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

[Evgenii Stepanov via llvm-commits]

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