On Sun, Jan 23, 2005 at 04:19:21AM +0100, Andi Kleen wrote:
> On Sat, Jan 22, 2005 at 09:10:40PM -0500, Chuck Ebbert wrote:
> > On Fri, 21 Jan 2005 at 15:41:06 -0600 Matt Mackall wrote:
> >
> > > Add rol32 and ror32 bitops to bitops.h
> >
> > Can you test this patch on top of yours? I did it on 2.6.10-ac10 but it
> > should apply OK. Compile tested and booted, but only random.c is using it
> > in my kernel.
>
> Does random really use variable rotates? For constant rotates
> gcc detects the usual C idiom and turns it transparently into
> the right machine instruction.
Nope, random doesn't. The only thing I converted in my sweep that did
were CAST5 and CAST6, which are fairly unique in doing key-based
rotations. On the other hand:
typedef unsigned int __u32;
static inline __u32 rol32(__u32 word, int shift)
{
return (word << shift) | (word >> (32 - shift));
}
int foo(int val, int rot)
{
return rol32(val, rot);
}
With 2.95:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 8b 4d 0c mov 0xc(%ebp),%ecx
6: 8b 45 08 mov 0x8(%ebp),%eax
9: d3 c0 rol %cl,%eax
b: c9 leave
c: c3 ret
With 3.3.5:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 8b 45 08 mov 0x8(%ebp),%eax
6: 8b 4d 0c mov 0xc(%ebp),%ecx
9: 5d pop %ebp
a: d3 c0 rol %cl,%eax
c: c3 ret
With gcc-snapshot:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 8b 45 08 mov 0x8(%ebp),%eax
6: 8b 4d 0c mov 0xc(%ebp),%ecx
9: d3 c0 rol %cl,%eax
b: 5d pop %ebp
c: c3 ret
So I think tweaks for x86 at least are unnecessary.
--
Mathematics is the supreme nostalgia of our time.
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