代码剖析（Code profiling）
程序员在优化软件性能时要注意应尽量优化软件中被频繁调用的部分，这样才能对程序进行有效优化。使用真实的数据，精确的分析应用程序在时间上的花费的行为就成为_代码剖析_。现在几乎所有的开发平台都支持代码剖析，本文要介绍的是linux下针对c/c++的GNU的gprof代码剖析工具。
PS：gprof不只能对c/c++，还可对Pascal和Fortran 77进行代码剖析。
gprof
GNU gprof 是一款linux平台上的程序分析软件（unix也有prof)。借助gprof可以获得C/C++程序运行期间的统计数据，例如每个函数耗费的时间，函数被调用的次数以及各个函数相互之间的调用关系。gprof可以帮助我们找到程序运行的瓶颈，对占据大量CPU时间的函数进行调优。
PS：gprof统计的只是用户态CPU的占用时间，不包括内核态的CPU时间。gprof对I/O瓶颈无能为力，耗时甚久的I/O操作很可能只占据极少的CPU时间。
如何使用gprof
gprof的使用很简单，遵循以下步骤即可：
使用编译标志-pg编译代码。
运行程序生成剖析数据。
运行gprof分析剖析数据，得到可视结果。
下面，我们来演练一下：
test.c:
#include <stdio.h>
void func();
void a() {
printf("Inside a()\n");
int i = 0;
for (; i < 0xffffff; ++i);
func();
return;
}
static void b() {
printf("Inside b()\n");
int i = 0;
for (; i < 0xffffff; ++i);
return;
}
int main() {
printf("Inside main()\n");
int i = 0;
for (; i < 0xfffff; ++i);
a();
b();
return 0;
}
PS: for循环被用来产生执行时间。
func.c:
#include <stdio.h>
void func() {
printf("Inside func\n");
int i = 0;
for (; i < 0xffffff; ++i);
return;
}
Step 1: 使用-pg标识编译上述代码
gcc文档中对-pg的描述：
-pg : Generate extra code to write profile information suitable for the analysis program gprof. You must use this option when compiling the source files you want data about, and you must also use it when linking.
也就是，在编译和链接的时候都要使用-pg标识，所以，一起用吧：
$ gcc -Wall -pg test.c func.c -o test
Step 2: 运行程序
$ ls
func.c makefile test test.c
$ ./test
Inside main()
Inside a()
Inside func
Inside b()
$ ls
func.c gmon.out makefile test test.c
这时，会发现目录下多了一个文件gmon.out，可以用gprof来分析它了。
Step 3: 使用gprof分析工具
gprof可以把gmon.out以人可读的方式解析出来，解析出的内容包括两个表（flat profile和call graph），一个包含函数执行时间，一个包含函数调用过程。
把这两个表重定向到analysis.txt：
$ gprof test gmon.out > analysis.txt
得到analysis.txt：
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls s/call s/call name
33.69 10.42 10.42 1 10.42 10.42 b
33.65 20.82 10.41 1 10.41 20.81 a
33.65 31.23 10.41 1 10.41 10.41 func
0.13 31.27 0.04 main
% the percentage of the total running time of the
time program used by this function.
cumulative a running sum of the number of seconds accounted
seconds for by this function and those listed above it.
self the number of seconds accounted for by this
seconds function alone. This is the major sort for this
listing.
calls the number of times this function was invoked, if
this function is profiled, else blank.
self the average number of milliseconds spent in this
ms/call function per call, if this function is profiled,
else blank.
total the average number of milliseconds spent in this
ms/call function and its descendents per call, if this
function is profiled, else blank.
name the name of the function. This is the minor sort
for this listing. The index shows the location of
the function in the gprof listing. If the index is
in parenthesis it shows where it would appear in
the gprof listing if it were to be printed.
Copyright (C) 2012-2014 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Call graph (explanation follows)
granularity: each sample hit covers 2 byte(s) for 0.03% of 31.27 seconds
index % time self children called name
<spontaneous>
[1] 100.0 0.04 31.23 main [1]
10.41 10.41 1/1 a [2]
10.42 0.00 1/1 b [3]
-----------------------------------------------
10.41 10.41 1/1 main [1]
[2] 66.6 10.41 10.41 1 a [2]
10.41 0.00 1/1 func [4]
-----------------------------------------------
10.42 0.00 1/1 main [1]
[3] 33.3 10.42 0.00 1 b [3]
-----------------------------------------------
10.41 0.00 1/1 a [2]
[4] 33.3 10.41 0.00 1 func [4]
-----------------------------------------------
This table describes the call tree of the program, and was sorted by
the total amount of time spent in each function and its children.
