On Thu, Jul 29, 2021 at 8:54 PM Palmer Dabbelt <pal...@dabbelt.com> wrote: > > On Tue, 27 Jul 2021 02:32:12 PDT (-0700), cmuell...@gcc.gnu.org wrote: > > Ok, so if I understand correctly Palmer and Andrew prefer > > overlap_op_by_pieces to be controlled > > by its own field in the riscv_tune_param struct and not by the field > > slow_unaligned_access in this struct > > (i.e. slow_unaligned_access==false is not enough to imply > > overlap_op_by_pieces==true). > > I guess, but I'm not really worried about this at that level of detail > right now. It's not like the tune structures form any sort of external > interface we have to keep stable, we can do whatever we want with those > fields so I'd just aim for encoding the desired behavior as simply as > possible rather than trying to build something extensible. > > There are really two questions we need to answer: is this code actually > faster for the C906, and is this what the average users wants under -Os.
I never mentioned -Os. My main goal is code compiled for -O2, -O3 or even -Ofast. And I want to execute code as fast as possible. Loading hot data from cache is faster when being done by a single load-word instruction than 4 load-byte instructions. Less instructions implies less pressure for the instruction cache. Less instructions implies less work for a CPU pipeline. Architectures, which don't have a penalty for unaligned accesses therefore observe a performance benefit. What I understand from Andrew's email is that it is not that simple and implementation might have a penalty for overlapping accesses that is high enough to avoid them. I don't have the details for C906, so I can't say if that's the case. > That first one is pretty easy: just running those simple code sequences > under a sweep of page offsets should be sufficient to determine if this > is always faster (in which case it's an easy yes), if it's always slower > (an easy no), or if there's some slow cases like page/cache line > crossing (in which case we'd need to think a bit). > > The second one is a bit tricker. In the past we'd said these sort of > "actively misalign accesses to generate smaller code" sort of thing > isn't suitable for -Os (as most machines still have very slow unaligned > accesses) but is suitable for -Oz (don't remember if that ever ended up > in GCC, though). That still seems like a reasonable decision, but if it > turns out that implementations with fast unaligned accesses become the > norm then it'd probably be worth revisiting it. Not sure exactly how to > determine that tipping point, but I think we're a long way away from it > right now. > > IMO it's really just premature to try and design an encoding of the > tuning paramaters until we have an idea of what they are, as we'll just > end up devolving down the path of trying to encode all possible hardware > and that's generally a huge waste of time. Since there's no ABI here we > can refactor this however we want as new tunings show up. I guess you mean that there needs to be a clear benefit for a supported machine in GCC. Either obviously (see below), by measurement results, or by decision of the machine's maintainer (especially if the decision is a trade-off). > > > I don't have access to pipeline details that give proof that there are cases > > where this patch causes a performance penalty. > > > > So, I leave this here as a summary for someone who has enough information > > and > > interest to move this forward: > > * the original patch should be sufficient, but does not have tests: > > https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575791.html > > * the tests can be taken from this patch: > > https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575864.html > > Note, that there is a duplicated "sw" in builtins-overlap-6.c, which > > should be a "sd". > > > > Thanks for the feedback! > > Cool. Looks like the C906 is starting to show up in the real world, so > we should be able to find someone who has access to one and cares enough > to at least run some simple benchamrks of these code sequences. IMO > that's a