From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joe Perches Date: Sat, 23 Sep 2017 01:33:30 +0000 Subject: Re: [PATCH] lib/lz4: make arrays static const, reduces object code size Message-Id: <1506130410.12311.50.camel@perches.com> List-Id: References: <20170921221939.20820-1-colin.king@canonical.com> <5aab65d9-6da6-c770-b5aa-9edbdda31dec@wanadoo.fr> <250fe35e-77d3-a9c0-5613-ce1c65f4cb7f@canonical.com> <1506100881.12311.41.camel@perches.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Arnd Bergmann Cc: Colin Ian King , Christophe JAILLET , Sven Schmidt <4sschmid@informatik.uni-hamburg.de>, Andrew Morton , kernel-janitors@vger.kernel.org, Linux Kernel Mailing List On Fri, 2017-09-22 at 23:39 +0200, Arnd Bergmann wrote: > On Fri, Sep 22, 2017 at 9:17 PM, Arnd Bergmann wrote: > > On Fri, Sep 22, 2017 at 7:21 PM, Joe Perches 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