[jmm-dev] State of play with modeling relaxed memory

[Jeremy Manson]

Doug,

I'm not getting a good read on your responses here - are you arguing that
we shouldn't support Category C anymore?  They don't really seem *that*
controversial; just because nothing actually performs these optimizations
doesn't mean that they aren't reasonable optimizations to perform.  It is
just that very few optimizers do any sort of whole program analysis.

(I'm not sure that there was anything wrong with our treatment of Category
C in the JSR133 model, either, although that is probably not entirely
relevant.)

Jeremy


On Mon, Mar 17, 2014 at 6:23 AM, Doug Lea <dl at cs.oswego.edu> wrote:

> On 03/12/2014 10:19 AM, Alan Jeffrey wrote:
>
>  So, assuming that TC1 (and similar examples) still stands,
>>
>
> Here are some notes on those test cases at
>   http://www.cs.umd.edu/~pugh/java/memoryModel/
> unifiedProposal/testcases.html
>
> They can be placed in three categories:
>
> A. Tests for SC outcomes (including no out-of-thin-air)
>   Cases 4, 5, 10, 12, 13, 14, 15
>
> B. Tests involving local analysis/reorderings only for rationales:
>   Cases 2, 3, 6, 7, 11, 16
>
> C. Tests additionally requiring non-local optimizations and/or
>    compiler-directed scheduling:
>   Cases 1, 8, 17, 18, 19, 20, plus those at
>     http://www.saraswat.org/Test-variants.html
>
> Unless I'm mis-remembering something (very possible), all listed
> outcomes of those in Categories A and B should be uncontroversial.
>
> The listed outcomes for those in Category C were sometimes a matter of
> whether anyone could come up with a good story for them.  No Java
> conformance test (and surely no existing program) relies on them.
> Some of these cases reflect the realization late in the JSR133 process
> that initial justification rules didn't capture non-local
> optimizations. There was internal debate about whether to tweak them
> versus restart using the constraint-based approach that Vijay Saraswat
> later developed into RAO. Since the JSR133 spec was at that point
> overdue, there wasn't much choice. But I don't see any reason to
> think that any model can handle Category C cases in a uniform way
> (which might not exactly match the listed outcomes) without using a
> RAO-like approach. Which probably holds independently of whether
> expressed via Pomsets, Event Structures, or Mazurkeiwicz traces or
> whatever.
>
> -Doug
>
>
>