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From: "asharif at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug gcov-profile/53547] New: Changing the source file between -fprofile-generate and -fprofile-use can lead to performance degradation
Date: Fri, 01 Jun 2012 00:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53547

             Bug #: 53547
           Summary: Changing the source file between -fprofile-generate
                    and -fprofile-use can lead to performance degradation
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: gcov-profile
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: asharif@gcc.gnu.org


Created attachment 27536
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27536
Test case.

Please take a look at the attached test case.

gcc -O2 -fprofile-generate test.c
rm -rf test.gcda && ./a.out
gcc -O2 -fprofile-use -DSTALE test.c

Note that with -DSTALE, a function is added which changes the function id
ordering within the module. With this re-ordering, the profile data no longer
matches up with the updated source file.

What is worse is that the performance actually *drops* below that of regular
-O2 (without -fprofile-use). This is because the summary information (sum_max,
etc.) is still valid while the edge count is considered to be 0 for all edges.
This leads to pessimistic inlining decisions.

Here is a patch that fixes solves the performance loss:
http://codereview.appspot.com/5989046/