[Bug c/54896] New: Some optimization slowness with GCC 4.7.2

[tammy at Cadence dot COM]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54896



             Bug #: 54896

           Summary: Some optimization slowness with GCC 4.7.2

    Classification: Unclassified

           Product: gcc

           Version: 4.7.2

            Status: UNCONFIRMED

          Severity: normal

          Priority: P3

         Component: c

        AssignedTo: unassig...@gcc.gnu.org

        ReportedBy: ta...@cadence.com





We have some code which consist from switch with many cases and non-random

memory writes. Look like there is nothing to optimize at all, but compiling

optimized version takes long time.



Attach is the code to generate source code to reproduce this problem,

generate_bigdata.c.





$ gcc generate_bigdata.c  

$ ./a.out 

  (This step will generate the testcase, BigData.c.)

$ time /gcc-4.7.2/bin/gcc -m32 BigData.c -c -o BigData.o

1.504u 0.077s 0:01.61 97.5%     0+0k 320+10024io 0pf+0w

$ time /gcc-4.7.2/bin/gcc -m32 -O1 BigData.c -c -o BigData.o

50.438u 0.191s 0:50.67 99.9%    0+0k 256+10024io 0pf+0w

$ time /gcc-4.7.2/bin/gcc -m32 -O4 BigData.c -c -o BigData.o

111.710u 3.977s 1:55.73 99.9%   0+0k 1024+9384io 0pf+0w



We tried with gcc44, the optimization takes long also but not as bad as 472.



Sure, code refactoring is best solution and disabling optimization for

particular file is good enough, but will be good idea to improve GCC too.