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

[Boris Brezillon]

Hi Marek,

On Wed, 11 Oct 2017 23:34:33 +0200
Marek Vasut <marek.vasut@xxxxxxxxx> wrote:

> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>  
> 
> Don't you lose type-checking with this conversion to macros ?
> 

Yes, we loose strict type checking, but if you look at the code, you'll
see that the macros do (valN).x[i], so, if valN is not a struct or
a union containing a field named x, the compiler will complain. That
should save us from devs passing random arguments to those macros.

Anyway, this code is not seeing a lot of changes lately, so I wouldn't
be so worried by the lack of strict type-checking implied by this
transition to macros. 

Regards,

Boris