[PATCH] D28404: IRGen: Add optnone attribute on function during O0

[Mehdi AMINI via Phabricator via cfe-commits]

mehdi_amini added a comment.

In https://reviews.llvm.org/D28404#640170, @probinson wrote:

> In https://reviews.llvm.org/D28404#640090, @mehdi_amini wrote:
>
> > In https://reviews.llvm.org/D28404#640046, @probinson wrote:
> >
> > > "I don't care" doesn't seem like much of a principle.
> >
> >
> > Long version is: "There is no use-case, no users, so I don't have much motivation to push it forward for the only sake of completeness". Does it sound enough of a principle like that?
>
>
> No.  You still need to have adequate justification for your use case, which I think you do not.


I don't follow your logic. 
IIUC, you asked about "why not supporting `O1/O2/O3`" ; how is *not supporting* these because their not useful / don't have use-case related to "supporting `O0` is useful"?

> 
> 
>>> Optnone does not equal -O0.  It is a debugging aid for the programmer, because debugging optimized code sucks.  If you have an LTO-built application and want to de-optimize parts of it to aid with debugging, then you can use the pragma, as originally intended.
>> 
>> Having to modifying the source isn't friendly. Not being able to honor -O0 during LTO is not user-friendly.
> 
> IMO, '-O0' and '-flto' are conflicting options and therefore not deserving of special support.

You're advocating for *rejecting* O0 built module at link-time? We'd still need to detect this though. Status-quo isn't acceptable.

Also, that's not practicable: what if I have an LTO static library for which I don't have the source, now if I build my own file with -O0 -flto I can't link anymore.

> In my experience, modifying source is by far simpler than hacking a build system to make a special case for compiler options for one module in an application.  (If you have a way to build Clang with everything done LTO except one module built with -O0, on Linux with ninja, I would be very curious to hear how you do that.)

Static library, separated projects, etc.
We have tons of users...

>>>   I don't think `-c -O0` should get this not-entirely-O0-like behavior.
>> 
>> What is "not-entirely"? And why do you think that?
> 
> "Not entirely" means that running the -O0 pipeline, and running an optimization pipeline but asking some subset of passes to turn themselves off, does not get you the same result.  And I think that because I'm the one who put 'optnone' upstream in the first place.  The case that particularly sticks in my memory is the register allocator, but I believe there are passes at every stage that do not turn themselves off for optnone.

That's orthogonal: you're saying we are not handling it correctly yet, I'm just moving toward *fixing* all these.


https://reviews.llvm.org/D28404