On 4/22/21 2:47 PM, Richard Biener wrote:
> On Thu, Apr 22, 2021 at 2:21 PM Martin Liška <mliska@suse.cz> wrote:
>>
>> 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.
> 
> I'd avoid warning if there's just a single ltrans unit.

That's doable and I've just done that in the attached patch.
Two disadvantages:
- one needs waiting for the warning after WPA
- source change (>1 LTRANS) can trigger the 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.
> 
> Note that drivers like lto-wrapper do not have fully initialized diagnostic
> machinery and use a "different" set of overloads (likewise gen* programs).

I managed printing the warning:

lto-wrapper: warning: jobserver is not available: ‘MAKEFLAGS’ environment variable is unset

lto-wrapper: note: see the ‘-flto’ option documentation for more information


and

lto-wrapper: warning: using serial compilation of 128 LTRANS jobs

lto-wrapper: note: see the ‘-flto’ option documentation for more information


> 
>>>
>>> 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?
> 
> Yes, that sounds reasonable.  I suspect that people might want to see
> -flto default to -flto=auto but then I don't think that's good because there's
> no system wide job scheduler limiting things (systemd-jobserver anyone?)

Done that.

Thoughts?
Martin

> 
> Richard.
> 
>> 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
>>>>>>
>>>>
>>