[patch, fortran] Move expressions from the mask in a forall header

[Thomas Koenig]

Hello world,

this patch moves expressions which do not depend on the
index variable(s) from FORALL headers (which also includes
DO CONCURRENT).

For the test case in do_concurrent_4.f90,

  do concurrent(i=1:n, a(i)>sum(a)/n)
    a(i) = a(i) * 0.5
  end do

Without the patch, this gets translated in a
straightforward manner to

  DO CONCURRENT main:i 1:10:1(> main:a(main:i) (/
_gfortran_sum_r4[[((main:a(FULL)) ((arg not-present)) ((arg
not-present)))]] 1.00000000e1))
  ASSIGN main:a(main:i) (* main:a(main:i) 5.00000000e-1)  END DO

With the patch and with front-end optimization on, this becomes

    ASSIGN block@7:__var_1 (/ _gfortran_sum_r4[[((main:a(FULL)) ((arg
not-present)) ((arg not-present)))]] 1.00000000e1)
    DO CONCURRENT main:i 1:10:1(> main:a(main:i) block@7:__var_1)
    ASSIGN main:a(main:i) (* main:a(main:i) 5.00000000e-1)    END DO

There is one fine point regarding the part of the patch used to check
if an expression is identical to the loop variable:

+  se = (*e)->symtree;
+
+  if (se == NULL)
+    return 0;
+
+  for (fa = (*current_code)->ext.forall_iterator; fa;
+       fa = fa->next)
+    {
+      if (se == fa->var->symtree)
+       return 1;
+    }
+  return 0;

Originally, this was

+  se = (*e)->symtree->n.sym;
+
+  for (fa = (*current_code)->ext.forall_iterator; fa; fa = fa->next)
+    {
+      si = fa->var->symtree->n.sym;
+      if (si == se)
+       return 1;
+    }
+

but this caused a regression in forall_5.f90 when
fa->var->symtree held the address 0x04 (which only
occurred when running the test suite).  I could not
figure out where this strange value was being generated,
so I setteled for comparing the symtree address instead
(and adding a NULL check just in case :-)

Regression-tested.  OK for trunk?

Regards

	Thomas

2014-08-17  Thomas Koenig  <tkoeng@gcc.gnu.org>

        PR fortran/60661
        * frontend-passes.c (optimize_forall_header):  Add prototype,
        new function.
        (optimize_code):  Call optimize_forall_header.
        (concurrent_iterator_check):   New function.
        (forall_header_varmove):  New function.

2014-08-17  Thomas Koenig  <tkoeng@gcc.gnu.org>

        PR fortran/60661
        * gfortran.dg/do_concurrent_4.f90:  New test.
        * gfortran.dg/do_concurrent_5.f90:  New test.

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