Re: [PATCH] lib/lz4: make arrays static const, reduces object code size

Joe Perches
2017-09-22T17:21:21Z

On Fri, 2017-09-22 at 09:48 +0200, Arnd Bergmann wrote:
> On Fri, Sep 22, 2017 at 1:11 AM, Colin Ian King
> <colin.king@canonical.com> wrote:
> > On 22/09/17 00:09, Christophe JAILLET wrote:
> > > Le 22/09/2017 à 00:19, Colin King a écrit :
> > > > From: Colin Ian King <colin.king@canonical.com>
> > > >
> > > > Don't populate the read-only arrays dec32table and dec64table on the
> > > > stack, instead make them both static const. Makes the object code
> > > > smaller by over 10K bytes:
> > >
> > > 10k? Wouaouh! This is way much more than what you usually win with such
> > > patches.
> >
> > Yes, I had to triple check it because it was an unbelievable win.
> >
>
> I wonder whether this should be reported as a gcc bug. I tried reproducing
> it here with gcc-7.1.1 and gcc-8.0.0, but I only see a 4K difference:
>
> text data bss dec hex filename
> 18220 176 0 18396 47dc build/tmp/lib/lz4/lz4_decompress-after.o
> 22297 0 0 22297 5719 build/tmp/lib/lz4/lz4_decompress-before.o

Perhaps not so much a gcc bug as an opportunity
for gcc to add an additional optimization.

gcc would have to verify that the const array is
not initialized with some variable or argument like:

int foo(int a)
{
const int array[] = {1, a};
...
}

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