[llvm] r301586 - [asan] Put ctor/dtor in comdat.

[Peter Collingbourne via llvm-commits]

(I mean the gold bug)

On Thu, May 4, 2017 at 12:15 PM, Peter Collingbourne <peter at pcc.me.uk>
wrote:

> Can we set the nounwind attribute on the constructor to work around the
> clang bug?
>
> Peter
>
> On Thu, May 4, 2017 at 12:11 PM, Evgenii Stepanov via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> I agree, I'll add a clang flag for this which will be disabled by default.
>>
>> On Thu, May 4, 2017 at 2:42 AM, Chandler Carruth <chandlerc at gmail.com>
>> wrote:
>> > On Thu, Apr 27, 2017 at 1:40 PM Evgeniy Stepanov via llvm-commits
>> > <llvm-commits at lists.llvm.org> wrote:
>> >>
>> >> Author: eugenis
>> >> Date: Thu Apr 27 15:27:23 2017
>> >> New Revision: 301586
>> >>
>> >> URL: http://llvm.org/viewvc/llvm-project?rev=301586&view=rev
>> >> Log:
>> >> [asan] Put ctor/dtor in comdat.
>> >>
>> >> When possible, put ASan ctor/dtor in comdat.
>> >>
>> >> The only reason not to is global registration, which can be
>> >> TU-specific. This is not the case when there are no instrumented
>> >> globals. This is also limited to ELF targets, because MachO does
>> >> not have comdat, and COFF linkers may GC comdat constructors.
>> >>
>> >> The benefit of this is a lot less __asan_init() calls: one per DSO
>> >> instead of one per TU. It's also necessary for the upcoming
>> >> gc-sections-for-globals change on Linux, where multiple references to
>> >> section start symbols trigger quadratic behaviour in gold linker.
>> >>
>> >> This is a second re-land of r298756. This time with a flag to disable
>> >> the whole thing to avoid a bug in the gold linker:
>> >>   https://sourceware.org/bugzilla/show_bug.cgi?id=19002
>> >
>> >
>> > Evgeniy, I strongly disagree with the default here.
>> >
>> > This bug in gold is likely to be encountered by anyone using a Linux
>> distro
>> > with binutils before v2.27 and using gold. Given how slow the BFD
>> linker is
>> > compared to gold, that's likely a *lot* of users of LLVM *if* their
>> version
>> > of binutils in that old.
>> >
>> > Unfortunately, both LTS versions of Ubuntu (and every version including
>> > Xenial and older) have too old of a version of gold. Debian stable has
>> too
>> > old of a version. Fedora 25 and older, OpenSuSE 42 and earlier, all
>> have too
>> > old of versions of gold.
>> >
>> > I think that this feature needs to be defaulted off. If you want to
>> detect
>> > LLD (which is likely to be installed *with* LLVM at least) or detect
>> > versions to enable this, great. But I think defaulting this on is
>> really the
>> > wrong thing for our users. We're essentially leaving a usability
>> landmine in
>> > ASan, with no advertisement, and just letting users figure out why and
>> how
>> > to update their linker. That doesn't seem right.
>> >
>> > Anyways, that's my 2 cents.
>> > -Chandler
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>>
>
>
>
> --
> --
> Peter
>



-- 
-- 
Peter
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