Gentoo Archives: gentoo-dev

From: Rich Freeman <rich0@g.o>
To: gentoo-dev <gentoo-dev@l.g.o>
Subject: Re: [gentoo-dev] Re: LTO use in the tree
Date: Mon, 28 Apr 2014 23:45:21
Message-Id: CAGfcS_nHLBRW9UaB-J_p+KzjO07NfYPfk0wJYu=i970rbutmaA@mail.gmail.com
In Reply to: Re: [gentoo-dev] Re: LTO use in the tree by Andrew Savchenko
1 On Mon, Apr 28, 2014 at 5:46 PM, Andrew Savchenko <bircoph@×××××.com> wrote:
2 > Hello,
3 >
4 > On Sun, 27 Apr 2014 07:23:11 -0400 Rich Freeman wrote:
5 >> And yet, in the same paragraph you mention -O3, which is
6 >> tantamount to just setting a flag and walking away. That turns
7 >> on 14 things you probably don't really need.
8 >
9 > Why 14 things? ...
10 >
11 > From my experience only three of them are harmful:
12 ...
13 > All other -O3 option have either no effect or measurable
14 > performance enhancements in the range of several percent.
15
16 You missed my point. I think running batch optimizations like -O2/3
17 only makes sense. The argument was that -flto doesn't always help,
18 and thus shouldn't always be used. My point was that convenience
19 options like -O2/3 were used because while the options don't always
20 help, they usually do, and nobody wants to bother with micromanaging
21 them.
22
23 Personally I use -O2 or -Os with a few additional options that are
24 less space-expensive than full -O3, on the premise that cache and
25 memory conservation probably buys you more than avoiding some jumps.
26 But, short of profiling every package any selection is going to be a
27 suboptimal choice based on averages.
28
29 I wasn't trying to say that there was something wrong with -O3/2/etc.
30
31 Rich