[llvm-dev] Redefining optnone to help LTO

[Mehdi Amini via llvm-dev]

Ultimately pushing a much stuff a possible to function attributes makes it the most natural. Think -ffast-math, -Os, -Oz, or subtargets attributes. Handling all this stuff at the function level has been proven the easiest to get straightfoward during LTO, and more flexible for “advanced” use-case when mixing modes in a single module.

The reasons to not address O1 vs O2 vs O3 is IMO that 1) it is complicated and 2) there isn’t much use-case for this, or I didn’t hear about anyone really caring a lot about it. So pragmatically, it makes sense to me to try to address first the low-hanging fruits and to solve what is valuable (I can’t build our kernel with ThinLTO because we don’t encode -ffreestanding correctly, etc.).

Now, if someone has a great idea on how to handle O1/O2/O3 consistently in a way that includes LTO, I’m willing to help on the implementation side!

— 
Mehdi



> On Jan 17, 2017, at 5:39 PM, Eric Christopher <echristo at gmail.com> wrote:
> 
> Honestly instead of optnone I'd prefer to do something else around storing optimization levels, but I think the connotations of that in LTO is going to be painful:
> 
> 1) What does it mean to merge two modules of different optimization levels?
> 2) What does it mean to inline two functions of different optimization levels?
> 3) What code generator should be used?
> 
> Etc.
> 
> Honestly I think this is a lot of stuff we don't have any good ideas for and that the current LTO scope doesn't really have. If there's really a need for it though...
> 
> (Also, if we don't want to store the actual optimization levels then replace "different optimization levels" above with "optnone and not-optnone" :)
> 
> -eric
> 
> 
> On Wed, Jan 11, 2017 at 8:34 AM Robinson, Paul via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> In D28404, Mehdi wanted to use the 'optnone' attribute as a way to record
> "I was compiled with -O0" in the IR, because it seems like a good idea to
> remember that fact in an LTO compilation and there is no way to remember
> that fact currently.  A couple of people felt it might be better to have
> this idea discussed on the dev list, where it might get better exposure,
> so I'm volunteering to get that discussion started.
> 
> While 'optnone' does cause lots of optimizations to bypass a function,
> exactly matching -O0 was not the motivation and never a hard requirement.
> The implementation makes a distinct effort to get close to the behavior
> of -O0, but it's not an exact match and for the intended purpose (allowing
> a given function to be un-optimized to help debugging) it worked fine.
> 
> Using 'optnone' to convey -O0 to LTO is something of a redefinition, or
> at least a re-purposing, of the attribute.  To get there from here, I
> think we would need a couple of things to happen, separately from the
> minor grunt work of adding 'optnone' to function IR at -O0.
> 
> 1) Update the LangRef definition of 'optnone' to reflect this intent.
> The current definition doesn't provide a motivation, and the description
> is (deliberately) a bit vague.  If we want 'optnone' to intentionally
> match -O0, that should be tightened up.
> 
> 2) Make a concerted effort to teach 'optnone' to targets.  Currently
> I know the X86 target is aware of it, but I'm not so sure about others.
> 
> 3) Take another look at what 'optnone' currently does *not* turn off,
> and see if there is something we can do about that.  In some cases this
> will not be practical, and we may just have to live with that.
> 
> (Okay, we need 3 things to happen.)
> 
> I won't say this is blocking Mehdi's work, but it would remove a
> point of contention and allow the review to proceed more smoothly.
> --paulr
> 
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