From: Linus Torvalds <email@example.com> To: Arnd Bergmann <firstname.lastname@example.org>, "Jason A. Donenfeld" <Jason@zx2c4.com> Cc: Eric Biggers <email@example.com>, linux-arch <firstname.lastname@example.org>, Vineet Gupta <email@example.com>, Russell King <firstname.lastname@example.org>, Herbert Xu <email@example.com>, "David S. Miller" <firstname.lastname@example.org>, Thomas Bogendoerfer <email@example.com>, Linux ARM <firstname.lastname@example.org>, Linux Kernel Mailing List <email@example.com>, "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" <firstname.lastname@example.org>, "open list:BROADCOM NVRAM DRIVER" <email@example.com> Subject: Re: [PATCH v2 07/13] asm-generic: unaligned always use struct helpers Date: Tue, 18 May 2021 06:12:03 -1000 [thread overview] Message-ID: <CAHk-=wjuoGyxDhAF8SsrTkN0-YfCx7E6jUN3ikC_tn2AKWTTsA@mail.gmail.com> (raw) In-Reply-To: <CAK8P3a3hbts4k+rrfnE8Z78ezCaME0UVgwqkdLW5NOps2YHUQQ@mail.gmail.com> On Tue, May 18, 2021 at 5:42 AM Arnd Bergmann <firstname.lastname@example.org> wrote: > > To be on the safe side, we could pass -fno-tree-loop-vectorize along > with -O3 on the affected gcc versions, or use a bigger hammer > (not use -O3 at all, always set -fno-tree-loop-vectorize, ...). I personally think -O3 in general is unsafe. It has historically been horribly buggy. It's gotten better, but this case clearly shows that "gotten better" really isn't that high of a bar. Very few projects use -O3, which is obviously part of why it's buggy. But the other part of why it's buggy is that vectorization is simply very complicated, and honestly, judging by the last report the gcc people don't care about being careful. They literally are ok with knowingly generating an off-by-one range check, because "it's undefined behavior". With that kind of mentality, I'm not personally all that inclined to say "sure, use -O3". We know it has bugs even for the well-defined cases. > -O3 is set for the lz4 and zstd compression helpers and for wireguard. I'm actually surprised wireguard would use -O3. Yes, performance is important. But for wireguard, correctness is certainly important too. Maybe Jason isn't aware of just how bad gcc -O3 has historically been? And -O3 has often generated _slower_ code, in addition to the bugs. It's not like it's a situation where "-O3 is obviously better than -O2". There's a reason -O2 is the default. And that tends to be even more true in the kernel than in many user space programs (ie smaller loops, generally much higher I$ miss rates etc). Jason? How big of a deal is that -O3 for wireguard wrt the normal -O2? There are known buggy gcc versions that aren't ancient. Of the other cases, that xor-neon.c case actually makes sense. For that file, it literally exists _only_ to get a vectorized version of the trivial xor_8regs loop. It's one of the (very very few) cases of vectorization we actually want. And in that case, we might even want to make things easier - and more explicit - for the compiler by making the xor_8regs loops use "restrict" pointers. That neon case actually wants and needs that tree-vectorization to DTRT. But maybe it doesn't need the actual _loop_ vectorization? The xor_8regs code is literally using hand-unrolled loops already, exactly to make it as simple as possible for the compiler (but the lack of "restrict" pointers means that it's not all that simple after all, and I assume the compiler generates conditionals for the NEON case? lz4 is questionable - yes, upstream lh4 seems to use -O3 (good), but it also very much uses unaligned accesses, which is where the gcc bug hits. I doubt that it really needs or wants the loop vectorization. zstd looks very similar to lz4. End result: at a minimum, I'd suggest using "-fno-tree-loop-vectorize", although somebody should check that NEON case. And I still think that using O3 for anything halfway complicated should be considered odd and need some strong numbers to enable. Linus
next prev parent reply other threads:[~2021-05-18 16:12 UTC|newest] Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-14 10:00 [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 01/13] asm-generic: use asm-generic/unaligned.h for most architectures Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 02/13] openrisc: always use unaligned-struct header Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 03/13] sh: remove unaligned access for sh4a Arnd Bergmann 2021-05-14 10:34 ` John Paul Adrian Glaubitz 2021-05-14 12:22 ` Arnd Bergmann 2021-05-15 15:36 ` John Paul Adrian Glaubitz 2021-05-15 20:10 ` Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 04/13] m68k: select CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 05/13] powerpc: use linux/unaligned/le_struct.h on LE power7 Arnd Bergmann 2021-05-14 11:48 ` Segher Boessenkool 2021-05-14 13:02 ` Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 06/13] asm-generic: unaligned: remove byteshift helpers Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 07/13] asm-generic: unaligned always use struct helpers Arnd Bergmann 2021-05-17 21:53 ` Eric Biggers 2021-05-18 7:25 ` Arnd Bergmann 2021-05-18 14:56 ` Linus Torvalds 2021-05-18 15:41 ` Arnd Bergmann 2021-05-18 16:12 ` Linus Torvalds [this message] 2021-05-18 18:09 ` Jason A. Donenfeld 2021-05-18 20:51 ` Arnd Bergmann 2021-05-18 21:31 ` Eric Biggers 2021-05-18 21:14 ` David Laight 2021-05-14 10:00 ` [PATCH v2 08/13] partitions: msdos: fix one-byte get_unaligned() Arnd Bergmann 2021-05-17 10:28 ` Christoph Hellwig 2021-05-17 10:44 ` Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 09/13] apparmor: use get_unaligned() only for multi-byte words Arnd Bergmann 2021-05-14 10:00 ` [PATCH v2 10/13] mwifiex: re-fix for unaligned accesses Arnd Bergmann 2021-05-15 6:22 ` Kalle Valo 2021-05-15 9:01 ` Arnd Bergmann 2021-05-15 18:23 ` Kalle Valo 2021-05-14 10:00 ` [PATCH v2 11/13] netpoll: avoid put_unaligned() on single character Arnd Bergmann 2021-05-14 10:01 ` [PATCH v2 12/13] asm-generic: uaccess: 1-byte access is always aligned Arnd Bergmann 2021-05-15 18:41 ` Randy Dunlap 2021-05-15 20:16 ` Arnd Bergmann 2021-05-14 10:01 ` [PATCH v2 13/13] asm-generic: simplify asm/unaligned.h Arnd Bergmann 2021-05-14 10:35 ` David Laight 2021-05-14 17:32 ` [PATCH v2 00/13] Unify asm/unaligned.h around struct helper Linus Torvalds 2021-05-14 18:51 ` Vineet Gupta 2021-05-14 19:22 ` Linus Torvalds 2021-05-14 19:45 ` Vineet Gupta 2021-05-14 20:19 ` Linus Torvalds 2021-05-14 19:31 ` Arnd Bergmann 2021-12-16 17:29 ` Ard Biesheuvel 2021-12-16 17:42 ` Linus Torvalds 2021-12-16 17:49 ` David Laight 2021-12-16 18:56 ` Segher Boessenkool 2021-12-17 12:34 ` David Laight 2021-12-17 13:35 ` Segher Boessenkool
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