[patch, fortran] Reduce stack use in blocked matmul

[Thomas Koenig]

Hi Andreas,

>> +      index_type t1_dim;
>> +      t1_dim = (a_dim1-1) * 256 + b_dim1;
>> +      if (t1_dim > 65536)
>> +	t1_dim = 65536;
>> +
>> +#pragma GCC diagnostic push
>> +#pragma GCC diagnostic ignored "-Wvla"
>> +      'rtype_name` t1[t1_dim]; /* was [256][256] */
> That does the wrong thing if b_dim1 == 0xDEADBEEF.
> 
> (gdb) p (a_dim1-1) * 256 + b_dim1
> $2 = -764456190

A look into the source code shows that b_dim1 is index_type,
which is 32 bits on 32-bit sytems and 64 bits on 64-bit system.

Now, consider if it is possible to declare an array on a 32-bit
system where the number of elements along one direction exceeds 2**31-1
(so sign extension would come into play), or if it would be
possible to declare an array on a 64-bit system where the number of
elements along one direction exceeds 2**63-1.

If you manage to come up with a legal Fortran testcas which
sets b_dim1 to 0xdeadbeef, I owe you a beer :-)

Regards

	Thomas