Profiling tools help you analyze your code's performance. Using a profiler such as gcov or gprof, you can find out some basic performance statistics, such as:
Once you know these things about how your code works when compiled, you can look at each module to see which modules should be optimized. gcov helps you determine where to work on optimization.
Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.
You should compile your code without optimization if you plan to use gcov because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look for `hot spots' where the code is using a great deal of computer time. Likewise, because gcov accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less helpful---they only report on the line where the macro call appears. If your complex macros behave like functions, you can replace them with inline functions to solve this problem.
gcov creates a logfile called sourcefile.gcov which indicates how many times each line of a source file sourcefile.c has executed. You can use these logfiles along with gprof to aid in fine-tuning the performance of your programs. gprof gives timing information you can use along with the information you get from gcov.
gcov works only on code compiled with GCC. It is not compatible with any other profiling or test coverage mechanism.
The format of the intermediate .gcov file is plain text with one entry per line
version:<gcc_version>
file:<source_file_name>
function:<start_line_number>,<end_line_number>,<execution_count>,<function_name>
lcount:<line number>,<execution_count>,<has_unexecuted_block>
branch:<line_number>,<branch_coverage_type>
Where the <branch_coverage_type> is
notexec (Branch not executed)
taken (Branch executed and taken)
nottaken (Branch executed, but not taken)
There can be multiple file entries in an intermediate gcov file. All entries following a file pertain to that source file until the next file entry. If there are multiple functions that start on a single line, then corresponding lcount is repeated multiple times.
Here is a sample when -i is used in conjunction with -b option:
version: 8.1.0 20180103
file:tmp.cpp
function:7,7,0,_ZN3FooIcEC2Ev
function:7,7,1,_ZN3FooIiEC2Ev
function:8,8,0,_ZN3FooIcE3incEv
function:8,8,2,_ZN3FooIiE3incEv
function:18,37,1,main
lcount:7,0,1
lcount:7,1,0
lcount:8,0,1
lcount:8,2,0
lcount:18,1,0
lcount:21,1,0
branch:21,taken
branch:21,nottaken
lcount:23,1,0
branch:23,taken
branch:23,nottaken
lcount:24,1,0
branch:24,taken
branch:24,nottaken
lcount:25,1,0
lcount:27,11,0
branch:27,taken
branch:27,taken
lcount:28,10,0
lcount:30,1,1
branch:30,nottaken
branch:30,taken
lcount:32,1,0
branch:32,nottaken
branch:32,taken
lcount:33,0,1
branch:33,notexec
branch:33,notexec
lcount:35,1,0
branch:35,taken
branch:35,nottaken
lcount:36,1,0
gcov should be run with the current directory the same as that when you invoked the compiler. Otherwise it will not be able to locate the source files. gcov produces files called mangledname.gcov in the current directory. These contain the coverage information of the source file they correspond to. One .gcov file is produced for each source (or header) file containing code, which was compiled to produce the data files. The mangledname part of the output file name is usually simply the source file name, but can be something more complicated if the -l or -p options are given. Refer to those options for details.
If you invoke gcov with multiple input files, the contributions from each input file are summed. Typically you would invoke it with the same list of files as the final link of your executable.
The .gcov files contain the : separated fields along with program source code. The format is
<execution_count>:<line_number>:<source line text>
Additional block information may succeed each line, when requested by command line option. The execution_count is - for lines containing no code. Unexecuted lines are marked ##### or =====, depending on whether they are reachable by non-exceptional paths or only exceptional paths such as C++ exception handlers, respectively. Given -a option, unexecuted blocks are marked $$$$$ or %%%%%, depending on whether a basic block is reachable via non-exceptional or exceptional paths. Executed basic blocks having a statement with zero execution_count end with * character and are colored with magenta color with -k option. The functionality is not supported in Ada.
Note that GCC can completely remove the bodies of functions that are not needed --- for instance if they are inlined everywhere. Such functions are marked with -, which can be confusing. Use the -fkeep-inline-functions and -fkeep-static-functions options to retain these functions and allow gcov to properly show their execution_count.
Some lines of information at the start have line_number of zero. These preamble lines are of the form
-:0:<tag>:<value>
The ordering and number of these preamble lines will be augmented as gcov development progresses --- do not rely on them remaining unchanged. Use tag to locate a particular preamble line.
The additional block information is of the form
<tag> <information>
The information is human readable, but designed to be simple enough for machine parsing too.
When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-boundary value.
