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From: "Arnd Bergmann" <arnd@arndb.de>
To: "Paul E. McKenney" <paulmck@kernel.org>,
	"John Paul Adrian Glaubitz" <glaubitz@physik.fu-berlin.de>
Cc: "Linus Torvalds" <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org,
	Linux-Arch <linux-arch@vger.kernel.org>,
	linux-alpha@vger.kernel.org,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Ivan Kokshaysky" <ink@jurassic.park.msu.ru>,
	"Matt Turner" <mattst88@gmail.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10
Date: Sat, 11 May 2024 22:08:50 +0200	[thread overview]
Message-ID: <8dd1c466-54e3-45c1-a19f-f81dd9dbf243@app.fastmail.com> (raw)
In-Reply-To: <a1331c86-dc07-4635-b169-623fcdd11824@paulmck-laptop>

On Sat, May 11, 2024, at 21:37, Paul E. McKenney wrote:
> On Sat, May 11, 2024 at 08:49:08PM +0200, John Paul Adrian Glaubitz wrote:
>
> The pre-EV56 Alphas have no byte store instruction, correct?
>
> If that is in fact correct, what code is generated for a volatile store
> to a single byte for those CPUs?  For example, for this example?
>
> 	char c;
>
> 	...
>
> 	WRITE_ONCE(c, 3);
>
> The rumor I heard is that the compilers will generate a non-atomic
> read-modify-write instruction sequence in this case, first reading the
> 32-bit word containing that byte into a register, then substituting the
> value to be stored into corresponding byte of that register, and finally
> doing a 32-bit store from that register.
>
> Is that the case, or am I confused?

I think it's slightly worse: gcc will actually do a 64-bit
read-modify-write rather than a 32-bit one, and it doesn't
use atomic ll/sc when storing into an _Atomic struct member:

echo '#include <stdatomic.h>^M struct s { _Atomic char c; _Atomic char s[7]; long l; }; void f(struct s *s) { atomic_store(&s->c, 3); }' | alpha-linux-gcc-14  -xc - -S -o- -O2 -mcpu=ev5

f:
	.frame $30,0,$26,0
$LFB0:
	.cfi_startproc
	.prologue 0
	mb
	lda $1,3($31)
	insbl $1,$16,$1
	ldq_u $2,0($16)
	mskbl $2,$16,$2
	bis $1,$2,$1
	stq_u $1,0($16)
	bis $31,$31,$31
	mb
	ret $31,($26),1
	.cfi_endproc
$LFE0:
	.end f

compared to -mcpu=ev56:

f:
	.frame $30,0,$26,0
$LFB0:
	.cfi_startproc
	.prologue 0
	mb
	lda $1,3($31)
	stb $1,0($16)
	bis $31,$31,$31
	mb
	ret $31,($26),1
	.cfi_endproc
$LFE0:
	.end f

      Arnd

  reply	other threads:[~2024-05-11 20:09 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-05-10 21:17 [GIT PULL] asm-generic cleanups for 6.10 Arnd Bergmann
2024-05-10 21:19 ` [GIT PULL] alpha: cleanups and build fixes " Arnd Bergmann
2024-05-10 21:40   ` John Paul Adrian Glaubitz
2024-05-10 22:28     ` Paul E. McKenney
2024-05-11 18:49       ` John Paul Adrian Glaubitz
2024-05-11 19:37         ` Paul E. McKenney
2024-05-11 20:08           ` Arnd Bergmann [this message]
2024-05-12  1:26             ` Paul E. McKenney
2024-05-12  6:02               ` John Paul Adrian Glaubitz
2024-05-12 14:44                 ` Paul E. McKenney
2024-05-13  3:50                   ` Akira Yokosawa
2024-05-13  4:03                     ` Paul E. McKenney
2024-05-13  6:28                       ` Arnd Bergmann
2024-05-13  9:26                     ` John Paul Adrian Glaubitz
2024-05-13 16:52                     ` Ulrich Teichert
2024-05-13 21:44                       ` John Paul Adrian Glaubitz
2024-05-13  7:10                   ` John Paul Adrian Glaubitz
2024-05-12  6:17   ` John Paul Adrian Glaubitz
2024-05-13 16:27   ` Linus Torvalds
2024-05-13 21:42     ` John Paul Adrian Glaubitz
2024-05-31  2:53       ` Maciej W. Rozycki
2024-05-13 17:05   ` pr-tracker-bot
2024-05-13 16:11 ` [GIT PULL] asm-generic cleanups " Linus Torvalds
2024-05-13 16:36   ` Arnd Bergmann
2024-05-20 22:35 ` pr-tracker-bot

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