From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible
Date: Wed, 14 Apr 2021 07:24:09 -0500 [thread overview]
Message-ID: <20210414122409.GV26583@gate.crashing.org> (raw)
In-Reply-To: <1618365589.67fxh7cot9.astroid@bobo.none>
On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote:
> Would be nice if we could let the compiler deal with it all...
>
> static inline unsigned long lr(unsigned long *mem)
> {
> unsigned long val;
>
> /*
> * This doesn't clobber memory but want to avoid memory operations
> * moving ahead of it
> */
> asm volatile("ldarx %0, %y1" : "=r"(val) : "Z"(*mem) : "memory");
>
> return val;
> }
(etc.)
That can not work reliably: the compiler can put random instructions
between the larx and stcx. this way, and you then do not have guaranteed
forward progress anymore. It can put the two in different routines
(after inlining and other interprocedural optimisations), duplicate
them, make a different number of copies of them, etc.
Nothing of that is okay if you want to guarantee forward progress on all
implementations, and also not if you want to have good performance
everywhere (or anywhere even). Unfortunately you have to write all
larx/stcx. loops as one block of assembler, so that you know exactly
what instructions will end up in your binary.
If you don't, it will fail mysteriously after random recompilations, or
have performance degradations, etc. You don't want to go there :-)
Segher
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>,
linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible
Date: Wed, 14 Apr 2021 07:24:09 -0500 [thread overview]
Message-ID: <20210414122409.GV26583@gate.crashing.org> (raw)
In-Reply-To: <1618365589.67fxh7cot9.astroid@bobo.none>
On Wed, Apr 14, 2021 at 12:01:21PM +1000, Nicholas Piggin wrote:
> Would be nice if we could let the compiler deal with it all...
>
> static inline unsigned long lr(unsigned long *mem)
> {
> unsigned long val;
>
> /*
> * This doesn't clobber memory but want to avoid memory operations
> * moving ahead of it
> */
> asm volatile("ldarx %0, %y1" : "=r"(val) : "Z"(*mem) : "memory");
>
> return val;
> }
(etc.)
That can not work reliably: the compiler can put random instructions
between the larx and stcx. this way, and you then do not have guaranteed
forward progress anymore. It can put the two in different routines
(after inlining and other interprocedural optimisations), duplicate
them, make a different number of copies of them, etc.
Nothing of that is okay if you want to guarantee forward progress on all
implementations, and also not if you want to have good performance
everywhere (or anywhere even). Unfortunately you have to write all
larx/stcx. loops as one block of assembler, so that you know exactly
what instructions will end up in your binary.
If you don't, it will fail mysteriously after random recompilations, or
have performance degradations, etc. You don't want to go there :-)
Segher
next prev parent reply other threads:[~2021-04-14 12:26 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 15:33 [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible Christophe Leroy
2021-04-08 15:33 ` Christophe Leroy
2021-04-08 15:33 ` [PATCH v1 2/2] powerpc/atomics: " Christophe Leroy
2021-04-08 15:33 ` Christophe Leroy
2021-04-12 22:08 ` Segher Boessenkool
2021-04-12 22:08 ` Segher Boessenkool
2021-04-13 16:36 ` Christophe Leroy
2021-04-13 16:36 ` Christophe Leroy
2021-04-12 21:54 ` [PATCH v1 1/2] powerpc/bitops: " Segher Boessenkool
2021-04-12 21:54 ` Segher Boessenkool
2021-04-13 16:33 ` Christophe Leroy
2021-04-13 16:33 ` Christophe Leroy
2021-04-13 21:58 ` Segher Boessenkool
2021-04-13 21:58 ` Segher Boessenkool
2021-04-14 2:01 ` Nicholas Piggin
2021-04-14 2:01 ` Nicholas Piggin
2021-04-14 12:24 ` Segher Boessenkool [this message]
2021-04-14 12:24 ` Segher Boessenkool
2021-04-14 12:42 ` Christophe Leroy
2021-04-14 12:42 ` Christophe Leroy
2021-04-14 15:19 ` Segher Boessenkool
2021-04-14 15:19 ` Segher Boessenkool
2021-04-14 15:32 ` David Laight
2021-04-14 15:32 ` David Laight
2021-04-14 17:20 ` Segher Boessenkool
2021-04-14 17:20 ` Segher Boessenkool
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