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

[Rich Freeman]

On Sun, Apr 27, 2014 at 7:41 AM, "C. Bergström"
<cbergstrom@×××××××××.com> wrote:
> On 04/27/14 06:23 PM, Rich Freeman wrote:
>> 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.. ...)

Sorry if it came across aggressively.  I was just pointing out that
the reason one sets CFLAGs generically is to avoid the trouble of
"optimizing the optimizer."  This always comes at a cost - I tend to
use -Os, but no doubt some packages would benefit from a different
global optimization, let alone specific optimizations.

That was just the point I wanted to make about LTO - I think it is of
general usefulness since it has the potential to help, and rarely
hurts.  The only problem with it is that the implementation is
immature.

>
> Can you name a single package that you use which receives a measurable
> benefit from LTO? (Just asking)

Alas, I cannot.  There are some general benchmarks out there, and they
seem to vary from little to no effect to significant.  More
CPU-intensive software seems the most likely to benefit.  No doubt the
benefits of LTO will improve as it matures.

Rich