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SubjectRe: [PATCH 13/15] x86: add cmpxchg_flag() variant
On 08/23/2011 01:45 PM, H. Peter Anvin wrote:
> On 08/23/2011 12:53 PM, Jeremy Fitzhardinge wrote:
>> Yes, that would ideal. The closest you can get is asm goto(), but the
>> syntax for that would be awful; something like:
>>
>> #define cmpxchg_jump(ptr, old, new, fail)\
>> asm goto (...)
>>
>>
>> :
>> :
>>
>> again:
>> old = *thingp;
>> new = frobulate(old);
>> cmpxchg_jump(thingp, old, new, again);
>> /* worked */
>>
>> Would this be useful enough?
>>
> Actually there is a trick:
>
> static inline bool ....
> {
> asm goto(... yes);
> no:
> return false;
> yes:
> return true;
> }
>
> ... which makes syntax a heckuva lot less awkward.

Yeah, but you'd need to define an inline for each type, since the
function isn't polymorphic. But it can be done with a macro.

However, having prototyped it, I dunno, it doesn't really seem like much
of a win for all the extra code it adds. I just can't get too excited
about an extra test instruction adjacent to a monster like a locked
cmpxchg. The jump variant avoids the test, but gcc still generates some
pretty bogus stuff:

lock; cmpxchgq %rbx,(%rcx); jne .L88 # D.24853, MEM[(volatile u64 *)top_p_26],
# 0 "" 2
#NO_APP
jmp .L87 #
.L88:
xorl %esi, %esi #
movq %rbx, %rdi # D.24853,
call free_pages #
.L87:
addq p2m_top_mfn(%rip), %r13 # p2m_top_mfn, D.24896
movq p2m_top_mfn_p(%rip), %rax # p2m_top_mfn_p, p2m_top_mfn_p


and adding unlikely()s doesn't help at all.

J


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