`-fipa-pta` drastically increases compilation time

[yshui]

I noticed this when building renderdoc. With -fipa-pta, the build lasted more than 8 hours, and I had to interrupt it. Without it, the build finished in a couple of minutes.

There are other examples, not as dramatic, usually it's single digit minutes -> half an hour-ish.

How much does -fipa-pta help performance? Is it worthwhile?

[TheMBL09898]

What -fipa-pta does: http://stackoverflow.com/questions/13066663/ddg#13076073

[yshui]

@TheMBL09898 Thanks, but I feel that doesn't answer the question I was asking.

[leg7]

Has this option been benchmarked? If the compilation times are hit so hard it better give a significant performance boost.

[TheMBL09898]

Since a typical program contains a large number of functions, with a huge number of possible call sequences, this kind of analysis can be very expensive.

Maybe you should disable the flag with package.cflags for the packages that are giving you trouble.

*FLAGS-="-fipa-pta"  media-gfx/renderdoc

But if it were me I'd just leave it on and perform the compilation overnight, as it supposedly "improves performance".

[mccarverone]

I just compiled renderdoc-1.20-r1 with the default parameters from gentooLTO in less than 2 minutes.

[yshui]

@mccarverone did you enable qt and python in the use flags?

[mccarverone]

Now I did and it actually did fail compiling at [283/288].

log.txt

[yshui]

Looks like cc1plus ate too much memory. Does it compile without -fipa-pta?

[mccarverone]

Yes it does.

So media-gfx/renderdoc *FLAGS-="${IPAPTA}" should be added to ipa-pta.conf. Would you submit a pull request?

[yshui]

@mccarverone i am more wondering if -fipa-pta should not be enabled at all. is there some representative benchmarks I could run?

[mccarverone]

Can't answer about that, sorry