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 ? -- Best regards, Marek Vasut