C语言 如何在 gcc 中禁用编译器优化？
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How to disable compiler optimizations in gcc?
提问by Neal
I am trying to learn assembly language. I have searched and found how to disassemble a .cfile but I think it produces some optimized version of the program. Is there any way so that I can see the exact assembly code which corresponds to my C file.
我正在努力学习汇编语言。我已经搜索并找到了如何反汇编.c文件，但我认为它会生成一些优化版本的程序。有什么方法可以让我看到与我的 C 文件相对应的确切汇编代码。
回答by pmr
The gcc option -Oenables different levels of optimization. Use -O0to disable them and use -Sto output assembly. -O3is the highest level of optimization.
gcc 选项-O启用不同级别的优化。使用-O0他们，使用禁用-S输出组件。-O3是最高级别的优化。
Starting with gcc 4.8 the optimization level -Ogis available. It enables optimizations that do not interfere with debugging and is the recommended default for the standard edit-compile-debug cycle.
从 gcc 4.8 开始，优化级别-Og可用。它启用不干扰调试的优化，并且是标准编辑-编译-调试循环的推荐默认值。
To change the dialect of the assembly to either intel or att use -masm=intelor -masm=att.
要将程序集的方言更改为 intel 或 att 使用-masm=intel或-masm=att.
You can also enable certain optimizations manually with -fname.
您还可以使用 手动启用某些优化-fname。
Have a look at the gcc manualfor much more.
查看gcc 手册了解更多信息。
回答by IanNorton
For gcc you want to omit any -O1 -O2or -O3options passed to the compiler or if you already have them you can append the -O0option to turn it off again. It might also help you to add -gfor debug so that you can see the c source and disassembled machine code in your debugger.
对于 gcc，您希望省略任何传递给编译器的-O1 -O2或-O3选项，或者如果您已经拥有它们，您可以附加-O0选项以再次将其关闭。它还可以帮助您添加-g进行调试，以便您可以在调试器中查看 c 源代码和反汇编的机器代码。
See also: http://sourceware.org/gdb/onlinedocs/gdb/Optimized-Code.html
另见：http: //sourceware.org/gdb/onlinedocs/gdb/Optimized-Code.html
回答by user3770818
To test without copy elision and see you copy/move constructors/operators in action add "-fno-elide-constructors".
要在没有复制省略的情况下进行测试并查看您在操作中复制/移动构造函数/运算符，请添加“-fno-elide-constructors”。
Even with no optimizations (-O0 ), GCC and Clang will still do copy elision, which has the effect of skipping copy/move constructors in some cases. See this questionfor the details about copy elision.
即使没有优化 (-O0)，GCC 和 Clang 仍然会执行复制省略，这在某些情况下会跳过复制/移动构造函数。有关复制省略的详细信息，请参阅此问题。
However, in Clang 3.4 it does trigger a bug (an invalid temporary object without calling constructor), which is fixed in 3.5.
然而，在 Clang 3.4 中它确实触发了一个错误（一个没有调用构造函数的无效临时对象），这个错误在 3.5 中得到了修复。
回答by MByD
回答by api
Long time ago, but still needed.
很久以前，但仍然需要。
info - https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
list - gcc -Q --help=optimizers test.c | grep enabled
disable as many as you like with:
gcc **-fno-web** -Q --help=optimizers test.c | grep enabled
回答by Nils Pipenbrinck
You can disable optimizations if you pass -O0 with the gcc command-line.
如果使用 gcc 命令行传递 -O0，则可以禁用优化。
E.g. to turn a .C file into a .S file call:
例如将 .C 文件转换为 .S 文件调用：
gcc -O0 -S test.c
gcc -O0 -S test.c
回答by mckenzm
You can also control optimisations internally with #pragma GCC push_options
您还可以使用 #pragma GCC push_options 在内部控制优化
#pragma GCC push_options
/* #pragma GCC optimize ("unroll-loops") */
.... code here .....
#pragma GCC pop_options
