From: Eric Biggers <ebiggers@kernel.org>
To: Arnd Bergmann <arnd@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
linux-arch <linux-arch@vger.kernel.org>,
Vineet Gupta <vgupta@synopsys.com>,
Russell King <linux@armlinux.org.uk>,
Herbert Xu <herbert@gondor.apana.org.au>,
"David S. Miller" <davem@davemloft.net>,
Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
"open list:HARDWARE RANDOM NUMBER GENERATOR CORE"
<linux-crypto@vger.kernel.org>,
"open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
Nobuhiro Iwamatsu <iwamatsu@debian.org>
Subject: Re: [PATCH v2 07/13] asm-generic: unaligned always use struct helpers
Date: Tue, 18 May 2021 14:31:12 -0700 [thread overview]
Message-ID: <YKQyICQuyJZsl+/j@gmail.com> (raw)
In-Reply-To: <CAK8P3a0QMjP-i7aw_CBRHPu7ffzX0p_vYF_SRtpd_iB8HW5TqQ@mail.gmail.com>
On Tue, May 18, 2021 at 10:51:23PM +0200, Arnd Bergmann wrote:
>
> > 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.
>
> Agreed. I think there is a fairly strong case for just using -O2 on lz4
> and backport that to stable.
> Searching for lz4 bugs with -O3 also finds several reports including
> one that I sent myself:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65709
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69702
>
> I see that user space zstd is built with -O3 in Debian, but it the changelog
> also lists "Improved : better speed on clang and gcc -O2, thanks to Eric
> Biggers", so maybe Eric has some useful ideas on whether we should
> just use -O2 for the in-kernel version.
>
In my opinion, -O2 is a good default even for compression code. I generally
don't see any benefit from -O3 in compression code I've written.
That being said, -O2 is what I usually use during development. Other people
could write code that relies on -O3 to be optimized well.
The Makefiles for lz4 and zstd use -O3 by default, which is a little concerning.
I do expect that they're still well-written enough to do well with -O2 too, but
it would require doing benchmarks to tell for sure. (As Arnd noted, it happens
that I did do such benchmarks on zstd about 5 years ago, and I found an issue
where some functions weren't marked inline when they should be, causing them to
be inlined at -O3 but not at -O2. That got fixed.)
- Eric
next prev parent reply other threads:[~2021-05-18 21:31 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
2021-05-18 18:09 ` Jason A. Donenfeld
2021-05-18 20:51 ` Arnd Bergmann
2021-05-18 21:31 ` Eric Biggers [this message]
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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