Re: [PATCH] LTO: fallback to -flto=N if -flto=jobserver does not work.

["Martin Liška" <mliska@suse.cz>]

On 4/22/21 1:19 PM, Richard Biener wrote:
> On Thu, Apr 22, 2021 at 11:02 AM Martin Liška <mliska@suse.cz> wrote:
>>
>> On 4/22/21 10:04 AM, Richard Biener wrote:
>>> On Wed, Apr 21, 2021 at 3:08 PM Martin Liška <mliska@suse.cz> wrote:
>>>>
>>>> When -flto=jobserver is used and we cannot detect job server, then we can
>>>> still fallbackto -flto=N mode.
>>>>
>>>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>>>>
>>>> Ready to be installed?
>>>
>>> I think this behavior needs to be documented - it falls back to a less
>>> conservative (possibly system overloading) mode - which IMHO is
>>> non-obvious and IMHO we shouldn't do.
>>
>> Sure, I'm sending corresponding patch. Note that it's quite common mistake
>> that '+' is missing in Makefile rule. That was motivation for my change.
> 
> Sure, but that change won't get this fixed.

It will as linker command line will contain (-flto=jobserver) and LTO will fallback to -flto=N.

> IMHO we should eventually
> emit diagnostic like
> 
> warning: could not find jobserver, compiling N jobs serially
> 
> once N > 1 (or 2?).

We do that now (for all N):
lto-wrapper: warning: jobserver is not available: ‘MAKEFLAGS’ environment variable is unset


> Likewise if people just use -flto and auto-detection
> finds nothing:

-flto != -flto=auto

Yes, -flto is a serial linking and we can emit a warning.

> 
> warning: using serial compilation of N LTRANS jobs
> note: refer to http://.... for how to use parallel compile
> 
> using the URL diagnostics to point to -flto=... documentation.

What about making that a proper warning (-Wlto)? We have diagnostics infrastructure
that prints URL links.

> 
> That is, teach users rather than second-guessing and eventually
> blowing things up.  IMHO only the jobserver mode is safe to
> automatically use.

Well, -flto=auto is also fine and document. I think there is no possibility
auto CPU deduction can fail. So -flto=jobserver (with missing make job server)
and -flto (equal to -flto=1) worth emitting a warning.

What do you think?
Martin

> 
> Richard.
> 
>> Martin
>>
>>>
>>> Richard.
>>>
>>>> Thanks,
>>>> Martin
>>>>
>>>> gcc/ChangeLog:
>>>>
>>>>         * lto-wrapper.c (run_gcc): When -flto=jobserver is used, but the
>>>>         makeserver cannot be detected, then use -flto=N fallback.
>>>> ---
>>>>  gcc/lto-wrapper.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/gcc/lto-wrapper.c b/gcc/lto-wrapper.c
>>>> index 03a5922f8ea..0b626d7c811 100644
>>>> --- a/gcc/lto-wrapper.c
>>>> +++ b/gcc/lto-wrapper.c
>>>> @@ -1585,8 +1585,9 @@ run_gcc (unsigned argc, char *argv[])
>>>>        if (jobserver && jobserver_error != NULL)
>>>>         {
>>>>           warning (0, jobserver_error);
>>>> -         parallel = 0;
>>>> +         /* Fall back to auto parallelism.  */
>>>>           jobserver = 0;
>>>> +         auto_parallel = 1;
>>>>         }
>>>>        else if (!jobserver && jobserver_error == NULL)
>>>>         {
>>>> --
>>>> 2.31.1
>>>>
>>