Add -fauto-profile option to Clang driver

[Diego Novillo]

Bah, forgot to actually CC Deaho.  Fixed.

On Tue, Oct 22, 2013 at 5:28 PM, Diego Novillo <dnovillo at google.com> wrote:
> On Tue, Oct 22, 2013 at 5:06 PM, Chandler Carruth <chandlerc at google.com> wrote:
>>
>> On Tue, Oct 22, 2013 at 1:58 PM, Bob Wilson <bob.wilson at apple.com> wrote:
>>>
>>> I really like GCC's -fprofile-generate and -fprofile-use, except I don't
>>> think we should reuse those names for something that works differently.  My
>>> overall preference would be something like this (using those names as
>>> placeholders):
>>>
>>> -fprofile-generate=<instrumentation-style>
>>> -fprofile-use=<profile-style>
>>>
>>> e.g., "-fprofile-use=auto".  That would at least unify the new options.
>>> In fact, we may even be able to reuse those option names with -fprofile-use
>>> being a synonym for -fprofile-use=gcc, which matches gcc's option.  I'm not
>>> at all familiar with how that option actually works in gcc, so I can't say
>>> whether that would make sense.
>>
>>
>> I don't think we can re-use '-fprofile-use' in a way different from GCC
>> here. GCC accepts it as "-fprofile-use=/path/..." and i could call my
>> profile file "auto" or "gcc" or "clang" and expect it to work.
>>
>> I think it is best for instrumentation-based profiling to use
>> '-fprofile-generate' and '-fprofile-use' just like GCC does, if with
>> different file formats, etc.
>
> Agreed.
>
>> I don't see in flags in upstream GCC regarding sample-driven profiling, but
>> "auto" I think is actively harmful in the name. There is nothing
>> intrinsically automatic about it. It is "external" in the sense that it
>> isn't from compiled-in instrumentation, but I don't see any reason for
>> "auto" to indicate that to the user.
>
> GCC (well the Google branch now) uses -fauto-profile for the
> sample-driven profiling. The external profiler actually generates a
> gcov file.  The two options -fprofile-generate and -fprofile-use are
> strictly for instrumentation based profiling.  Our internal users are
> already using -fauto-profile in their builds.
>
> Dehao (CCd) tells me that the option was initially named
> -fsample-profile, but they then renamed it to -fauto-profile.
>
>> I think for now, we should put this functionality behind a specific flag
>> whose name is indicative of the user's expected behavior. The best idea I've
>> seen is "-fsample-profile=/path/..." but I'd love to hear better
>> suggestions.
>
> I don't really have better names.  -fsample-profile and -fauto-profile
> are both the same to me.  The name -fauto-profile has the slight
> advantage that it can serve as a more general flag name, with the
> actual profiling style automatically detected by the format of the
> input file. But, I don't really care all that much.
>
> Having said that, I can also see -fprofile-use=filename being smart
> enough to know what type of profile it's getting by examining the
> signature of "filename".  This is contemplated in my LLVM patch.  The
> pass instantiates a different reader according to the type of file it
> detects (right now it does nothing of the sort, however).
>
>
> Diego.