我有下面这段code的,我在C.其写下相当简单,因为它正好位移位 X 为每一个循环。
I have the following piece of code that I wrote in C. Its fairly simple as it just right bit-shifts x for every loop of for.
int main() { int x = 1; for (int i = 0; i > -2; i++) { x >> 2; } }现在正在发生奇怪的是,当我刚刚编译它没有任何的优化或第一级优化( -O ),它运行得很好(我定时可执行文件及其有关 1.4S 与 -O 和 5.4S 无任何优化。
Now the strange thing that is happening is that when I just compile it without any optimizations or with first level optimization (-O), it runs just fine (I am timing the executable and its about 1.4s with -O and 5.4s without any optimizations.
现在,当我加入 -O2 或 -O3 开关编译和时间生成的可执行文件,这不是' ŧ停止(我已经测试高达 60 )。
Now when I add -O2 or -O3 switch for compilation and time the resulting executable, it doesn't stop (I have tested for up to 60s).
这是什么可能导致此任何想法?
Any ideas on what might be causing this?
推荐答案优化循环产生无限循环是根据签署整数溢出你的结果。有符号整数溢出是ç未定义行为,不应该依赖。不仅可以将它混淆开发它也可以由编译器优化出
The optimized loop is producing an infinite loop which is a result of you depending on signed integer overflow. Signed integer overflow is undefined behavior in C and should not be depended on. Not only can it confuse developers it may also be optimized out by the compiler.
大会(不优化):的gcc -std = C99 -S -O0的main.c
Assembly (no optimizations): gcc -std=c99 -S -O0 main.c
_main: LFB2: pushq %rbp LCFI0: movq %rsp, %rbp LCFI1: movl $1, -4(%rbp) movl $0, -8(%rbp) jmp L2 L3: incl -8(%rbp) L2: cmpl $-2, -8(%rbp) jg L3 movl $0, %eax leave ret大会(优化的3级):GCC -std = C99 -S -O3的main.c
_main: LFB2: pushq %rbp LCFI0: movq %rsp, %rbp LCFI1: L2: jmp L2 #<- infinite loop更多推荐
编译器优化方案导致运行速度慢
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