对于怀疑在“gdb”的尾巴优化code [英] Doubt regarding a tail optimized code under 'gdb'

问题描述

考虑在C尾递归阶乘实现：

Consider a tail recursive factorial implementation in C:

#include <stdio.h>

unsigned long long factorial(unsigned long long fact_so_far, unsigned long long count, unsigned long long max_count){

if (max_count==0 || max_count==1 || count >= max_count)
        return fact_so_far;
else
{
        printf("%llu  %p \n", count, &factorial);
        return factorial(fact_so_far * count, ++count, max_count);
}

}


int main(int argc, char **argv)
{
        unsigned long long n;
        scanf("%llu", &n);
        printf("\n Factorial %llu \n",factorial(1,0,n));
        return 0;

}

我放在'因子'断点，我在gdb的执行上面。断点是永远不会命中。

I place a breakpoint in 'factorial' and I run the above under 'gdb'. The breakpoint is never hit.

假设它的尾部调用优化（我用gcc编译-O2它），它应该命中断点，一旦ATLEAST，IIRC。

Assuming that its tail call optimised (I have compiled it using gcc -O2), it should hit the breakpoint, atleast once, IIRC.

编辑：我得到的最终结果没有击中任何断点。对于如

I get the final result without hitting any breakpoint. For eg,

(gdb) b factorial
Breakpoint 1 at 0x8048429: file factorial-tail.c, line 3.
(gdb) run
Starting program: /home/amit/quest/codes/factorial-tail 
5
0  0x8048420 
1  0x8048420 
2  0x8048420 
3  0x8048420 
4  0x8048420 

 Factorial 120 

Program exited normally.
(gdb)

我在哪里去了？

推荐答案

这可能是阶乘函数内联到主。如果发生这种情况，将会有用于从其他.c文件调用阶乘的第二个副本;这就是你的断点。尝试通过-fno内联。

It could be that the factorial function is inlined into main. If this happens, there will be a second copy of factorial used for calls from other .c files; that's where your breakpoint was. Try passing -fno-inline.

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