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From: "xunxun1982 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/50905] New: Gcc4.6.x's -ftree-parallelize-loops is effective only when using "-O2/-O3 -ffast-math"
Date: Fri, 28 Oct 2011 19:37:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50905

             Bug #: 50905
           Summary: Gcc4.6.x's -ftree-parallelize-loops is effective only
                    when using "-O2/-O3 -ffast-math"
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: xunxun1982@gmail.com


Created attachment 25648
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25648
the test

I found that Gcc4.6.1 and Gcc4.6.2 -ftree-parallelize-loops is effective only
when using "-O2/-O3 -ffast-math" on Windows or on Ubuntu. 
Is this right?

Using my test,

$gcc -ftree-parallelize-loops=2 -c main.c
$nm main.o
                 U clock
0000000000000000 T main
                 U printf
                 U puts
                 U sin
$gcc -O2 -ftree-parallelize-loops=2 -c main.c
$nm main.o
0000000000000000 r .LC4
0000000000000008 r .LC6
                 U __printf_chk
                 U clock
0000000000000000 T main
                 U puts
                 U sin
$gcc -ffast-math -ftree-parallelize-loops=2 -c main.c
$nm main.o
                 U clock
0000000000000000 T main
                 U printf
                 U puts
                 U sin
$gcc -O2 -ftree-parallelize-loops=2 -ffast-math -c main.c
$nm main.o
0000000000000000 r .LC5
                 U GOMP_parallel_end
                 U GOMP_parallel_start
                 U __printf_chk
                 U clock
0000000000000000 T main
0000000000000000 t main._loopfn.0
                 U omp_get_num_threads
                 U omp_get_thread_num
                 U puts
                 U sin
Only -O2 -ftree-parallelize-loops=2 -ffast-math achieve my desired result.

But I think it should generate the similar symbols below when using
-ftree-parallelize-loops=2 alone:
                 U GOMP_parallel_end
                 U GOMP_parallel_start
                 U omp_get_num_threads
                 U omp_get_thread_num

Am I right?
Or the -ftree-parallelize-loops is such option that should be used with
"-O2/-O3 -ffast-math"?

Thanks.