[PATCH] [LAA] Merge memchecks for accesses separated by a constant offset

[silviu.baranga at arm.com]

Hi Adam,

Sorry for delaying the commit, but I have a last minute change for this. It seems that this change would cause a regression in the TSVC/Symbolics-flt benchmark.

The regression happens when executing a code that looks like this:

struct Data{

  int a[LEN];
  int B[LEN];

};

void test(int m) {

  for (int i = 0; i < m; ++i)
    Data.a[i + m] = Data.a[i] + Data.b[i];

}

a[i] and b[i] get merged according to the algorithm. However the resulting interval covers a[i + m], and the runtime check fails.
I've limited the merging algorithm to only merge when the distance to the max/min is less then a constant (I've chosen 512 for this), which should prevent this issues from happening (at least for loops with more than 512 iterations). Does this sound reasonable?

That benchmark was the only regression that I've seen (lnt, spec2k,spec2k), but I've not seen any improvements either - so either further tuning is required or we would need to turn on the interleaved accesses to see any gains.

Thanks,
Silviu


http://reviews.llvm.org/D10386

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