From: Matteo Croce <mcroce@linux.microsoft.com> To: Nick Kossifidis <mick@ics.forth.gr> Cc: linux-riscv <linux-riscv@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>, Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>, Atish Patra <atish.patra@wdc.com>, Emil Renner Berthing <kernel@esmil.dk>, Akira Tsukamoto <akira.tsukamoto@gmail.com>, Drew Fustini <drew@beagleboard.org>, Bin Meng <bmeng.cn@gmail.com>, David Laight <David.Laight@aculab.com>, Guo Ren <guoren@kernel.org> Subject: Re: [PATCH v3 3/3] riscv: optimized memset Date: Wed, 23 Jun 2021 02:08:30 +0200 [thread overview] Message-ID: <CAFnufp2w1TGtaBjfTtsBpDatgAtATRZbB4MURV3tLh1fi-W1JQ@mail.gmail.com> (raw) In-Reply-To: <17cd289430f08f2b75b7f04242c646f6@mailhost.ics.forth.gr> On Tue, Jun 22, 2021 at 3:07 AM Nick Kossifidis <mick@ics.forth.gr> wrote: > > Στις 2021-06-17 18:27, Matteo Croce έγραψε: > > + > > +void *__memset(void *s, int c, size_t count) > > +{ > > + union types dest = { .u8 = s }; > > + > > + if (count >= MIN_THRESHOLD) { > > + const int bytes_long = BITS_PER_LONG / 8; > > You could make 'const int bytes_long = BITS_PER_LONG / 8;' and 'const > int mask = bytes_long - 1;' from your memcpy patch visible to memset as > well (static const...) and use them here (mask would make more sense to > be named as word_mask). > I'll do > > + unsigned long cu = (unsigned long)c; > > + > > + /* Compose an ulong with 'c' repeated 4/8 times */ > > + cu |= cu << 8; > > + cu |= cu << 16; > > +#if BITS_PER_LONG == 64 > > + cu |= cu << 32; > > +#endif > > + > > You don't have to create cu here, you'll fill dest buffer with 'c' > anyway so after filling up enough 'c's to be able to grab an aligned > word full of them from dest, you can just grab that word and keep > filling up dest with it. > I tried that, but this way I have to wait 8 bytes more before starting the memset. And, the machine code needed to generate 'cu' is just 6 instructions on riscv: slli a5,a0,8 or a5,a5,a0 slli a0,a5,16 or a0,a0,a5 slli a5,a0,32 or a0,a5,a0 so probably it's not worth it. > > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > > + /* Fill the buffer one byte at time until the destination > > + * is aligned on a 32/64 bit boundary. > > + */ > > + for (; count && dest.uptr % bytes_long; count--) > > You could reuse & mask here instead of % bytes_long. > Sure, even if the machine code will be the same. > > + *dest.u8++ = c; > > +#endif > > I noticed you also used CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS on your > memcpy patch, is it worth it here ? To begin with riscv doesn't set it > and even if it did we are talking about a loop that will run just a few > times to reach the alignment boundary (worst case scenario it'll run 7 > times), I don't think we gain much here, even for archs that have > efficient unaligned access. It doesn't _now_, but maybe in the future we will have a CPU which handles unaligned accesses correctly! -- per aspera ad upstream
WARNING: multiple messages have this Message-ID (diff)
From: Matteo Croce <mcroce@linux.microsoft.com> To: Nick Kossifidis <mick@ics.forth.gr> Cc: linux-riscv <linux-riscv@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch <linux-arch@vger.kernel.org>, Paul Walmsley <paul.walmsley@sifive.com>, Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>, Atish Patra <atish.patra@wdc.com>, Emil Renner Berthing <kernel@esmil.dk>, Akira Tsukamoto <akira.tsukamoto@gmail.com>, Drew Fustini <drew@beagleboard.org>, Bin Meng <bmeng.cn@gmail.com>, David Laight <David.Laight@aculab.com>, Guo Ren <guoren@kernel.org> Subject: Re: [PATCH v3 3/3] riscv: optimized memset Date: Wed, 23 Jun 2021 02:08:30 +0200 [thread overview] Message-ID: <CAFnufp2w1TGtaBjfTtsBpDatgAtATRZbB4MURV3tLh1fi-W1JQ@mail.gmail.com> (raw) In-Reply-To: <17cd289430f08f2b75b7f04242c646f6@mailhost.ics.forth.gr> On Tue, Jun 22, 2021 at 3:07 AM Nick Kossifidis <mick@ics.forth.gr> wrote: > > Στις 2021-06-17 18:27, Matteo Croce έγραψε: > > + > > +void *__memset(void *s, int c, size_t count) > > +{ > > + union types dest = { .u8 = s }; > > + > > + if (count >= MIN_THRESHOLD) { > > + const int bytes_long = BITS_PER_LONG / 8; > > You could make 'const int bytes_long = BITS_PER_LONG / 8;' and 'const > int mask = bytes_long - 1;' from your memcpy patch visible to memset as > well (static const...) and use them here (mask would make more sense to > be named as word_mask). > I'll do > > + unsigned long cu = (unsigned long)c; > > + > > + /* Compose an ulong with 'c' repeated 4/8 times */ > > + cu |= cu << 8; > > + cu |= cu << 16; > > +#if BITS_PER_LONG == 64 > > + cu |= cu << 32; > > +#endif > > + > > You don't have to create cu here, you'll fill dest buffer with 'c' > anyway so after filling up enough 'c's to be able to grab an aligned > word full of them from dest, you can just grab that word and keep > filling up dest with it. > I tried that, but this way I have to wait 8 bytes more before starting the memset. And, the machine code needed to generate 'cu' is just 6 instructions on riscv: slli a5,a0,8 or a5,a5,a0 slli a0,a5,16 or a0,a0,a5 slli a5,a0,32 or a0,a5,a0 so probably it's not worth it. > > +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS > > + /* Fill the buffer one byte at time until the destination > > + * is aligned on a 32/64 bit boundary. > > + */ > > + for (; count && dest.uptr % bytes_long; count--) > > You could reuse & mask here instead of % bytes_long. > Sure, even if the machine code will be the same. > > + *dest.u8++ = c; > > +#endif > > I noticed you also used CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS on your > memcpy patch, is it worth it here ? To begin with riscv doesn't set it > and even if it did we are talking about a loop that will run just a few > times to reach the alignment boundary (worst case scenario it'll run 7 > times), I don't think we gain much here, even for archs that have > efficient unaligned access. It doesn't _now_, but maybe in the future we will have a CPU which handles unaligned accesses correctly! -- per aspera ad upstream _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv
next prev parent reply other threads:[~2021-06-23 0:09 UTC|newest] Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-06-17 15:27 [PATCH v3 0/3] riscv: optimized mem* functions Matteo Croce 2021-06-17 15:27 ` Matteo Croce 2021-06-17 15:27 ` [PATCH v3 1/3] riscv: optimized memcpy Matteo Croce 2021-06-17 15:27 ` Matteo Croce 2021-06-18 14:06 ` kernel test robot 2021-06-18 14:06 ` kernel test robot 2021-06-18 14:06 ` kernel test robot 2021-06-21 14:26 ` Christoph Hellwig 2021-06-21 14:26 ` Christoph Hellwig 2021-06-22 8:19 ` David Laight 2021-06-22 8:19 ` David Laight 2021-06-22 22:53 ` Matteo Croce 2021-06-22 22:53 ` Matteo Croce 2021-06-22 22:00 ` Matteo Croce 2021-06-22 22:00 ` Matteo Croce 2021-06-22 0:14 ` Nick Kossifidis 2021-06-22 0:14 ` Nick Kossifidis 2021-06-22 23:35 ` Matteo Croce 2021-06-22 23:35 ` Matteo Croce 2021-06-23 9:48 ` Nick Kossifidis 2021-06-23 9:48 ` Nick Kossifidis 2021-06-17 15:27 ` [PATCH v3 2/3] riscv: optimized memmove Matteo Croce 2021-06-17 15:27 ` Matteo Croce 2021-06-21 14:28 ` Christoph Hellwig 2021-06-21 14:28 ` Christoph Hellwig 2021-06-22 0:46 ` Nick Kossifidis 2021-06-22 0:46 ` Nick Kossifidis 2021-06-30 4:40 ` kernel test robot 2021-06-30 4:40 ` kernel test robot 2021-06-30 4:40 ` kernel test robot 2021-06-17 15:27 ` [PATCH v3 3/3] riscv: optimized memset Matteo Croce 2021-06-17 15:27 ` Matteo Croce 2021-06-21 14:32 ` Christoph Hellwig 2021-06-21 14:32 ` Christoph Hellwig 2021-06-22 1:07 ` Nick Kossifidis 2021-06-22 1:07 ` Nick Kossifidis 2021-06-22 8:38 ` David Laight 2021-06-22 8:38 ` David Laight 2021-06-23 1:14 ` Matteo Croce 2021-06-23 1:14 ` Matteo Croce 2021-06-23 9:05 ` David Laight 2021-06-23 9:05 ` David Laight 2021-06-23 0:08 ` Matteo Croce [this message] 2021-06-23 0:08 ` Matteo Croce 2021-06-22 1:09 ` [PATCH v3 0/3] riscv: optimized mem* functions Nick Kossifidis 2021-06-22 1:09 ` Nick Kossifidis 2021-06-22 2:39 ` Guo Ren 2021-06-22 2:39 ` Guo Ren
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