On Thu, Nov 15, 2012 at 9:25 AM, Dmitry Vyukov <dvyu...@google.com> wrote: > On Thu, Nov 15, 2012 at 9:19 PM, Jakub Jelinek <ja...@redhat.com> wrote: >> On Thu, Nov 15, 2012 at 09:05:13AM -0800, Konstantin Serebryany wrote: >>> +dvyukov, +glider, +samsonov >>> >>> Sorry I am lagging behind e-mail, but I am sure Dmitry, Alexander or >>> Alexey may submit the patch upstream. >>> Please make sure to comment the reason for using a separate typedef. >>> >>> We need our custom unwinder based on frame pointers to remain the >>> default choice on x86[_64] because this is a hotspot >>> and replacing it with any library call (especially if that call does >>> not use frame pointers but instead uses debug info) will slow down >>> the tool significantly. >>> The asan docs explicitly say that you need -fno-omit-frame-pointers to >>> get reasonable bug reports. >> >> The backtrace at asan crash time definitely isn't a hotspot, so that IMHO is >> a place which definitely should use a proper and accurrate library >> unwinding. > > Asan collects stack traces on every malloc/free call.
Right. A simple exercise: run 400.perlbench or 483.xalancbmk with asan. If the stack traces in malloc/free are enabled (which is the default), they consume 10-30% of time (don't remember the exact numbers). If we replace fp-based unwinder with something else, this will become 80%. Most programs we care about (e.g. Chrome or Firefox) behave very similar to 483.xalancbmk, i.e. they are very malloc intensive and have deep calls stacks. --kcc > > >> Frame pointers aren't emitted by default on either x86_64 or >> i?86, system libraries likely won't have them, unless people rebuild >> everything with -fsanitize=address -fno-omit-frame-pointer (unlikely to >> happen) etc. So relying on frame pointers is definitely very risky. > > If a user has asan build where he adds -fsanitize=address, it seems > trivial to add -fno-omit-frame-pointer as well. > >> Even >> libasan itself is right now built with, even explicit, -fomit-frame-pointer. > > That's the right thing. Asan does not unwind itself and performance critical.
