[PATCH] Matrix Flattening and Transposing optimizations

Paolo Bonzini paolo.bonzini@lu.unisi.ch
Wed Mar 28 10:49:00 GMT 2007


>>     a = (int **)  malloc(dim1 * sizeof(int *));
>>     for (i=0; i<dim1; i++)
>>        a[i] = (int *) malloc (dim2 * sizeof(int));
> 
> Do people really do this...
> 
>>     a = (int *) malloc (dim1 * dim2 *sizeof(int));
> 
> ... as opposed to this?
> 
> In what sorts of contexts do you expect this to trigger?

Any code written:

1) for portability with MS-DOS, where you couldn't malloc more than 64kb
in one shot;

2) for compilers which could not do decent strength reduction, by people
who didn't bother doing the strength reduction themselves and just thought
"oh, it's faster this way"; double indirection is faster even with GCC at
-O0 (I didn't test -O2 -fno-ivopts or something like that).

3) by people who got accustomed to that idiom because of 1) or 2) above,
and didn't bother to revisit their assumptions.

Paolo



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