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Re: PATCH: PR other/55333: libsanitizer StackTrace::FastUnwindStack wrong x32

On Thu, Nov 15, 2012 at 9:25 AM, Dmitry Vyukov <> wrote:
> On Thu, Nov 15, 2012 at 9:19 PM, Jakub Jelinek <> 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.

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.


>> 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.

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