Re: JavaMemoryModel: Idiom for safe, unsynchronized reads

From: Doug Lea (
Date: Mon Jun 28 1999 - 12:37:39 EDT

Consider the use of simple classes like String and Integer. One of the
original motivations for declaring these classes as immutable wass for
the sake of thread safety.

Should there be two versions, one of which uses synch in its constructor
and one not? Or should people put synch blocks only when constructing
those that the believe might be accessible across threads? Or what?
No answers along these lines strike me as tolerable.

I think the only choice here is for the semantics to guarantee safety.
Compilers can optimize away barriers when they can prove lack of
visibility across threads. Luckily there has been some significant
progress along these lines, that apply even to hostile programming
environments :-) such as alphas. OOPSLA will have three papers on
escape analysis techniques that I believe could be used to kill a
large enough percentage of unnecessary barriers so that this is no
longer a massive impediment to obtaining good performance. (This is
just a belief though. I haven't seen any empirical results wrt
barriers; only locks.)

Doug Lea, Computer Science Department, SUNY Oswego, Oswego, NY 13126 USA 315-341-2688 FAX:315-341-5424  
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