linux-kernel - Re: [GIT PULL] alpha: cleanups and build fixes for 6.10

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Date: Sat, 11 May 2024 22:08:50 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Paul E. McKenney" <paulmck@...nel.org>,
 "John Paul Adrian Glaubitz" <glaubitz@...sik.fu-berlin.de>
Cc: "Linus Torvalds" <torvalds@...ux-foundation.org>,
 linux-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 linux-alpha@...r.kernel.org,
 "Richard Henderson" <richard.henderson@...aro.org>,
 "Ivan Kokshaysky" <ink@...assic.park.msu.ru>,
 "Matt Turner" <mattst88@...il.com>,
 "Alexander Viro" <viro@...iv.linux.org.uk>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10

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