Gentoo Archives: gentoo-amd64

From: DJ Cozatt <ygdrasil@×××××××.net>
To: gentoo-amd64@l.g.o
Subject: [gentoo-amd64] Re: Digest of gentoo-amd64@lists.gentoo.org issue 367 (13009-13035)
Date: Fri, 08 Jul 2011 21:03:54
Message-Id: 1310157998.2676.16.camel@random.mshome
1 On Tue, 2011-07-05 at 13:02 +0000, gentoo-amd64+help@l.g.o
2 wrote:
3 > On Thu, 30 Jun 2011 21:36:38 -0700
4 > Mark Knecht <markknecht@×××××.com> wrote:
5 >
6 > >
7 > > I think it's completely appropriate for this list. This
8 > distro expects
9 > > that we put CFLAG options in make.conf so I need to hear
10 > about this
11 > > stuff even if I don't have to background to completely
12 > understand
13 > > what's really causing the problem.
14 > >
15 >
16 > In this case, or in the case of any program where
17 > "-fno-strict-aliasing"
18 > could make a difference, the maintainer of the program would
19 > include
20 > the option in the ebuild. The user would not have to worry
21 > too much
22 > about it.
23 >
24 > But yes, it is always good to know about the compiler flags.
25 >
26 > To see exactly what compile flags are being used in your
27 > programs, here
28 > is a neat method I picked up from somewhere. Just open a
29 > terminal and
30 > enter the following command:
31 >
32 > echo 'int main(){return 0;}' > test.c && gcc -v -Q $CFLAGS
33 > test.c -o test && rm test.c test
34 >
35 > In place of $CFLAGS just substitute any option of interest.
36 > There will
37 > be a flood of output, but just scroll back a few lines to find
38 > the "options
39 > passed:" and "options enabled:" sections.
40 >
41 > For example, using "-O2" for $CFLAGS indicates that
42 > "-fstrict-aliasing" is
43 > used, but it is not used with "-O1."
44 >
45 > It also shows that with "-O2" the option "-mno-sse4" is used,
46 > and so if
47 > you want to use SSE4 for certain programs (e.g. video, audio)
48 > you will
49 > need to specifically enable it.
50 >
51 > There may be an even slicker way to reveal the flags, but this
52 > is the
53 > only way I know.
54 >
55 > Frank Peters
56 >
57
58 An example of the issue discussed
59 https://bugs.gentoo.org/show_bug.cgi?id=347818
60
61 see in particular comment #3 they are understaffed and have a ton to do
62 but in the norm don't care to here it.
63
64 https://bugs.gentoo.org/show_bug.cgi?id=339485
65
66 Is a discussion/flame about the report upstream qa messages.
67 Help me out here guys and weigh in. (dons flame suit)
68
69 Further on the conversations wandered-to in the topic 'optimization'
70 in the kernel config menu under the heading 'General Setup' lies
71 [*] Optimize trace point call sites
72
73 CONFIG_JUMP_LABEL:
74
75 If it is detected that the compiler has support for "asm goto", the
76 kernel will compile trace point locations with just a nop instruction.
77 When trace points are enabled, the nop will be converted to a jump to
78 the trace function. This technique lowers overhead and stress on the
79 branch prediction of the processor.
80
81 I've had this checked for a good deal of time and see no issues because
82 of it.
83
84 David J Cozatt
85 aka user99

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