linux-kernel - Re: [PATCH v2 2/2] powerpc/32be: use stmw/lmw for registers save/restore in asm

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Message-ID: <20180517142732.GU17342@gate.crashing.org>
Date:   Thu, 17 May 2018 09:27:32 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe LEROY <christophe.leroy@....fr>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] powerpc/32be: use stmw/lmw for registers save/restore in asm

On Thu, May 17, 2018 at 03:27:37PM +0200, Christophe LEROY wrote:
> Le 17/05/2018 à 15:15, Segher Boessenkool a écrit :
> >>I guess we've been enabling this for all 32-bit targets for ever so it
> >>must be a reasonable option.
> >
> >On 603, load multiple (and string) are one cycle slower than doing all the
> >loads separately, and store is essentially the same as separate stores.
> >On 7xx and 7xxx both loads and stores are one cycle slower as multiple
> >than as separate insns.
> 
> That's in theory when the instructions are already in the cache.
> 
> But loading several instructions into the cache takes time.

Yes, of course, that's why I wrote:

> >load/store multiple are nice for saving/storing registers.

:-)


Segher