[PATCH] Properly detect working jobserver in gcc driver.

[Martin Liška]

On 8/2/19 11:15 AM, Jan Hubicka wrote:
>> On Fri, Aug 2, 2019 at 10:50 AM Jakub Jelinek <jakub@redhat.com> wrote:
>>>
>>> On Fri, Aug 02, 2019 at 10:47:10AM +0200, Martin Liška wrote:
>>>>> Can you strace if other fds are opened and not closed in the spot you had it
>>>>> before?  Advantage of doing it there is that it will not be done for all the
>>>>> -E/-S/-c compilations when the linker is not spawned.
>>>>
>>>> I've used the same trick which you used and I'm attaching the output.
>>>> I believe it's fine, I can't see any opened fd by GCC.
>>>
>>> LGTM.
>>

Hi.

>> Btw, we discussed yesterday on the phone and the conclusion was to
>> make -flto auto-detect a job-server (but not fall back to # of threads)
>> and add -flto=auto to auto-detect a job-server and fall back to # of threads.
>> That basically makes -flto=jobserver the default behavior which means
>> we should document -flto=1 as a way to override jobserver detection.
> 
> And concerning to the yesterday discussion, my preference would still be
> -flto to first try presence of jobserver and default to number of
> threads otherwise. It seems like user friendly default to me and other
> tools with reasonable parallelism support usually behaves this way, too.

I also like the default as Honza defined.

> 
> Sure one can use -flto=auto everywhere but then one needs to use
> different options for differnt compilers.  It would be nice to make
> -flto to do right hting for majority of users including those like me
> who cut&paste command lines from Make output and execute them by hand.

Note that our ambition is to ideally backport the patches to our gcc9 package.
The auto-detection of job-server with -flto is a behavior change that will happen
in the gcc9 package.

To be honest I don't like the invention of 'auto' value for -flto command.
That would be useful just for gcc9 right now :/

Martin

> 
> The fork bombing is IMO relatively rare and it was not what Jakub ran
> into (it was broken jobserv detection). 
> After all whole distro built with Martin's orriginal approach of
> -flto=<nthreads> which forkbombs on every possible occasion. 
> 
> Said that, I could live with more conservative defaults.
> Honza
>