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p u b l i c s t a t i c float Compa re E x c h a nge ( ref float loc ation l , float va l u e , float compa rand ); p u b l i c stat i c T Compa r e E x c h a n ge < T > ( ref T l o c a t ion l , T value, T compa rand ) where T : c l a s s ;
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Notice that 64-bit compare-exchange operations are available, even on 32-bit processors, thanks to the CMPXCHG88 instruction supported broadly by all modern Intel and AMD processors This is exposed through I n t e r
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Due to this last point, it is sometimes possible to atomically load and store nonatomic-sized memory locations In fact, the CLR offers a p u b l i c stat i c l o n g R e a d ( ref l o n g l o c a t i o n ) method on the I nt e r l o c ked class that exploits this fact It internally just uses a Compa r e E x c h a nge that over writes the value if it's currently 0, but otherwise leaves it as is, enabling you to read its current contents as an atomic operation, even on 32-bit machines You can use this capability to generally perform 64-bit atomic reads and writes on 32-bit processors, avoiding tom reads, and can even conditionalize its use to avoid the cost of an unnecessary interlocked instruction on actual 64-bit machines In C++, you'd #i fdef out uses of Interloc ked E x c h a nge64 to become ordinary loads and stores on 64-bit machines, and in managed code you can use a fast runtime check:
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stat i c void AtomicWrit e ( ref long location , long va l u e ) { if ( I ntPt r S i z e 4) Interlocked E x c h a nge ( ref locat ion , v a l u e ) ; else location } stat i c long Atom i c Read ( ref long location )
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If we're lucky, the if check will be optimized away by the JIT compiler, since I n t Pt r S i z e (aka, s i zeof ( vo i d * is a constant known at JIT com pile time Notice that the At om i c Re a d function has been written out long hand, to use I n t e r l o c ked Compa r e E x c h a n ge, rather than being defined in terms of the existing I n t e r loc ked Re ad function This is just for illustration purposes We specify a value of e for the comparand and value so that unless the current value of the target is e there is no actual write performed But if one is performed, the value is unchanged Because Compa r e E x c h a nge returns the value seen, we just return that Using this trick for loads is patently not the most efficient way to per form a read operation: an interlocked operation unconditionally acquires the target address' s cache line in exclusive mode, possibly invalidating other processors' cache lines in the process and causing cache coherence traffic and contention This is particularly wasteful because we don't need to write at all If many such reads are used close together, this technique can become more expensive (on 32 bit) than using a simple spin lock to protect the sequence As with any lock free technique, use this with care, and meas ure, measure, measure But if you are primarily targeting 64-bit and can tol erate worse performance on 32-bit architectures, this is a perfectly fine approach
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