Messages in this thread | | | From | Arnd Bergmann <> | Date | Fri, 22 Sep 2017 23:39:15 +0200 | Subject | Re: [PATCH] lib/lz4: make arrays static const, reduces object code size |
| |
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
Arnd
| |