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

[roger at eyesopen dot com]

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(:).

Whilst its true that we could hope for the tree-ssa optimizers to start
recognizing memset and memcpy idioms as an optimization pass, it makes
sense that the f90 array lowering/scalarizing machinery recognize these easy
cases itself.  I'd like to hope this idiom is common enough, that this
change improves some common benchmarks, but I've not done actual timings
myself.


The following patch has been tested on x86_64-unknown-linux-gnu with a full
"make bootstrap", including gfortran, and regression tested with a
top-level "make -k check" with no new failures.

Ok for mainline?

2006-12-18  Roger Sayle  <roger@eyesopen.com>

        * trans-expr.c (is_zero_initializer_p): Determine whether a given
        constant expression is a zero initializer.
        (gfc_trans_zero_assign): New function to attempt top optimize
        "a(:) = 0.0" as a call to __builtin_memset (a, 0, sizeof(a));
        (gfc_trans_assignment): Special case array assignments to zero
        initializer constants, using gfc_trans_zero_assign.

        * gfortran.dg/array_memset_1.f90: New test case.

Roger
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