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Doubt regarding a tail optimized code under ‘gdb ’
- From: Amit Saha <amitsaha dot in at gmail dot com>
- To: gdb at sourceware dot org
- Date: Mon, 18 May 2009 22:16:46 +0530
- Subject: Doubt regarding a tail optimized code under ‘gdb ’
Hello all,
Out of sheer curiosity, I tried to run a tail optimised code under
'gdb'. Consider a tail recursive factorial implementation in C:
<code>
#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;
}
</code>
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, at
least once. 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? Any pointers would be appreciated? I am using
'gdb-6.8-debian' and gcc-4.3.3.
Best Regards,
Amit
--
Journal: http://amitksaha.wordpress.com
IRC: cornucopic on #scheme, #lisp, #math, #linux
"Recursion is the basic iteration mechanism in Scheme"
--- http://c2.com/cgi/wiki?TailRecursion