文章目錄

• 參考
• 簡介
• 源碼
• 生成的匯編 -O2
• 不帶-O2

參考

gcc manual
http://gcc.gnu.org/pub/gcc/summit/2003/GENERIC%20and%20GIMPLE.pdf

簡介

在做sprintf的優(yōu)化時(shí)，有一個(gè)判斷標(biāo)準(zhǔn)，具體的判斷標(biāo)準(zhǔn)是什么？需要看GCC的代碼。
ISO C90 函數(shù)： abort, abs, acos, asin, atan2, atan, calloc, ceil, cosh, cos, exit, exp, fabs, floor, fmod, fprintf, fputs, frexp, fscanf, isalnum, isalpha, iscntrl, isdigit, isgraph, islower, isprint, ispunct, isspace, isupper, isxdigit, tolower, toupper, labs, ldexp, log10, log, malloc, memchr, memcmp, memcpy, memset, modf, pow, printf, putchar, puts, scanf, sinh, sin, snprintf, sprintf, sqrt, sscanf, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strpbrk,
strrchr, strspn, strstr, tanh, tan, vfprintf, vprintf and vsprintf，統(tǒng)統(tǒng)都會被認(rèn)為是內(nèi)建的函數(shù)，即使沒有built-in前綴，除非指明編譯選項(xiàng)‘-fno-builtin’ is specified (or ‘-fno-builtin-function’ is specified for an individual function). 對應(yīng)的內(nèi)建函數(shù)有前綴“_builtin”。這樣gcc在實(shí)現(xiàn)的內(nèi)部就會根據(jù)一些標(biāo)準(zhǔn)做優(yōu)化。

源碼

[root@10 test]# cat discr.c #include <stdio.h> #include <stdlib.h> void fun1(int c, int b) {char abc[300];sprintf(abc,"abcde000000,123123123123123123123"); 、、、其實(shí)這里就當(dāng)前的語境來說，沒有必要做sprintf操作。system(abc);//printf("func1\n");//char *a;//*a=3; } void fun() {system("abcde");fun1(1,2); } int main() {fun(); }

生成的匯編 -O2

[root@10 test]# gcc -O2 discr.c；使用了sse的寄存器xmm0.

(gdb) disass fun1 Dump of assembler code for function fun1:0x00000000004005d0 <+0>: sub $0x138,%rsp0x00000000004005d7 <+7>: movdqa 0x101(%rip),%xmm0 # 0x4006e00x00000000004005df <+15>: mov $0x33,%eax0x00000000004005e4 <+20>: mov %rsp,%rdi0x00000000004005e7 <+23>: mov %ax,0x20(%rsp)0x00000000004005ec <+28>: movaps %xmm0,(%rsp)0x00000000004005f0 <+32>: movdqa 0xf8(%rip),%xmm0 # 0x4006f00x00000000004005f8 <+40>: movaps %xmm0,0x10(%rsp)0x00000000004005fd <+45>: callq 0x4004a0 <system@plt>0x0000000000400602 <+50>: add $0x138,%rsp0x0000000000400609 <+57>: retq

不帶-O2

(gdb) disass fun1 Dump of assembler code for function fun1:0x00000000004005d6 <+0>: push %rbp0x00000000004005d7 <+1>: mov %rsp,%rbp0x00000000004005da <+4>: sub $0x140,%rsp0x00000000004005e1 <+11>: mov %edi,-0x134(%rbp)0x00000000004005e7 <+17>: mov %esi,-0x138(%rbp)0x00000000004005ed <+23>: lea -0x130(%rbp),%rax0x00000000004005f4 <+30>: mov $0x4006e8,%esi0x00000000004005f9 <+35>: mov %rax,%rdi0x00000000004005fc <+38>: mov $0x0,%eax0x0000000000400601 <+43>: callq 0x4004e0 <sprintf@plt>0x0000000000400606 <+48>: lea -0x130(%rbp),%rax0x000000000040060d <+55>: mov %rax,%rdi0x0000000000400610 <+58>: callq 0x4004d0 <system@plt>0x0000000000400615 <+63>: nop0x0000000000400616 <+64>: leaveq0x0000000000400617 <+65>: retq End of assembler dump.

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