Re: JavaMemoryModel: What's "control dependence" mean?

From: Richard Jones (
Date: Sat Sep 15 2001 - 14:00:38 EDT

> That said, how big is the impact in practice? My impression is that
> most null pointer checks, etc. can be resolved using purely local
> information (and indeed that doing so was crucial to performance).
> It's a pain to insert the ordering constraints, sure, but what real
> performance impact does it have? For example, I suspect, but don't
> know, that the main benefit of related field analysis is in
> eliminating comparisons and conditional traps/branches, and the extra
> flexibility in instruction scheduling isn't a huge win. Does anyone
> have data either way? Keith?
> -Jan-Willem Maessen
> -------------------------------
> JavaMemoryModel mailing list -

FWIW Jalapeno arranges object layouts so that null pointer checks are handled
by hardware (at least on powerpc linux and AIX - I'm not sure about intel
linux). Essentially, trying to access an object through a null pointer
causes jalapeno to dereference an address that is a small negative integer,
resulting in a trap.

Richard Jones
JavaMemoryModel mailing list -

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