On 11/12/2010 01:08 PM, Mathieu Desnoyers wrote:
* Paolo Bonzini ([email protected]) wrote:
These are useful to flip single bits.
I'm trying to figure out the semantic of these operations.
are they
uatomic_add / uatomic_or or uatomic_add_return / uatomic_or_return ?
They return the old value, consistently with their names (see also the
test_uatomic.c change). There are three possibilities:
- return nothing
- return the old value
- return the new value
Return nothing can be useful because it can be optimized on x86 as "lock
orl (mem), reg/imm". However, there are no other return-nothing atomic
ops in uatomic_*.h so I decided not to provide this.
Returning the new value doesn't make sense for and/or since you cannot
revert the operation (unlike uatomic_add which can be implemented from
uatomic_add_return).
So I chose the second.
Paolo
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