Each entry in this table consists of several lines. The line with the
index number at the left hand margin lists the current function.
The lines above it list the functions that called this function,
and the lines below it list the functions this one called.
This line lists:
index A unique number given to each element of the table.
Index numbers are sorted numerically.
The index number is printed next to every function name so
it is easier to look up where the function is in the table.
% time This is the percentage of the `total' time that was spent
in this function and its children. Note that due to
different viewpoints, functions excluded by options, etc,
these numbers will NOT add up to 100%.
self This is the total amount of time spent in this function.
children This is the total amount of time propagated into this
function by its children.
called This is the number of times the function was called.
If the function called itself recursively, the number
only includes non-recursive calls, and is followed by
a `+' and the number of recursive calls.
name The name of the current function. The index number is
printed after it. If the function is a member of a
cycle, the cycle number is printed between the
function's name and the index number.
For the function's parents, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the function into this parent.
children This is the amount of time that was propagated from
the function's children into this parent.
called This is the number of times this parent called the
function `/' the total number of times the function
was called. Recursive calls to the function are not
included in the number after the `/'.
name This is the name of the parent. The parent's index
number is printed after it. If the parent is a
member of a cycle, the cycle number is printed between
the name and the index number.
If the parents of the function cannot be determined, the word
`<spontaneous>' is printed in the `name' field, and all the other
fields are blank.
For the function's children, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the child into the function.
children This is the amount of time that was propagated from the
child's children to the function.
called This is the number of times the function called
this child `/' the total number of times the child
was called. Recursive calls by the child are not
listed in the number after the `/'.
name This is the name of the child. The child's index
number is printed after it. If the child is a
member of a cycle, the cycle number is printed
between the name and the index number.
If there are any cycles (circles) in the call graph, there is an
entry for the cycle-as-a-whole. This entry shows who called the
cycle (as parents) and the members of the cycle (as children.)
The `+' recursive calls entry shows the number of function calls that
were internal to the cycle, and the calls entry for each member shows,
for that member, how many times it was called from other members of
the cycle.
Copyright (C) 2012-2014 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Index by function name
[2] a [4] func
[3] b [1] main
使用-a参数屏蔽静态（私有）函数信息：
$ gprof -a test gmon.out > analysis.txt
使用-b参数屏蔽冗余信息：
$ gprof -b test gmon.out > analysis.txt
得到如下信息：
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls s/call s/call name
33.69 10.42 10.42 1 10.42 10.42 b
33.65 20.82 10.41 1 10.41 20.81 a
33.65 31.23 10.41 1 10.41 10.41 func
0.13 31.27 0.04 main
Call graph
granularity: each sample hit covers 2 byte(s) for 0.03% of 31.27 seconds
index % time self children called name
<spontaneous>
[1] 100.0 0.04 31.23 main [1]
10.41 10.41 1/1 a [2]
10.42 0.00 1/1 b [3]
-----------------------------------------------
10.41 10.41 1/1 main [1]
[2] 66.6 10.41 10.41 1 a [2]
10.41 0.00 1/1 func [4]
-----------------------------------------------
10.42 0.00 1/1 main [1]
[3] 33.3 10.42 0.00 1 b [3]
-----------------------------------------------
10.41 0.00 1/1 a [2]
[4] 33.3 10.41 0.00 1 func [4]
-----------------------------------------------
Index by function name
[2] a [4] func
[3] b [1] main
使用-p参数只打印flat profile信息：
$ gprof -p test gmon.out > analysis.txt
使用-p(function)参数只打印function函数信息：
只打印函数a()的flat profile信息
$ gprof -pa test gmon.out > analysis.txt
使用-P参数屏蔽flat profile信息：
$ gprof -P test gmon.out > analysis.txt
使用-q参数只打印call graph信息：
$ gprof -q test gmon.out > analysis.txt
使用-q(function)参数只打印function函数的call graph信息：
只打印函数a()的call graph信息
$ gprof -qa test gmon.out > analysis.txt
使用-Q参数屏蔽call graph信息：
$ gprof -Q test gmon.out > analysis.txt
可以组合使用这个参数。
参考：
http://www.cnblogs.com/rocketfan/archive/2009/11/15/1603465.html
http://www.thegeekstuff.com/2012/08/gprof-tutorial/
http://blog.csdn.net/leichelle/article/details/8208530
http://www.ibm.com/developerworks/cn/linux/l-gnuprof.html
著作权归作者所有。
商业转载请联系作者获得授权,非商业转载请注明出处。
原文: https://fooyou.github.io/document/2015/07/22/performance-tools-for-linux-cplusplus.html
