[Mono-dev] Questions about the linear IR

[Rasmus Halland]

Thank you for the explanations, guys!

On Thu, Jul 31, 2008 at 9:13 AM, Massimiliano Mantione <massi at ximian.com> wrote:
>
> On Thu, 2008-07-31 at 01:23 +0200, Zoltan Varga wrote:
>> With the linear IR, most things are easy, and only a few things are
>> hard, like optimizations which transform multiple operations into one,
>> like transforming a load+operation+store into an operating taking a
>> memory operand on x86.
>
> But I would add another thing: with the old representation that kind
> of transformations were easy only if the "expression" you wanted to
> transform was represented as a tree.
> If, for some reason, the operation was "linearized", using local
> variables as intermediate results instead of the tree nodes, then the
> optimization simply did not take place.
> Or the jit developer had to code twice: once for the tree case and
> once for the "linear" case.
>
> It all boils down to this: in a tree based representation, tree nodes
> are values, that ultimately need to be stored somewhere.
> But also local variables (assigned at the tree roots) contain values.
> So, if you have an optimization pass that deals with values, you must
> handle both locals and tree nodes, so your data structures get quirky
> and you end up with double codepaths for the same logic all over the
> place.
>
> I had a prototype for global value numbering and full alias analysis for
> the old jit (my work on HSSA before doing inlining), but the code was
> terrible because of this.
>
> Ciao,
>  Massi
>
>
>