On 4/22/21 1:19 PM, Richard Biener wrote:
> On Thu, Apr 22, 2021 at 11:02 AM Martin Liška <mli...@suse.cz> wrote:
>>
>> On 4/22/21 10:04 AM, Richard Biener wrote:
>>> On Wed, Apr 21, 2021 at 3:08 PM Martin Liška <mli...@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
>>>>
>>
