On Thu, 16 Aug 2007, Linus Torvalds wrote:

On Fri, 17 Aug 2007, Paul Mackerras wrote:

quoted

I'm really surprised it's as much as a few K.  I tried it on powerpc
and it only saved 40 bytes (10 instructions) for a G5 config.

One of the things that "volatile" generally screws up is a simple

	volatile int i;

	i++;

which a compiler will generally get horribly, horribly wrong.

In a reasonable world, gcc should just make that be (on x86)

	addl $1,i(%rip)

on x86-64, which is indeed what it does without the volatile. But with the 
volatile, the compiler gets really nervous, and doesn't dare do it in one 
instruction, and thus generates crap like

        movl    i(%rip), %eax
        addl    $1, %eax
        movl    %eax, i(%rip)

instead. For no good reason, except that "volatile" just doesn't have any 
good/clear semantics for the compiler, so most compilers will just make it 
be "I will not touch this access in any way, shape, or form". Including 
even trivially correct instruction optimization/combination.

Apart from having to fetch more bytes for the instructions (which does
matter), execution time is probably the same on modern processors, as they
convert the single instruction to RISC-style load, modify, store anyway.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds