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• gcc的profile特性，gcc 的 -pg 选项将在每个函数的入口处加入对mcount的代码调用;
• 如果ftrace编写了自己的mcount stub函数，则可借此实现trace功能;

//$ cat foo.c void foo(void) {
   	int a = 1;}

编译：

gcc foo.c -pg -S -o foo-pg.sgcc foo.c -pg -c -o foo-pg.ogcc foo.c -S -o foo.sgcc foo.c -c -o foo.o

foo.s文件：

;$ cat foo.s foo:	pushq	%rbp	movq	%rsp, %rbp	movl	$1, -4(%rbp)	popq	%rbp	ret

foo-pg.s文件：

;$ cat foo-pg.s foo:	pushq	%rbp	movq	%rsp, %rbp	subq	$16, %rsp	call	mcount	movl	$1, -4(%rbp)	leave	ret

查看两个.o文件

[rongtao@localhost demo]$ readelf -s foo.o Symbol table '.symtab' contains 9 entries:   Num:    Value          Size Type    Bind   Vis      Ndx Name     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND      1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS foo.c     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1      3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2      4: 0000000000000000     0 SECTION LOCAL  DEFAULT    3      5: 0000000000000000     0 SECTION LOCAL  DEFAULT    5      6: 0000000000000000     0 SECTION LOCAL  DEFAULT    6      7: 0000000000000000     0 SECTION LOCAL  DEFAULT    4      8: 0000000000000000    13 FUNC    GLOBAL DEFAULT    1 foo[rongtao@localhost demo]$ readelf -s foo-pg.o Symbol table '.symtab' contains 10 entries:   Num:    Value          Size Type    Bind   Vis      Ndx Name     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND      1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS foo.c     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1      3: 0000000000000000     0 SECTION LOCAL  DEFAULT    3      4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4      5: 0000000000000000     0 SECTION LOCAL  DEFAULT    6      6: 0000000000000000     0 SECTION LOCAL  DEFAULT    7      7: 0000000000000000     0 SECTION LOCAL  DEFAULT    5      8: 0000000000000000    22 FUNC    GLOBAL DEFAULT    1 foo     9: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND mcount

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