RFR: JDK-8246751 Mac OS build settings should use -O3

[Magnus Ihse Bursie]

On 2020-06-09 08:59, David Holmes wrote:
> On 9/06/2020 8:26 am, Claes Redestad wrote:
>> On 2020-06-09 00:12, Sergey Bylokhov wrote:
>>> On 6/8/20 3:06 pm, Claes Redestad wrote:
>>>> Right, -O3 is now likely the default for most files.
>>>>
>>>> I'm not sure you suggest the reported ~2% difference between -Os and
>>>> -O2/-O3 as an argument against changing to -O3 by default. It might be
>>>> statistically significant - or just noise - but we should at any rate
>>>> be wary of overfitting to a particular (micro-)benchmark.
>>>
>>> I guess that should be the opposite way round. Some good numbers 
>>> other than
>>> noise should be used as an argument of changing -0s to -03 by default.
>>
>> I think if the differences are all in the noise then that could just as
>> well be used as an argument for _harmonizing_ the build optimization
>> strategy with the other platforms. While not easy to quantify,
>> harmonizing theoretically reduces risk of obscure platform specific
>> issues, since, while different, different platforms will use similar
>> optimizations schemes at -O3.
>
> On the third hand "if ain't broke ..."
>
> Seems to me that any change to build flags should be backed up by 
> a-priori evidence that it is a worthwhile change.
We inherited a build system with a lot of randomness, arbitrarily set 
values, and copy/pasted code that had diverged over the year. For MacOS 
especially, we inherited the Apple port, which was done with little or 
no consideration to the JDK as a whole, but more as a hack to get Java 
running on MacOS.

Our approach was to mimick the old build system "bug by bug" (as our 
slogan were at the time), and then, when the new framework were in 
place, tidy up the code, fix the bugs, and harmonize the components over 
time.

The odd choice of optimization target for macOS is part of this legacy. 
The method of harmonizing it with all the other platforms is part of our 
strategy.

If -Os really is a superior optimization flag, it sure is for more 
platforms than macOS; at least all compilations using clang. That might 
be the case, but if so, we should change it for all platforms, not just 
one. As things stand right now, it's macOS that is the odd man out. And 
if -Os and -O3 is rougly the same, we should move macOS to where the 
rest of the platforms are, not the other way around.

/Magnus



>
> David
> -----
>
>>>
>>>>
>>>> It'd also be good to re-run with the new default: when most 
>>>> compilation
>>>> units change to -O3, which optimization level is ideal for an 
>>>> individual
>>>> compilation unit (such as unsafe.cpp) might very well change, since
>>>> what optimizations are available through the build chain can shift
>>>> considerably.
>>>
>>> Hope we will get some good improvements, otherwise it is better to 
>>> use -0s.
>>>
>>
>> I hope so too!
>>
>> /Claes