when is the .da file created!

[smartgong]

hi, every expert!
    I will come to ask another silly quesion:).
I have watched the source files of GCC for some times.
But I donot know which file create the file(.da), which is Gcov data files.
   I know the other two gcov data files (.bb, .bbg) could be created by profile.c. but as for .da ???
    There are some files that write and read the files. But where and when are these files created?/
   
   strange,!! 
 
    thanks for your kind help !
                                    yours smartgong

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