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@vger.kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
9e343e87d2