[Mono-devel-list] SSA and try/catch/finally regions

[Zoltan Varga]

                                         Hi,

  A simpler solution would be to mark those variables used in
catch/finally blocks as volatile.
This is already done.

                            Zoltan

On Apr 4, 2005 1:43 PM, Massimiliano Mantione <massi at ximian.com> wrote:
> 
> I'm going to do some work in the SSA construction code, so that
> we will have proper SSA "versions" to represent field accesses.
> 
> While working on that code, I thought it would be nice to enable
> SSA also when we currently cannot do it.
> 
> Now we disable SSA for the following reasons:
> - Aliasing risks (the address of a local has been taken)
> - try/catch/finally regions
> 
> The first issue is addressed by alias analysis, which (in a
> limited form, but enough for this purpose) is already done.
> 
> The second one is related to the fact that try and catch/finally
> blocks currently are not properly connected in the CFG.
> But the real issue is not just how to connect the blocks, but how
> to take into account the fact that when we reach a catch/finally
> region the value of some variable is unpredictable.
> 
> I explain with a simple example:
> 
>                 int v = 0;
>                 try {
>                         v = 1;
>                         DoSomething ();
>                         v = 2;
>                         DoSomeMore ();
>                         v = 3;
>                         DoEvenMore ();
>                 } finally {
>                         Console.WriteLine (v);
>                 }
> 
> Here, the value of 'v' in the finally BB depends from which of the
> "Do..." methods (eventually) throws an exception.
> So, simply connecting the try BB to the finally BB in the CFG (and,
> by the way, also the finally BB to its "next" BB, which we now don't
> do!) is not enough.
> 
> My idea is to use a sequence of OP_DUMMY_STORE instructions at the
> beginning of catch/finally regions, one for each variable that is
> assigned in the corresponding try region (and is actually used in
> the catch/finally region, of course).
> 
> The complexity of doing this is low, and I think I can easily embed
> this kind of scanning in the alias analysis pass (which already does
> a full pass on the whole compiled method, looking for all load and
> store instructions).
> Maybe handling nested try regions will give me some trouble, but in
> the end it should be easy...
> 
> In this way SSA would know that those variables have an unknown
> value in those blocks.
> 
> Do you think this is a reasonable approach?
> 
> Ciao,
>   Massi
> 
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