[Wheel-builders] manylinux compile/optimization options

[Vlad K.]

On 2017-12-06 19:29, Marcel Martin wrote:
> 
> Why do you think so? As far as I know, using -O3 is perfectly normal 
> and
> safe!

Because experience has shown that blanket -O3 can lead to undefined 
behavior or "optimizations" that actually result with worse performing 
code. I have certainly seen broken code with global -O3, and it is 
illadvised on Gentoo as a global option. Instead individual packages 
enable -O3 if it's known that the flag will produce adequate and faster 
code.

Also Phoronix has some tests that show -O3 better in some cases, but 
worse in others, for example:

https://www.phoronix.com/scan.php?page=article&item=clear-gcc6-opts&num=2

and previously

https://www.phoronix.com/scan.php?page=article&item=gcc_47_optimizations&num=1



Both on FreeBSD and Gentoo pip will compile without any -O flags, even 
though Python (on both) is compiled with -O2. However on FreeBSD the 
compiler is clang, and talking to some developers I was informed that 
-O3 is not a solution, but using LLVM's LTO is. LTO is also a subject in 
some experimental Gentoo builds.

There may be individual python ports that explicitly enable -O3, but I 
haven't checked for those. With that, I wanted to understand this 
behavior for Pillow so I can submit a change requesting Pillow be 
compiled with -O3 for now, until LTO is understood better (and 
available, I think it requires LLVM newer than the one in FreeBSD base), 
because from what I've seen the difference is seriously significant, and 
so far I see no breakage, running Pillow compiled with -O3 in 
production.




-- 
Vlad K.