Record the initial Map in the Store IC if it is not the same as the transitioned Map

Record the initial Map in the Store IC if it is not the same as the transitioned Map

[Jakob Kummerow, 2013-04-21 17:33:00]

Toon, this is your turf.

Weiliang, I think the code is working this way intentionally. Keeping an IC
monomorphic is generally desirable; going polymorphic unnecessarily can hurt
performance quite a bit. 
Do you have any particular case where the changed version works better? Have you
made sure that it doesn't cause regressions elsewhere?

[Toon Verwaest, 2013-04-22 17:28:39]

This is indeed working as intended. We'll avoid deopts by transitioning the
boilerplate. See mvstanton's upcoming work.

[weiliang.lin2, 2013-04-23 03:17:54]

On 2013/04/21 17:33:00, Jakob wrote:
> Toon, this is your turf.
> 
> Weiliang, I think the code is working this way intentionally. Keeping an IC
> monomorphic is generally desirable; going polymorphic unnecessarily can hurt
> performance quite a bit. 
> Do you have any particular case where the changed version works better? Have
you
> made sure that it doesn't cause regressions elsewhere?

Thanks Jakob, 
sunspider access-nsieve.js suffers from the intentional design. The example code
is as follow. 
function nsieve(m, isPrime){
   var i, k, count;

   for (i=2; i<=m; i++) { isPrime[i] = true; }
   count = 0;

   for (i=2; i<=m; i++){
      if (isPrime[i]) {
         for (k=i+i; k<=m; k+=i) isPrime[k] = false;
         count++;
      }
   }
   return count;
}
function sieve() {
    for (var i = 1; i <= 3; i++ ) {
        var m = (1<<i)*10000;
        var flags = Array(m+1);
        nsieve(m, flags);
    }
}

The assignment of "isPrime[i] = true" changes the isPrime's Map from
fast_holey_smi to fast_holey. Original code will keep monomorphic here  and
discard the initial fast_holey_smi Map. So crankshaft generates Map check
against only fast_holey. But it will deopt when next call of nsieve with
fast_holey_smi parameter. 

Yes, I will go to measure whether my fix will cause regression elsewhere.

[Toon Verwaest, 2013-04-23 08:02:52]

Honestly that single sunspider benchmark isn't pressing enough to land an
intermediate patch that will get reverted later anyway.