Em Thu, 30 Nov 2017 15:06:15 +0100 Arnd Bergmann <arnd@xxxxxxxx> escreveu: > 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 Yeah, I saw the old versions and I'm following this thread. > > 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. My main concern here is to avoid someone to undo the changes. Adding a quick note on each of those changes is helpful, in order to warn people and refrain undoing. So, adding a quick comment works for me. Regards, Mauro
