GCC 的优化控制

      最近在玩新的Qualnet模拟器, 在Linux下用gdb调试出现奇怪的问题. 在跟踪时无法正确地在指定的位置设定断点, 也没有按程序的流程跳转. 在尝试不同编译器以及不同平台后, 发现是Gcc的优化选项出现了问题.

       Qualnet的默认编译包括了一个-O3的优化参数, 以减少编译所用的时间, 但同时也会给gdb的调试带来问题. 在去除参数后, 问题解决.

       Gcc有关优化参数的说明如下, 如果不想看英文的, 记住以后在调试程序的时候, 记得把优化的选项关了就行了….不然出现什么奇怪的情况, 可就不好找问题在哪里了.

3.10 Options That Control Optimization

These options control various sorts of optimizations.

Without any optimization option, the compiler’s goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.

The compiler performs optimization based on the knowledge it has of the program. Optimization levels -O and above, in particular, enable unit-at-a-time mode, which allows the compiler to consider information gained from later functions in the file when compiling a function. Compiling multiple files at once to a single output file in unit-at-a-time mode allows the compiler to use information gained from all of the files when compiling each of them.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.

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 of compilation time.

-O turns on the following optimization flags:


-O also turns on -fomit-frame-pointer on machines where doing so does not interfere with debugging.

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.

-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:

          -falign-functions  -falign-jumps
          -falign-loops  -falign-labels
          -fcse-follow-jumps  -fcse-skip-blocks
          -fgcse  -fgcse-lm
          -freorder-blocks  -freorder-functions
          -fsched-interblock  -fsched-spec
          -fschedule-insns  -fschedule-insns2
          -fstrict-aliasing -fstrict-overflow

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.

Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the -finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload and -ftree-vectorize options.
Reduce compilation time and make debugging produce the expected results. This is the default.
Optimize for size. -Os enables all -O2 optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

-Os disables the following optimization flags:

          -falign-functions  -falign-jumps  -falign-loops
          -falign-labels  -freorder-blocks  -freorder-blocks-and-partition
          -fprefetch-loop-arrays  -ftree-vect-loop-version

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

更详细的,请查阅 http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Optimize-Options.html#Optimize-Options

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