in LDS, then it sounds like LLVM needs to be fixed to disable this >>>>>> optimization for certain address spaces. On the other hand, if you're >>>>>> doing inttoptr(NULL) + offset, where "offset" is the result of a >>>>>> ptrtoint somewhere, you should be doing inttoptr(offset) instead, and >>>>>> then LLVM should never misbehave. >>>>> >>>>> >>>>> >>>>> I don't think that using inttoptr before every load and store would be >>>>> better than volatile. The must be a better solution. >>>> >>>> >>>> >>>> Can't we just allocate the required LDS memory explicitly like we did >>>> for >>>> the LDS-based derivative computations? >>>> >>>> It may require shuffling around a bit how/when we calculate the required >>>> sizes, but it doesn't seem impossible. >>> >>> >>> We want to share the same declaration in TCS main and epilog parts. >>> >>> Does LLVM know that LDS declarations are pre-initialized? >>> Do sized LDS declarations affect SIMD-occupancy-based optimizations? >>> Because Mesa always declares 64kB of LDS and the real value is >>> determined at runtime. >> >> >> I don't know about the latter, but for the former, if you declare the >> LDS variable as having external linkage, LLVM should assume that it >> might be initialized beforehand -- exactly like a global non-static >> variable in C. > > > Makes sense. > > I don't think LLVM is really looking at LDS size too closely for anything, > since LDS is per-thread group. But it's been a while since I checked. > > So just declaring a 64/32 KB memory block and then potentially not using all > of it is probably fine and is probably the best short-term solution (if it > works). > > It's a good point though that "shuffling around the computation of the > required sizes" is potentially much more involved than I was thinking at > first. It looks like if we wanted to be perfectly honest with LLVM about > what's going on (and I believe we should, in the long run), we'd have to > teach it a notion of "per-thread LDS memory". That requires more thought.
We can't tell LLVM the size of LDS, because we don't know it - it's computed from 2 independent shaders (LS and HS, or ES and GS). Marek _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
