[patch, fortran] Fix PR 82567

[Thomas Koenig <tkoenig@netcologne.de>]

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Hello world,

this patch fixes a regression with long compile times,
which came about due to our handling of array constructors
at compile time.  This, togeteher with a simplification in
front end optimization, led to long compile times and large
code.

Regression-tested. OK for trunk and the other affected branches?

Regards

	Thomas

2917-10-17  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/82567
         * frontend-passes.c (combine_array_constructor): If an array
         constructor is all constants and has more elements than a small
         constant, don't convert a*[b,c] to [a*b,a*c] to reduce compilation
         times.

2917-10-17  Thomas Koenig  <tkoenig@gcc.gnu.org>

         PR fortran/82567
         * gfortran.dg/array_constructor_51.f90: New test.

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! { dg-do compile }
! { dg-additional-options "-ffrontend-optimize -fdump-tree-original" }
! PR 82567 - long compile times caused by large constant constructors
! multiplied by variables

  SUBROUTINE sub()
  IMPLICIT NONE
  
  INTEGER, PARAMETER :: n = 1000
  REAL, ALLOCATABLE :: x(:)
  REAL :: xc, h
  INTEGER :: i
 
  ALLOCATE( x(n) )
  xc = 100.
  h = xc/n
  x = h*[(i,i=1,n)]
  
end
! { dg-final { scan-tree-dump-times "__var" 0 "original" } }

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Index: frontend-passes.c
===================================================================
--- frontend-passes.c	(Revision 253768)
+++ frontend-passes.c	(Arbeitskopie)
@@ -1635,6 +1635,8 @@ combine_array_constructor (gfc_expr *e)
   gfc_constructor *c, *new_c;
   gfc_constructor_base oldbase, newbase;
   bool scalar_first;
+  int n_elem;
+  bool all_const;
 
   /* Array constructors have rank one.  */
   if (e->rank != 1)
@@ -1674,12 +1676,38 @@ combine_array_constructor (gfc_expr *e)
   if (op2->ts.type == BT_CHARACTER)
     return false;
 
-  scalar = create_var (gfc_copy_expr (op2), "constr");
+  /* This might be an expanded constructor with very many constant values. If
+     we perform the operation here, we might end up with a long compile time,
+     so an arbitrary length bound is in order here.  If the constructor
+     constains something which is not a constant, it did not come from an
+     expansion, so leave it alone.  */
 
+#define CONSTR_LEN_MAX 42
+
   oldbase = op1->value.constructor;
+
+  n_elem = 0;
+  all_const = true;
+  for (c = gfc_constructor_first (oldbase); c; c = gfc_constructor_next(c))
+    {
+      if (c->expr->expr_type != EXPR_CONSTANT)
+	{
+	  all_const = false;
+	  break;
+	}
+      n_elem += 1;
+    }
+
+  if (all_const && n_elem > CONSTR_LEN_MAX)
+    return false;
+
+#undef CONSTR_LEN_MAX
+
   newbase = NULL;
   e->expr_type = EXPR_ARRAY;
 
+  scalar = create_var (gfc_copy_expr (op2), "constr");
+
   for (c = gfc_constructor_first (oldbase); c;
        c = gfc_constructor_next (c))
     {