[fortran PATCH] Implement a(:,:) = 0.0 using memset

[Tim Prince]

roger@eyesopen.com wrote:
> The following patch makes use of the recently added gfc_full_array_ref_p
> function to provide the optimization of using memset when assigning an
> entire array to zero.  Currently, the source code below:
> 
>   integer :: a(20)
>   a(:) = 0;
> 
> we currently generate the following with -fdump-tree-original
> 
>   int8 S.0;
> 
>   S.0 = 1;
>   while (1)
>     {
>       if (S.0 > 20) goto L.1; else (void) 0;
>       (*a)[NON_LVALUE_EXPR <S.0> + -1] = 0;
>       S.0 = S.0 + 1;
>     }
>   L.1:;
> 
> with the patch below, we now generate this instead.
> 
>   (void) __builtin_memset ((void *) a, 0, 80);
> 
> 
> This can then take advantage of GCC's intrinsic expansion machinery,
> including Jan's recent improvements for x86.  I'm keen to hear if there
> are any corner cases that I've overlooked and aren't covered by the
> gfortran
> testsuite.  Perhaps if someone could run NIST, polyhedron and the usual
> suspects to confirm there are no issues.
> 
> Once this is in the tree, and there are no major issues, there are some
> obvious extensions and improvements that can be made a follow-up patches:
> [1] Avoid using memset for small array sizes, such that the tree-ssa
> optimizers would unroll the loop and reveal the assignments via SRA.
> [2] Allow reverse order initialization, such as a(20:1:-1) = 0.
> [3] Extend the infrastructure to support sequentially consecutive
> assignments that don't cover the entire array a(20:40) = 0.0.
> [4] Extend infrastructure for arbitrary (run-time) length expressions,
> such as a(1:n) = 0.0.
> [5] Generalize this optimization to use memcpy (or memmove?) for array
> assignments, a(:) = b(:).
> 

What can builtin_memset() do better than vectorized code, on platforms 
which matter to you?  Non-temporal store would be advantageous when the 
array is larger than cache size, or will never be referenced while still 
in cache, but will builtin_memset() be able to make such decisions 
efficiently, if at all?