linux-kernel - Re: [PATCH] mtd: cfi: convert inline functions to macros

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Message-ID: <b109f253-ac22-82b0-6716-6a069eb3e4f2@gmail.com>
Date:   Wed, 11 Oct 2017 23:34:33 +0200
From:   Marek Vasut <marek.vasut@...il.com>
To:     Arnd Bergmann <arnd@...db.de>,
        David Woodhouse <dwmw2@...radead.org>,
        Brian Norris <computersforpeace@...il.com>,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Richard Weinberger <richard@....at>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>
Cc:     stable@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mtd: cfi: convert inline functions to macros

On 10/11/2017 03:54 PM, Arnd Bergmann wrote:
> The map_word_() functions, dating back to linux-2.6.8, try to perform
> bitwise operations on a 'map_word' structure. This may have worked
> with compilers that were current then (gcc-3.4 or earlier), but end
> up being rather inefficient on any version I could try now (gcc-4.4 or
> higher). Specifically we hit a problem analyzed in gcc PR81715 where we
> fail to reuse the stack space for local variables.
> 
> This can be seen immediately in the stack consumption for
> cfi_staa_erase_varsize() and other functions that (with CONFIG_KASAN)
> can be up to 2200 bytes. Changing the inline functions into macros brings
> this down to 1280 bytes.  Without KASAN, the same problem exists, but
> the stack consumption is lower to start with, my patch shrinks it from
> 920 to 496 bytes on with arm-linux-gnueabi-gcc-5.4, and saves around
> 1KB in .text size for cfi_cmdset_0020.c, as it avoids copying map_word
> structures for each call to one of these helpers.
> 
> With the latest gcc-8 snapshot, the problem is fixed in upstream gcc,
> but nobody uses that yet, so we should still work around it in mainline
> kernels and probably backport the workaround to stable kernels as well.
> We had a couple of other functions that suffered from the same gcc bug,
> and all of those had a simpler workaround involving dummy variables
> in the inline function. Unfortunately that did not work here, the
> macro hack was the best I could come up with.
> 
> It would also be helpful to have someone to a little performance testing
> on the patch, to see how much it helps in terms of CPU utilitzation.
> 
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715
> Cc: stable@...r.kernel.org
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Don't you lose type-checking with this conversion to macros ?

-- 
Best regards,
Marek Vasut