linux-kernel - Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

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Date:   Wed, 14 Apr 2021 07:24:09 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v1 1/2] powerpc/bitops: Use immediate operand when possible

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