[llvm-dev] [RFC] Turn the MachineOutliner on by default in AArch64 under -Oz

[Friedman, Eli via llvm-dev]

On 4/23/2018 1:41 PM, Jessica Paquette wrote:
> Hi Eli,
>
>> I just tried some tests, and I'm seeing a bunch of failures on SPEC 
>> at -O3; looks like mostly crashes at runtime.   I can try to reduce a 
>> testcase if you need it.
> If you could do that, that would be great. Our testing has been 
> primarily for -Oz and -O2, so I haven’t looked at -O3 at all.

Okay, I'll try to come up with something soon.

>
>> I don't think this is really the right approach.  With LTO, you can 
>> have a mix of functions, some of which are minsize, and some of which 
>> are not.  Or with profile info, we might want to outline only cold 
>> code (I guess this isn't implemented yet, but potentially future 
>> work).  Tying whether we run the outliner to a command-line flag 
>> restricts the possible uses; either the entire module gets outlining, 
>> or none of it does.
> I’m worried that walking the entire list of functions in the module 
> when nothing has the minsize attribute would incur unnecessary 
> compile-time overhead. If that’s a reasonable thing to do though, I’m 
> fine with that approach. It’d be a less invasive change, and would 
> give us the desired LTO behaviour for free.

Walking the list of functions is very cheap, relatively speaking; I'm 
not concerned about the cost of that.  The cost I'd be concerned about 
is the cost of running a ModulePass at that point in the pipeline; IIRC 
the last time someone tried it, there were bug reports about memory 
usage (see https://bugs.llvm.org/show_bug.cgi?id=36123 .)

-Eli

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