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

From	Joe Perches <joe@perches.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] lib/lz4: make arrays static const, reduces object code size
Date	2017-09-23 03:40 +0200
Message-ID	<usHAd-55o-3@gated-at.bofh.it> (permalink)
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On Fri, 2017-09-22 at 23:39 +0200, Arnd Bergmann wrote:
> On Fri, Sep 22, 2017 at 9:17 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> > On Fri, Sep 22, 2017 at 7:21 PM, Joe Perches <joe@perches.com> wrote:
> > > On Fri, 2017-09-22 at 09:48 +0200, Arnd Bergmann wrote:
> > > > On Fri, Sep 22, 2017 at 1:11 AM, Colin Ian King
> > > >    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};
> > >         ...
> > > }
> > 
> > It depends. With a 10KB different in .text size, my guess is that this
> > is a case where gcc does the right optimization in principle, but
> > fails to do what was intended in some corner cases.
> 
> I found the problem: "gcc -fsanitze=kernel-address --param asan-stack=1"
> produces lots of expensive checks here with gcc-5 or higher.
> 
> Disabling it makes a big difference:
> 
> upstream:
> gcc-7.0.0 --fsanitze=kernel-address --param asan-stack=1: 31789 bytes
> gcc-7.0.0: 16535 bytes
> 
> patched:
> gcc-7.0.0 --fsanitze=kernel-address --param asan-stack=1: 20351 bytes
> gcc-7.0.0: 14490 bytes

I think you are looking at a different issue.

There seems still a difference in size between
current and Colin's patch in compiled object size.



>       Arnd

Thread

[PATCH] lib/lz4: make arrays static const, reduces object code size Colin King <colin.king@canonical.com> - 2017-09-22 00:20 +0200
  Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Christophe JAILLET <christophe.jaillet@wanadoo.fr> - 2017-09-22 01:10 +0200
    Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Colin Ian King <colin.king@canonical.com> - 2017-09-22 01:20 +0200
      Re: [PATCH] lib/lz4: make arrays static const, reduces object code size Arnd Bergmann <arnd@arndb.de> - 2017-09-22 09:50 +0200
        Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Joe Perches <joe@perches.com> - 2017-09-22 19:30 +0200
          Re: [PATCH] lib/lz4: make arrays static const, reduces object code size Arnd Bergmann <arnd@arndb.de> - 2017-09-22 21:20 +0200
            Re: [PATCH] lib/lz4: make arrays static const, reduces object code size Arnd Bergmann <arnd@arndb.de> - 2017-09-22 21:40 +0200
            Re: [PATCH] lib/lz4: make arrays static const, reduces object code size Arnd Bergmann <arnd@arndb.de> - 2017-09-22 23:40 +0200
              Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Colin Ian King <colin.king@canonical.com> - 2017-09-22 23:50 +0200
              Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Joe Perches <joe@perches.com> - 2017-09-23 03:40 +0200
            Re: [PATCH] lib/lz4: make arrays static const, reduces object code  size Joe Perches <joe@perches.com> - 2017-09-23 03:40 +0200

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