Re: The application performance built from gcc 7.1 is very worse compared to gcc 6.3

[Xi Ruoyao <ryxi at stu dot xidian dot edu dot cn>]

On 2017-06-01 15:59 +0800, Nan Xiao wrote:
> Hi all,
> 
> Greetings from me!
> 
> My OS is ArchLinux, and on this Monday, I upgrade gcc from 6.3.1 to
> 7.1.1 (both packages are released by ArchLinux). But after rebuilding
> my program, I find the performance downgrades drastically.
> 
> For single-thread version, the program which is built with "gcc 6.3.1"
> would use ~85 seconds to process a transaction; while with "7.1.1", it
> grows to ~115 seconds.
> For multiple-thread version using OpenMP, the  program which is built
> with "gcc 6.3.1" would use  only ~11 seconds to process a transaction;
> while with "7.1.1", it grows to ~50 seconds.
> 
> The compile and link flags are " -g -O2 -fopenmp -fprofile-arcs
> -ftest-coverage -fPIC   -std=gnu++11 ".
> 
> I also download and build gcc 7.1 myself from
> https://gcc.gnu.org/gcc-7/, the single-thread program can use ~95
> seconds, while OpenMP version still consumes ~50 seconds. So compared
> to program built using gcc 6.3, the performance is very worse.
> 
> I have checked the release note of gcc
> (https://gcc.gnu.org/gcc-7/changes.html), but can't find some valuable
> information.
> 
> Could anyone give some comments? Thanks very much in advance!
> 
> Best Regards
> Nan Xiao

Sadly, GCC has some annoying performance regressions.  But I think at
first you could change your compile flags:

(1) "-fprofile-arcs" slows down your program since it add code into
    your program to record branch probabilities.  If you want to optimize,
    use "-fbranch-probabilities".  (See the man page of GCC.)
(2) "-O2 -ftest-coverage" may produce strange coverage result.
    If you want to test coverage with gcov, it's better to disable all
    optimization (with -O0).

Which package are you building?  If it's open source, please send a link
here and maybe someone can reduce it for a test case.  Then the developers
of GCC may fix the regression.  But if possible I suggest you to create
a test case (simple enough) by yourself and report it to
<http://gcc.gnu.org/bugzilla>.

And please tell us your architecture (i686 or x86_64).
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University