is +=1 thread safe

[Diez B. Roggisch]

Duncan Booth schrieb:
> Marc 'BlackJack' Rintsch <bj_666 at gmx.net> wrote:
> 
>> On Thu, 01 May 2008 15:33:09 -0700, Gary Herron wrote:
>>
>>> Of course it's not thread safe.   For the same reason and more basic, 
>>> even the expression i++ is not thread safe in C++.
>>>
>>> Any such calculation, on modern processors, requires three operations: 
>>>   retrieve value of i into a register,
>>>   increment the register
>>>   write the value into i.
>> There are no modern processors with an opcode for incrementing a memory
>> location!?  At least my C64 can do that.  ;-)
>>
> The operation i++ in C/C++ implies two things: it increments the memory 
> location and returns the new result. That means there are at least two ways 
> to generate code for this:
> 
>    increment memory
>    load new value
> 
> or
> 
>    load value
>    incremenent
>    store
> 
> Neither of these is going to be thread-safe (even on a C64) unless you 
> protect the operations somehow. The latter is probably preferred as it only 
> implies two operations to memory whereas the former implies three.

This is not entirely right. increment is usually atomic (at least for 
single-core processors), so you don't suffer from a load/store being 
interrupted.

Of course it can happen that the previous or later loading of the value 
for computational purposes is interrupted by another thread. But you 
have the guarantee that all incs (and decs) are counted, in contrast to 
what the OP observed with python.

Diez