Add -fauto-profile option to Clang driver

[Diego Novillo]

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.