g++编译时加了优化代码选项是不是会改变代码顺序？ - 未名空间MITBBS历史存档

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g++编译时加了优化代码选项是不是会改变代码顺序？

g++编译时加了优化代码选项是不是会改变代码顺序？# Unix - 噫吁兮，危乎高哉

s*s2005-07-02 07:07

1 楼

无意中发现的，gdb调试
如果有-O2，用gdb调试时，逐行执行和源文件顺序不是完全一致
如果不用优化选项，还是一致的。

c*n2005-07-02 07:07

2 楼

sure, never combine "-g" with "-O*".

that's how it optimized your code.

【在 s*****s 的大作中提到】

: 无意中发现的，gdb调试
: 如果有-O2，用gdb调试时，逐行执行和源文件顺序不是完全一致
: 如果不用优化选项，还是一致的。

e*m2005-07-02 07:07

3 楼

You could/should combine them together in a lot of case.
you could then stripe the informtion if you need.

【在 c****n 的大作中提到】

: sure, never combine "-g" with "-O*".
:
: that's how it optimized your code.

c*n2005-07-02 07:07

4 楼

can I ask you when you should have to combine them?

【在 e****m 的大作中提到】

: You could/should combine them together in a lot of case.
: you could then stripe the informtion if you need.

a*e2005-07-02 07:07

5 楼

when you need to make sure the compiler works correctly when
these two options are used together. LOL.

【在 c****n 的大作中提到】

: can I ask you when you should have to combine them?

c*r2005-07-02 07:07

6 楼

when you are using GNU CC and want to debug optimized programs.

【在 c****n 的大作中提到】

: can I ask you when you should have to combine them?

c*r2005-07-02 07:07

7 楼

Read the gcc manual:
"Unlike most other C compilers, GCC allows you to use -g with -O.
The shortcuts taken by optimized code may occasionally produce sur-
prising results: some variables you declared may not exist at all;
flow of control may briefly move where you did not expect it; some
statements may not be executed because they compute constant
results or their values were already at hand; some statements may
execute in different places because they were moved out of loops."

【在 s*****s 的大作中提到】

: 无意中发现的，gdb调试
: 如果有-O2，用gdb调试时，逐行执行和源文件顺序不是完全一致
: 如果不用优化选项，还是一致的。

c*n2005-07-02 07:07

8 楼

looks weird, it's surely that one can combine two options together
and get no error. but what's the benefit from this combination? Do
we really need to dubug a optimized code???

【在 c*r 的大作中提到】

:
: Read the gcc manual:
: "Unlike most other C compilers, GCC allows you to use -g with -O.
: The shortcuts taken by optimized code may occasionally produce sur-
: prising results: some variables you declared may not exist at all;
: flow of control may briefly move where you did not expect it; some
: statements may not be executed because they compute constant
: results or their values were already at hand; some statements may
: execute in different places because they were moved out of loops."