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

[Peter Collingbourne via llvm-commits]

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
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