pretty low interest bar, so I don't see any harm in waiting -- > when the hardware is common then I'm sure someone will care enough to > give this a shot, and until then it's not really impacting anyone either > way. > > The -Os thing is a bigger discussion, and while I'm happy to have it I > don't really think we're even close to these being common enough yet. I > saw your memmove patch and think the same rationale might apply there, > but I haven't looked closely and won't have time to for a bit as I've > got to get around to the other projects. The cpymemsi patch is also targeting -O2 or higher for fast code execution. And it is one of the cases where there is an obvious performance benefit for all machines that have slow_unaligned_access==false. At the moment the cpymemsi expansion for RISC-V is implemented as if there is no machine with slow_unaligned_access==false. And in fact there is a machine already in GCC mainline with this property: C906. Machines that can do fast unaligned accesses should not be wasting their cycles with load-store-pairs of bytes, if they can do load-store pairs of words. > > > On Tue, Jul 27, 2021 at 3:48 AM Palmer Dabbelt <pal...@dabbelt.com> wrote: > >> > >> On Mon, 26 Jul 2021 03:05:21 PDT (-0700), Andrew Waterman wrote: > >> > On Thu, Jul 22, 2021 at 10:27 AM Palmer Dabbelt <pal...@dabbelt.com> > >> > wrote: > >> >> > >> >> On Thu, 22 Jul 2021 06:29:46 PDT (-0700), gcc-patches@gcc.gnu.org wrote: > >> >> > Could you add a testcase? Otherwise LGTM. > >> >> > > >> >> > Option: -O2 -mtune=thead-c906 -march=rv64gc -mabi=lp64 > >> >> > void foo(char *dst){ > >> >> > __builtin_memset(dst, 0, 15); > >> >> > } > >> >> > >> >> I'd like to see: > >> >> > >> >> * Test results. This is only on for one target right now, so relying on > >> >> it to just work on others isn't a good idea. > >> >> * Something to demonstrate this doesn't break -mstrict-align. > >> >> * Some sort of performance analysis. Most machines that support > >> >> unaligned access do so with some performance degredation, > >> > > >> > Also, some machines that gracefully support misaligned accesses under > >> > most circumstances nevertheless experience a perf degradation when the > >> > load depends on two stores that overlap partially but not fully. This > >> > transformation will obviously trigger such behavior from time to time. > >> > >> Ya, I thought I wrote a response to this but I guess it's just in a > >> buffer somewhere. The code sequences this is generating are really the > >> worst case for unaligned stores: one of them is always guaranteed to be > >> misaligned, and it partially overlaps with a store one cycle away. > >> > >> We're really only saving a handful of instructions at best here, so > >> there's not much room for error when it comes to these sorts of things. > >> Even if this difficult case is handled fast we're only talking about > >> saving 2 cycles, which is pretty borderline as the single-issue in-order > >> machines I've worked with that do support misaligned accesses tend to > >> take at least a few cycles of performance hit on misaligned accesses. > >> Even if misaligned accesses are single cycle, some back of the envelope > >> calculations says that a pipeline flush when crossing a cache line would > >> definitely push this negative and one per page crossing would be in the > >> margins (depending on how we assume the original accesses are aligned). > >> > >> This is way too subtle of a thing to analyze without at least some > >> knowledge of how this pipeline works, whether that's from a benchmark or > >> just a pipeline description. > >> > >> > Note, I'm not objecting to this patch; I'm only responding to Palmer's > >> > comment. It makes sense to enable this kind of optimization for > >> > -mtune=native etc., just not for standard software distributions. > >> > >> IMO there are really two cases here: -mtune=c906 and -Os (-mtune=native > >> is sort of a red herring, it's just syntax). For -mtune=c906 I'm happy > >> enabling this as long as it's actually correct and improves performance, > >> but that'll