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From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-04-04 15:30:59

Alexander Terekhov wrote:
> Peter Dimov wrote:
> [...]
>> xadd is branchless; it just returns the old value, whereas inc
>> doesn't. MSVC always generates lock xadd, even for
>> _InterlockedIncrement, BTW.
> Well, maybe. But you need value neither for increments nor decrements.
> (I mean that for decrements you can simply rely on ZF flag). Oder?

I don't need the value, but it doesn't matter; it's just as fast. Or just as
slow. x86 isn't very predictable (I don't know how familiar you are with
it); some instructions look better on paper but are (or were) actually
slower (on 586 and above) than an equivalent RISC-style read/modify/write
sequence, inc included. And of course the rules change with every generation
(in some cases, sub-generation) of CPUs.

Basically, the only way to know which is faster is to measure it.

> Nah. For the sake of killing C/C++ volatiles rather sooner than later,
> I strongly suggest that you hide that load in asm. Just add load+cmp
> followed by ZF branch prior to "lock dec" which also sets ZF, IIRC.

Eh, __asm__ is no better than a volatile. Both are non-portable. Yes, dec
sets ZF (and SF). That's why InterlockedDecrement on 386 returns +1, 0 or -1
instead of the actual value. :-)

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