un-requested compiler optimisations

[David Nadlinger]

On 4/14/11 8:24 PM, spir wrote:
> If it does optimise, then it is definitely a compiler bug. Since you
> *explicitely* ask for a double reverse, it *must* just do it.
> Suppressing them is here just breaking the language's semantics!
> […]

Sigh… First, as Steve already pointed out, retro() specifically 
shortcuts reversing a range twice to the original range.

Second, your remarks about optimization are not quite correct, at least 
for most of today's high-level languages. A compiler is free to optimize 
a program in whatever possible way, as long as it can prove the result 
equivalent to the original. And as burning CPU-cycles is not part of the 
program semantics, a modern optimizing compiler is free to optimize the 
loop and factorial call away altogether. If you don't believe me, have a 
look at the attached sample program and the asm generated for main() 
(some recent Clang version, clang -O3).

David


---
#include <stdio.h>

int factorial(int a) {
   int p = 1;
   for (int i = 2; i <= a; ++i) {
     p *= i;
   }
   return p;
}

int main (int argc, char const *argv[]) {
   int t;
   for (long i = 0; i < 10000000; ++i) {
     t = factorial(9);
   }
   printf("%d\n", t);
   return 0;
}


_main:
0000000100000ee0	pushq	%rbp
0000000100000ee1	movq	%rsp,%rbp
0000000100000ee4	leaq	0x00000041(%rip),%rdi
0000000100000eeb	movl	$0x00058980,%esi ; 9! in hex
0000000100000ef0	xorb	%al,%al
0000000100000ef2	callq	0x100000f02; symbol stub for: _printf
0000000100000ef7	xorl	%eax,%eax
0000000100000ef9	popq	%rbp
0000000100000efa	ret