直接上程序：
setjmp和longjmp是处理函数嵌套调用的，goto语句不能跨越函数，所以不选择goto。
#include <setjmp.h>
int setjmp(jmp_buf env); //返回值：若第一次直接调用则直接返回0,若从longjmp调用则返回下面的val
void longjmp(jmp_buf env, int val);
对程序进行不带优化编译：
[henry@localhost c]$ gcc -g youhua.c -o youhua
对程序进行带优化的编译：
[henry@localhost c]$ gcc -g -O youhua.c -o youhua_after
对比上面结果可以看到，全局、静态、volatile变量不受优化的影响。
- 不进行优化时，上面定义的5个变量包括register变量都直接从内存中取值。
- 进行优化后，register变量和局部变量gcc都是从寄存器中取的值。
gcc都做了什么优化呢？首先可以看到变量从内存取值优化到从寄存器取值。一下是manual的部分翻译。
gcc有几个优化等级：
O0，O1，O2，O3
-O0表示没有优化,-O1为缺省值，-O3优化级别最高
'-O '
'-O1 '
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function. With `-O ', the compiler tries to reduce code size and execution
time, without performing any optimizations that take a great deal ##编译器试着减少代码段的大小和代码执行时间，如果没有执行一些
of compilation time. 优化结果将花费大量编译时间。 `-O ' turns on the following optimization flags:
-fdefer-pop 延迟到必要时在函数栈种pop参数
-fdelayed-branch
-fguess-branch-probability
-fcprop-registers
-floop-optimize
-fif-conversion
-fif-conversion2
-ftree-ccp
-ftree-dce
-ftree-dominator-opts
-ftree-dse
-ftree-ter
-ftree-lrs
-ftree-sra
-ftree-copyrename
-ftree-fre
-ftree-ch
-funit-at-a-time
-fmerge-constants `-O ' also turns on `-fomit-frame-pointer ' on machines where doing ## ’-O‘也打开-fomit-frame-pointer标志当机器 so does not interfere with debugging. 这样做不会影响干涉调试。 `-O ' doesn 't turn on `-ftree-sra ' for the Ada compiler. This
option must be explicitly specified on the command line to be enabled for the Ada compiler.`-O2 '
`-O2 '
Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.The compiler does not perform loop unrolling or function inlining when you specify `-O2 '.As compared to `-O ',this option increases both compilation time and the performance of the generated code.
进一步的优化。GCC会支持所有不涉及时间空间交换的所有支持的优化选项。当你加入-o2选项时，编译器不会进行循环展开和函数内联。与-O选项相比，这个选项会增加编辑时间和合成码的性能。
`-O2' turns on all optimization flags specified by `-O'. It also turns on the following optimization flags:
-fthread-jumps
-fcrossjumping
-foptimize-sibling-calls
-fcse-follow-jumps
-fcse-skip-blocks
-fgcse
-fgcse-lm
-fexpensive-optimizations
-fstrength-reduce
-frerun-cse-after-loop
-frerun-loop-opt
-fcaller-saves
-fpeephole2
-fschedule-insns
-fschedule-insns2
-fsched-interblock
-fsched-spec
-fregmove
-fstrict-aliasing
-fdelete-null-pointer-checks
-freorder-blocks
-freorder-functions
-falign-functions
-falign-jumps
-falign-loops
-falign-labels
-ftree-vrp
-ftree-pre
Please note the warning under `-fgcse' about invoking `-O2' on programs that use computed gotos.
`-O3'
Optimize yet more.`-O3 ' turns on all optimizations specified by `-O2' and also turns on the `-finline-functions ',`-funswitch-loops' and `-fgcse-after-reload' options.
再一次的优化，-O3选项会添加所有-O2中添加的选项，并且添加`-finline-functions ',`-funswitch-loops' and `-fgcse-after-reload' 这三个选项
`-O0'
Do not optimize.This is the default.
-Os相当于-O2.5。是使用了所有-O2的优化选项，但又不缩减代码尺寸的方法。
详细的说明如下：
Level 2.5 (-Os)
The special optimization level (-Os or size) enables all -O2 optimizations that do not increase code size; it puts the emphasis on size over speed. This includes all second-level optimizations, except for the alignment optimizations. The alignment optimizations skip space to align functions, loops, jumps and labels to an address that is a multiple of a power of two, in an architecture-dependent manner. Skipping to these boundaries can increase performance as well as the size of the resulting code and data spaces; therefore, these particular optimizations are disabled. The size optimization level is enabled as:
-Os这个特殊的优化等级，能够实现-O2的全部不增加代码段大小优化，他强调程序的大小而不是程序的运行速度，他包含了所有第二等级的优化，除了对齐优化，这些对齐优化在体系结构的依赖性的程序中，跳过一些线性结构，循环，跳转和标签的空间，到一个指数为2的多项式和的地址。跳过这些界限可以提高性能，以及由此产生的代码和数据空间的大小，因此，这些特定的优化被禁用。
完！
参考：·[1]
apue