On Fri, Feb 05, 2010 at 01:54:42PM -0800, H. Peter Anvin wrote:
> On 02/05/2010 04:11 AM, Borislav Petkov wrote:
> > +
> > +unsigned int __arch_hweight16(unsigned int w)
> > +{
> > + unsigned int res = 0;
> > +
> > + asm volatile("xor %%dh, %%dh\n\t"
> > + __arch_hweight_alt(32)
> > + : "=di" (res)
> > + : "di" (w)
> > + : "ecx", "memory");
> > +
>
> This is wrong in more ways than I can shake a stick at.
Thanks for reviewing it though - how else would I learn :).
> a) "di" doesn't mean the DI register - it means the DX register (d) or
> an immediate (i). Since you don't have any reference to either %0 or %1
> in your code, you have no way of knowing which one it is. The
> constraint for the di register is "D".
right.
> b) On 32 bits, the first argument register is in %eax (with %edx used
> for the upper half of a 32-bit argument), but on 64 bits, the first
> argument is in %rdi, with the return still in %rax.
Sure, it is right there in arch/x86/include/asm/calling.h. Shame on me.
> c) You call a C function, but you don't clobber the set of registers
> that a C function would clobber. You either need to put the function in
> an assembly wrapper (which is better in the long run), or clobber the
> full set of registers that is clobbered by a C function (which is better
> in the short term) -- which is eax, edx, ecx on 32 bits, but rax, rdi,
> esi, rdx, rcx, r8, r9, r10, r11 on 64 bits.
I think you mean rsi instead of esi here.
Well, the example Brian pointed me to - __mutex_fastpath_lock - lists
the full set of clobbered registers. Please elaborate on the assembly
wrapper for the function, wouldn't I need to list all the clobbered
registers there too or am I missing something?
> d) On the other hand, you do *not* need a "memory" clobber.
Right, in this case we have all non-barrier like inlines so no memory
clobber, according to the comment above alternative() macro.
Thanks.
--
Regards/Gruss,
Boris.
-
Advanced Micro Devices, Inc.
Operating Systems Research Center
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