The speed differences between registers and memory

assembly.diff

--- memory.s    2010-08-11 11:14:35.503392530 -0400
+++ register.s  2010-08-11 11:14:29.988393279 -0400
@@ -1,19 +1,20 @@
-       .file   "memory.c"
+       .file   "register.c"
        .text
 .globl main
        .type   main, @function
 main:
        pushl   %ebp
        movl    %esp, %ebp
-       subl    $16, %esp
-       movl    $0, -4(%ebp)
+       pushl   %ebx
+       movl    $0, %ebx
        jmp     .L2
 .L3:
-       addl    $1, -4(%ebp)
+       addl    $1, %ebx
 .L2:
-       cmpl    $499999999, -4(%ebp)
+       cmpl    $499999999, %ebx
        jle     .L3
-       leave
+       popl    %ebx
+       popl    %ebp
        ret
        .size   main, .-main
        .ident  "GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"

benchmarks.txt

$ gcc memory.c && time ./a.out

real    0m2.183s
user    0m2.092s
sys     0m0.004s

$ gcc register.c && time ./a.out

real    0m0.315s
user    0m0.300s
sys     0m0.004s

memory.c

void main () {
  int loop;
  for (loop=0; loop<500000000; loop++);
}

memory.s

	.file	"memory.c"
	.text
.globl main
	.type	main, @function
main:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$16, %esp
	movl	$0, -4(%ebp)
	jmp	.L2
.L3:
	addl	$1, -4(%ebp)
.L2:
	cmpl	$499999999, -4(%ebp)
	jle	.L3
	leave
	ret
	.size	main, .-main
	.ident	"GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
	.section	.note.GNU-stack,"",@progbits

original.c

#include <stdio.h>

int main (int argc, char *argv[]) {
 int i = 47;
 int loop;
 for (loop=0; loop<500000000; loop++)
  i + i;
 return 0;
}

register.c

void main () {
  register int loop;
  for (loop=0; loop<500000000; loop++);
}

register.s

	.file	"register.c"
	.text
.globl main
	.type	main, @function
main:
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%ebx
	movl	$0, %ebx
	jmp	.L2
.L3:
	addl	$1, %ebx
.L2:
	cmpl	$499999999, %ebx
	jle	.L3
	popl	%ebx
	popl	%ebp
	ret
	.size	main, .-main
	.ident	"GCC: (Ubuntu 4.4.3-4ubuntu5) 4.4.3"
	.section	.note.GNU-stack,"",@progbits