On Thu, Nov 30, 2017 at 1:49 PM, Mauro Carvalho Chehab <mchehab@xxxxxxxxxx> wrote: >> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 >> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> >> --- >> I'm undecided here whether there should be a comment pointing >> to PR81715 for each file that the bogus local variable workaround >> to prevent it from being cleaned up again. It's probably not >> necessary since anything that causes actual problems would also >> trigger a build warning. > > This kind of sucks, and it is completely unexpected... why val is > so special that it would require this kind of hack? It's explained in the gcc bug report: basically gcc always skipped one optimization on inline function arguments that it does on normal variables. Without KASAN and asan-stack, we didn't notice because the impact was fairly small, but I ended up finally getting to the bottom of it in September, and it finally got fixed. I had an older version of the patch that was much more invasive before we understood what exactly is happening, see https://lkml.org/lkml/2017/3/2/484 > Also, there's always a risk of someone see it and decide to > simplify the code, returning it to the previous state. > > So, if we're willing to do something like that, IMHO, we should have > some macro that would document it, and fall back to the direct > code if the compiler is not gcc 5, 6 or 7. Older compilers are also affected and will produce better code with my change, the difference is just smaller without asan-stack (added ion gcc-5) is disabled, since that increases the stack space used by each variable to (IIRC) 32 bytes. The fixed gcc-8 produces identical code with and without my change. I don't think that a macro would help here at all, but if you prefer, I could add a link to that gcc bug in each function that has the problem. Arnd