need at least some hardware-oriented analysis -- whether > >> it's a benchmark or just a confirmation that these sequences are > >> actually expected to run fast. > >> > >> -Os is a different case, though. Last time this came up we decided that > >> -Os shouldn't purposefully misalign accesses, even when that saves code > >> size, as that's too likely to hit pathological performance cases. I > >> don't know if the weights have changed here: the C906 is currently by > >> far the cheapest chip (which likely means it's going to be the most > >> popular), but it's unclear as to whether it's even RISC-V compliant and > >> I don't know of many people who've actually gotten one. IMO it's too > >> early to flip -Os over to this behavior, we at least need to know that > >> this chip is going to be sufficiently RISC-V-ey that standard software > >> will even run on it much less be popular. > >> > >> > > >> > > >> >> it's unclear > >> >> that this conversion will actually manifst a performance improvement. > >> >> I don't have a C906 and don't know what workloads people care about > >> >> running on one, but I'm certainly not convinced this is a win -- > >> >> what's listed here looks to be the best case, and that's only saving > >> >> two instructions to generate a pretty pathological sequence > >> >> (misaligned access that conflicts with a prior store). > >> > >> Ah, I guess there it was ;) > >> > >> >> > >> >> Jojo: do you have any description of the C906 pipeline? Specifically in > >> >> this case it'd be good to know how it handles unaligned accesses. > >> >> > >> >> > > >> >> > On Thu, Jul 22, 2021 at 8:53 PM Christoph Muellner via Gcc-patches > >> >> > <gcc-patches@gcc.gnu.org> wrote: > >> >> >> > >> >> >> This patch enables the overlap-by-pieces feature of the by-pieces > >> >> >> infrastructure for inlining builtins in case the target has set > >> >> >> riscv_slow_unaligned_access_p to false. > >> >> >> > >> >> >> To demonstrate the effect for targets with fast unaligned access, > >> >> >> the following code sequences are generated for a 15-byte memset-zero. > >> >> >> > >> >> >> Without overlap_op_by_pieces we get: > >> >> >> 8e: 00053023 sd zero,0(a0) > >> >> >> 92: 00052423 sw zero,8(a0) > >> >> >> 96: 00051623 sh zero,12(a0) > >> >> >> 9a: 00050723 sb zero,14(a0) > >> >> >> > >> >> >> With overlap_op_by_pieces we get: > >> >> >> 7e: 00053023 sd zero,0(a0) > >> >> >> 82: 000533a3 sd zero,7(a0) > >> >> >> > >> >> >> gcc/ChangeLog: > >> >> >> > >> >> >> * config/riscv/riscv.c (riscv_overlap_op_by_pieces): New > >> >> >> function. > >> >> >> (TARGET_OVERLAP_OP_BY_PIECES_P): Connect to > >> >> >> riscv_overlap_op_by_pieces. > >> >> >> > >> >> >> Signed-off-by: Christoph Muellner <cmuell...@gcc.gnu.org> > >> >> >> --- > >> >> >> gcc/config/riscv/riscv.c | 11 +++++++++++ > >> >> >> 1 file changed, 11 insertions(+) > >> >> >> > >> >> >> diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c > >> >> >> index 576960bb37c..98c76ba657a 100644 > >> >> >> --- a/gcc/config/riscv/riscv.c > >> >> >> +++ b/gcc/config/riscv/riscv.c > >> >> >> @@ -5201,6 +5201,14 @@ riscv_slow_unaligned_access (machine_mode, > >> >> >> unsigned int) > >> >> >> return riscv_slow_unaligned_access_p; > >> >> >> } > >> >> >> > >> >> >> +/* Implement TARGET_OVERLAP_OP_BY_PIECES_P. */ > >> >> >> + > >> >> >> +static bool > >> >> >> +riscv_overlap_op_by_pieces (void) > >> >> >> +{ > >> >> >> + return !riscv_slow_unaligned_access_p; > >> >> >> +} > >> >> >> + > >> >> >> /* Implement TARGET_CAN_CHANGE_MODE_CLASS. */ > >> >> >> > >> >> >> static bool > >> >> >> @@ -5525,6 +5533,9 @@ riscv_asan_shadow_offset (void) > >> >> >> #undef TARGET_SLOW_UNALIGNED_ACCESS > >> >> >> #define TARGET_SLOW_UNALIGNED_ACCESS riscv_slow_unaligned_access > >> >> >> > >> >> >> +#undef TARGET_OVERLAP_OP_BY_PIECES_P > >> >> >> +#define TARGET_OVERLAP_OP_BY_PIECES_P riscv_overlap_op_by_pieces > >> >> >> + > >> >> >> #undef TARGET_SECONDARY_MEMORY_NEEDED > >> >> >> #define TARGET_SECONDARY_MEMORY_NEEDED riscv_secondary_memory_needed > >> >> >> > >> >> >> -- > >> >> >> 2.31.1 > >> >> >>
