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I got motivated from tail call optimization question What Is Tail Call Optimization? So, I decided to see how can I do it in plain C. So, I wrote the 2 factorial programs, 1st where tail call optimization can be applied. I call this fact function as fact(n, 1). unsigned long long int fact(int n, int cont) { if(n == 0) return cont; else return fact(n-1, n * cont); } 2nd is the normal recursion where multiple stack frames are required. unsigned long long int fact(int n) { if(n == 0) return 1; else return n * fact(n-1); } This is the assembly generated by a 32 bit compiler for the former with -O2 0x8048470 <fact>: push %ebp 0x8048471 <fact+1>: mov %esp,%ebp 0x8048473 <fact+3>: mov 0x8(%ebp),%edx 0x8048476 <fact+6>: mov 0xc(%ebp),%eax 0x8048479 <fact+9>: test %edx,%edx 0x804847b <fact+11>: je 0x8048488 <fact+24> 0x804847d <fact+13>: lea 0x0(%esi),%esi 0x8048480 <fact+16>: imul %edx,%eax 0x8048483 <fact+19>: sub $0x1,%edx 0x8048486 <fact+22>: jne 0x8048480 <fact+16> 0x8048488 <fact+24>: mov %eax,%edx 0x804848a <fact+26>: sar $0x1f,%edx 0x804848d <fact+29>: pop %ebp 0x804848e <fact+30>: ret This is the assembly generated by a 32 bit compiler for latter with -O2. 0x8048470 <fact>: push %ebp 0x8048471 <fact+1>: mov %esp,%ebp 0x8048473 <fact+3>: push %edi 0x8048474 <fact+4>: push %esi 0x8048475 <fact+5>: push %ebx 0x8048476 <fact+6>: sub $0x14,%esp 0x8048479 <fact+9>: mov 0x8(%ebp),%eax 0x804847c <fact+12>: movl $0x1,-0x18(%ebp) 0x8048483 <fact+19>: movl $0x0,-0x14(%e

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