[webkit-dev] Load elimination

[Peter Siket]

Sorry!  I sent this to the wrong mail list but I still welcome any 
opinions about it. :)

Peter

Peter Siket wrote:
> Hi All!
>
> I would like to create a small filter which eliminates unnecessary 
> LIR_ld instructions. It seems that there are several unnecessary
> loads which can not be simply eliminated by the CSE filter due to its 
> elimination is not as simple as the elimination of other CSE
> instructions like LIR_add.
>
> For example:
>     class C {
>         var x : int  = 5;
>         var y : int = 7;
>     }
>
>     var o : C = new C();
>
>     function f(o) {
>         var j = 0;
>         for (var i = 0; i < 20; i++) {
>             j += o.x + o.y + (o.x + o.y) * (o.x + o.y);
>         }
>         return j;
>     }
>
>    
> The values of o.x and o.y are loaded every time when they are used but 
> only one of them would be enough.
> Since the loads are done by different LIR instructions the add 
> instructions will also be executed three times.
> If we could eliminate the loads the CSE filter would eliminate the 
> second and third adds too.
>
> In the first implementation of the filter I check only the LIR_ld 
> instructions created by the getslotvalue
> function of the Interpreter and all the store insertions. Between the 
> first and the second loads of o.x all the
> possible stores, which can modify the value of o.x should be checked. If 
> anything modified the o.x between the two
> loads than the second load could not be eliminated.
>
> It passes on almost all of the acceptance tests but fails in 28 cases. 
> It seems that not only the store instructions
> but some IL primitive functions can also modify the objects so that is 
> why it fails in test. If I handled this situation
> too conservatively and let the elimination of a load instruction only if 
> there were no call instruction between the
> first and the second load than there would be almost nothing to eliminate.
>
> I think that it could work if I could decrease the number of possible 
> primitive-function calls when the load must not be
> eliminated but unfortunately I am not yet familiar with the tamarin in 
> detail to see everything.
>
> I would welcome any opinion, observation, comment or anything about this. :)
>
> Thanks,
>    Peter Siket
>  
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