When using gcov, you must first compile your program with two special GCC options: -fprofile-arcs -ftest-coverage. This tells the compiler to generate additional information needed by gcov (basically a flow graph of the program) and also includes additional code in the object files for generating the extra profiling information needed by gcov. These additional files are placed in the directory where the object file is located.
Running the program will cause profile output to be generated. For each source file compiled with -fprofile-arcs, an accompanying .gcda file will be placed in the object file directory.
Running gcov with your program's source file names as arguments will now produce a listing of the code along with frequency of execution for each line. For example, if your program is called tmp.cpp, this is what you see when you use the basic gcov facility:
$ g++ -fprofile-arcs -ftest-coverage tmp.cpp
$ a.out
$ gcov tmp.cpp -m
File 'tmp.cpp'
Lines executed:92.86% of 14
Creating 'tmp.cpp.gcov'
The file tmp.cpp.gcov contains output from gcov. Here is a sample:
-: 0:Source:tmp.cpp
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:template<class T>
-: 4:class Foo
-: 5:{
-: 6: public:
1*: 7: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
#####: 7: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
1: 7: Foo(): b (1000) {}
------------------
2*: 8: void inc () { b++; }
------------------
Foo<char>::inc():
#####: 8: void inc () { b++; }
------------------
Foo<int>::inc():
2: 8: void inc () { b++; }
------------------
-: 9:
-: 10: private:
-: 11: int b;
-: 12:};
-: 13:
-: 14:template class Foo<int>;
-: 15:template class Foo<char>;
-: 16:
-: 17:int
1: 18:main (void)
-: 19:{
-: 20: int i, total;
1: 21: Foo<int> counter;
-: 22:
1: 23: counter.inc();
1: 24: counter.inc();
1: 25: total = 0;
-: 26:
11: 27: for (i = 0; i < 10; i++)
10: 28: total += i;
-: 29:
1*: 30: int v = total > 100 ? 1 : 2;
-: 31:
1: 32: if (total != 45)
#####: 33: printf ("Failure\n");
-: 34: else
1: 35: printf ("Success\n");
1: 36: return 0;
-: 37:}
Note that line 7 is shown in the report multiple times. First occurrence presents total number of execution of the line and the next two belong to instances of class Foo constructors. As you can also see, line 30 contains some unexecuted basic blocks and thus execution count has asterisk symbol.
When you use the -a option, you will get individual block counts, and the output looks like this:
-: 0:Source:tmp.cpp
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:template<class T>
-: 4:class Foo
-: 5:{
-: 6: public:
1*: 7: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
#####: 7: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
1: 7: Foo(): b (1000) {}
------------------
2*: 8: void inc () { b++; }
------------------
Foo<char>::inc():
#####: 8: void inc () { b++; }
------------------
Foo<int>::inc():
2: 8: void inc () { b++; }
------------------
-: 9:
-: 10: private:
-: 11: int b;
-: 12:};
-: 13:
-: 14:template class Foo<int>;
-: 15:template class Foo<char>;
-: 16:
-: 17:int
1: 18:main (void)
-: 19:{
-: 20: int i, total;
1: 21: Foo<int> counter;
1: 21-block 0
-: 22:
1: 23: counter.inc();
1: 23-block 0
1: 24: counter.inc();
1: 24-block 0
1: 25: total = 0;
-: 26:
11: 27: for (i = 0; i < 10; i++)
1: 27-block 0
11: 27-block 1
10: 28: total += i;
10: 28-block 0
-: 29:
1*: 30: int v = total > 100 ? 1 : 2;
1: 30-block 0
%%%%%: 30-block 1
1: 30-block 2
-: 31:
1: 32: if (total != 45)
1: 32-block 0
#####: 33: printf ("Failure\n");
%%%%%: 33-block 0
-: 34: else
1: 35: printf ("Success\n");
1: 35-block 0
1: 36: return 0;
1: 36-block 0
-: 37:}
In this mode, each basic block is only shown on one line --- the last line of the block. A multi-line block will only contribute to the execution count of that last line, and other lines will not be shown to contain code, unless previous blocks end on those lines. The total execution count of a line is shown and subsequent lines show the execution counts for individual blocks that end on that line. After each block, the branch and call counts of the block will be shown, if the -b option is given.
Because of the way GCC instruments calls, a call count can be shown after a line with no individual blocks. As you can see, line 33 contains a basic block that was not executed.
