re PR fortran/82567 ([6/7/8] gfortran takes a long time to compile a simple implied-do with -Optimization.)

2017-10-18  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.

2017-10-18  Thomas Koenig  <tkoenig@gcc.gnu.org>

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

From-SVN: r253872

4 files changed, 62 insertions, 1 deletions

diff --git a/gcc/fortran/ChangeLog b/gcc/fortran/ChangeLog
index 8b9f90e..2d903e2 100644
--- a/gcc/fortran/ChangeLog
+++ b/gcc/fortran/ChangeLog
@@ -1,5 +1,13 @@
 2017-10-18  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.
+
+2017-10-18  Thomas Koenig  <tkoenig@gcc.gnu.org>
+
 	PR fortran/79795
 	* resolve.c (resovle_symbol): Change gcc_assert to
 	sensible error message.
diff --git a/gcc/fortran/frontend-passes.c b/gcc/fortran/frontend-passes.c
index ae4fba6..fcfaf95 100644
--- a/gcc/fortran/frontend-passes.c
+++ b/gcc/fortran/frontend-passes.c
@@ -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
+     and actually longer execution time, so a 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 4
 
   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))
     {
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a792c35..8e866af 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2017-10-18  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
+	PR fortran/82567
+	* gfortran.dg/array_constructor_51.f90: New test.
+
+2017-10-18  Thomas Koenig  <tkoenig@gcc.gnu.org>
+
 	PR fortran/79795
 	* gfortran.dg/assumed_size_2.f90: New test.
 
diff --git a/gcc/testsuite/gfortran.dg/array_constructor_51.f90 b/gcc/testsuite/gfortran.dg/array_constructor_51.f90
new file mode 100644
index 0000000..4c3cdf7
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/array_constructor_51.f90
@@ -0,0 +1,20 @@
+! { 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" } }