On Saturday 18 August 2007 05:13, Linus Torvalds wrote:
> On Sat, 18 Aug 2007, Satyam Sharma wrote:
> > No code does (or would do, or should do):
> >
> > x.counter++;
> >
> > on an "atomic_t x;" anyway.
>
> That's just an example of a general problem.
>
> No, you don't use "x.counter++". But you *do* use
>
> if (atomic_read(&x) <= 1)
>
> and loading into a register is stupid and pointless, when you could just
> do it as a regular memory-operand to the cmp instruction.
It doesn't mean that (volatile int*) cast is bad, it means that current gcc
is bad (or "not good enough"). IOW: instead of avoiding volatile cast,
it's better to fix the compiler.
> And as far as the compiler is concerned, the problem is the 100% same:
> combining operations with the volatile memop.
>
> The fact is, a compiler that thinks that
>
> movl mem,reg
> cmpl $val,reg
>
> is any better than
>
> cmpl $val,mem
>
> is just not a very good compiler.
Linus, in all honesty gcc has many more cases of suboptimal code,
case of "volatile" is just one of many.
Off the top of my head:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28417
unsigned v;
void f(unsigned A) { v = ((unsigned long long)A) * 365384439 >> (27+32); }
gcc-4.1.1 -S -Os -fomit-frame-pointer t.c
f:
movl $365384439, %eax
mull 4(%esp)
movl %edx, %eax <===== ?
shrl $27, %eax
movl %eax, v
ret
Why is it moving %edx to %eax?
gcc-4.2.1 -S -Os -fomit-frame-pointer t.c
f:
movl $365384439, %eax
mull 4(%esp)
movl %edx, %eax <===== ?
xorl %edx, %edx <===== ??!
shrl $27, %eax
movl %eax, v
ret
Progress... Now we also zero out %edx afterwards for no apparent reason.
--
vda
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