When you use the -b option, your output looks like this:
-: 0:Source:tmp.cpp
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:template<class T>
-: 4:class Foo
-: 5:{
-: 6: public:
1*: 7: Foo(): b (1000) {}
------------------
Foo<char>::Foo():
function Foo<char>::Foo() called 0 returned 0% blocks executed 0%
#####: 7: Foo(): b (1000) {}
------------------
Foo<int>::Foo():
function Foo<int>::Foo() called 1 returned 100% blocks executed 100%
1: 7: Foo(): b (1000) {}
------------------
2*: 8: void inc () { b++; }
------------------
Foo<char>::inc():
function Foo<char>::inc() called 0 returned 0% blocks executed 0%
#####: 8: void inc () { b++; }
------------------
Foo<int>::inc():
function Foo<int>::inc() called 2 returned 100% blocks executed 100%
2: 8: void inc () { b++; }
------------------
-: 9:
-: 10: private:
-: 11: int b;
-: 12:};
-: 13:
-: 14:template class Foo<int>;
-: 15:template class Foo<char>;
-: 16:
-: 17:int
function main called 1 returned 100% blocks executed 81%
1: 18:main (void)
-: 19:{
-: 20: int i, total;
1: 21: Foo<int> counter;
call 0 returned 100%
branch 1 taken 100% (fallthrough)
branch 2 taken 0% (throw)
-: 22:
1: 23: counter.inc();
call 0 returned 100%
branch 1 taken 100% (fallthrough)
branch 2 taken 0% (throw)
1: 24: counter.inc();
call 0 returned 100%
branch 1 taken 100% (fallthrough)
branch 2 taken 0% (throw)
1: 25: total = 0;
-: 26:
11: 27: for (i = 0; i < 10; i++)
branch 0 taken 91% (fallthrough)
branch 1 taken 9%
10: 28: total += i;
-: 29:
1*: 30: int v = total > 100 ? 1 : 2;
branch 0 taken 0% (fallthrough)
branch 1 taken 100%
-: 31:
1: 32: if (total != 45)
branch 0 taken 0% (fallthrough)
branch 1 taken 100%
#####: 33: printf ("Failure\n");
call 0 never executed
branch 1 never executed
branch 2 never executed
-: 34: else
1: 35: printf ("Success\n");
call 0 returned 100%
branch 1 taken 100% (fallthrough)
branch 2 taken 0% (throw)
1: 36: return 0;
-: 37:}
For each function, a line is printed showing how many times the function is called, how many times it returns and what percentage of the function's blocks were executed.
For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.
For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message ``never executed'' is printed.
For a call, if it was executed at least once, then a percentage indicating the number of times the call returned divided by the number of times the call was executed will be printed. This will usually be 100%, but may be less for functions that call "exit" or "longjmp", and thus may not return every time they are called.
The execution counts are cumulative. If the example program were executed again without removing the .gcda file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs.
The data in the .gcda files is saved immediately before the program exits. For each source file compiled with -fprofile-arcs, the profiling code first attempts to read in an existing .gcda file; if the file doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file.
if (a != b)
c = 1;
else
c = 0;
can be compiled into one instruction on some machines. In this case, there is no way for gcov to calculate separate execution counts for each line because there isn't separate code for each line. Hence the gcov output looks like this if you compiled the program with optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;
The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.
Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at all.
If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file that needs it. If fileA.o and fileB.o both contain out of line bodies of a particular inlineable function, they will also both contain coverage counts for that function. When fileA.o and fileB.o are linked together, the linker will, on many systems, select one of those out of line bodies for all calls to that function, and remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function body. Hence when instrumented, all but one use of that function will show zero counts.
If the function is inlined in several places, the block structure in each location might not be the same. For instance, a condition might now be calculable at compile time in some instances. Because the coverage of all the uses of the inline function will be shown for the same source lines, the line counts themselves might seem inconsistent.
Long-running applications can use the "__gcov_reset" and "__gcov_dump" facilities to restrict profile collection to the program region of interest. Calling "__gcov_reset(void)" will clear all profile counters to zero, and calling "__gcov_dump(void)" will cause the profile information collected at that point to be dumped to .gcda output files. Instrumented applications use a static destructor with priority 99 to invoke the "__gcov_dump" function. Thus "__gcov_dump" is executed after all user defined static destructors, as well as handlers registered with "atexit". If an executable loads a dynamic shared object via dlopen functionality, -Wl,--dynamic-list-data is needed to dump all profile data.
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