Re: [gentoo-dev] Re: LTO use in the tree

[C. Bergström]

On 04/27/14 06:23 PM, Rich Freeman wrote:
> On Sat, Apr 26, 2014 at 10:37 PM, "C. Bergström"
> <cbergstrom@×××××××××.com> wrote:
>> #2 The only reference to anything which the compiler could impact is
>> "Use Boyer-Moore (and unroll its inner loop a few times)." Finding out which
>> flag controls that for ${CC} would have some importance. It's almost
>> certainly combined with -O3 and or some standalone loop related
>> optimization. (Nothing depending on LTO). If they were really clever or
>> determined  - there's probably a few GCC or other pragma which could give a
>> hint about unrolling.
> So, I'll certainly agree that package-specific CFLAG tuning will
> always be superior to just setting some flag at the system level and
> walking away.
>
> And yet, in the same paragraph you mention -O3, which is tantamount to
> just setting a flag and walking away.  That turns on 14 things you
> probably don't really need.
I was trying to give a simplified example... no need to nitpick my reply 
(Every compiler defines -O3 differently and even the flag to unroll 
loops and that threshold may be different.. ...)
>
> I run -flto at the system level since in my experience it only causes
> problems with a handful of packages, and when it does provide a
> benefit I get it.
Can you name a single package that you use which receives a measurable 
benefit from LTO? (Just asking)

I don't disagree about enabling it, filing bug reports or many other 
things. I'm just curious if you have any hard numbers... (You seem 
passionate and sorry if this seems like I'm putting you on the spot)

/*
Side note
IPA (aka whole program and LTO) is by far the hardest optimizations I've 
ever personally had to debug/engineer/tune in a compiler. Making it 
robust needs passionate users who file good reduced test cases. While 
for a single source you have creduce or delta - what options are there 
for automated reduction of whole program problems..
*/