Re: [Libunwind-devel] unw_backtrace(): Performance slowdown for ARM?

[Chris January]

Hello Luke,

On 18/01/18 07:53, Luke Diamand wrote:
> On 17 January 2018 at 15:55, Dave Watson <address@hidden> wrote:
> > > > I didn't notice anything in particular looking at the commit logs.
> > > > I'll try and repro when I get a chance
> > > arm_find_proc_info() seems to start out trying eh_frame tables. Isn't
> > > it always best on ARM to start looking in the exidx tables?
> > Sounds reasonable to me, although I'm not an arm expert.  Did the
> > default change somewhere?  I couldn't find anything looking at the
> > history.
> I'm definitely not an expert!
>
> But the exidx tables are quite a bit smaller than the DWARF tables: on
> the code I'm working on, I see ~2MB on the MIPS target vs ~0.5MB on
> the ARM target for the unwind tables. And I don't think there's even
> an option in gcc to choose anything else.
>
> And this presentation by Linaro (who _are_ experts!) says "Unwinding
> on ARM is different - no .eh_frame but .ARM.exidx and .ARM.extab".
>
> https://wiki.linaro.org/KenWerner/Sandbox/libunwind?action=AttachFile&do=get&target=libunwind-LDS.pdf
Whether the compiler generates a .eh_frame section or an .ARM.exidx
section depends on the toolchain so a generic unwinder such as libunwind
needs to support both.

On aarch32 ARM libunwind also defaults to --enable-debug-frame=yes. This
can cause libunwind to open and read the executable and any shared
libraries every unwind if the .eh_frame section is missing or does not
cover all addresses, which matches your comment that it is opening and
closing files a lot. If you know you have .ARM.exidx sections then try
configuring libunwind with --enable-debug-frame=no.

Yours,
Chris January

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