关于“gdb”下尾部优化代码的疑问
考虑 C 中的尾递归阶乘实现：
#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;
}
我在“阶乘”中放置一个断点，然后在“gdb”下运行上述代码。 断点永远不会被命中。
假设它的尾部调用已优化（我已经使用 gcc -O2 编译了它），它应该至少命中断点一次，IIRC。
编辑：我在没有达到任何断点的情况下得到了最终结果。 例如，
(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)
我哪里错了？
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;
}
I place a breakpoint in 'factorial' and I run the above under 'gdb'. The breakpoint is never hit.
Assuming that its tail call optimised (I have compiled it using gcc -O2), it should hit the breakpoint, atleast once, IIRC.
EDIT: 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)
Where am I going wrong?
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阶乘函数可能内联到 main 中。 如果发生这种情况，将会有第二个阶乘副本用于从其他 .c 文件进行调用； 那就是你的断点所在的地方。 尝试传递 -fno-inline。
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.
对我来说效果很好。 您是否打算在编译时使用 -g 标志来添加调试信息？ 您还记得必须输入一个数字才能计算阶乘吗？
Works fine for me. You are membering to use the -g flag to add debug info when you compile? And you are remembering that you have to enter a number to calculate the factorial of?