diff --git a/gcc/fortran/ChangeLog b/gcc/fortran/ChangeLog
index cf74c46933e..d79478d949c 100644
--- a/gcc/fortran/ChangeLog
+++ b/gcc/fortran/ChangeLog
@@ -1,3 +1,11 @@
+2011-12-29  Thomas Koenig  <tkoenig@gcc.gnu.org>
+
+	* dependency.c (gfc_dep_compare_functions):  Document
+	new behavior for REALs and complex.  Add comment to cases
+	where only INTEGERs are handled.  Compare REAL and COMPLEX
+	constants, returning 0 and -2 only.  Add assert to make
+	sure that only integer constants are compared.
+
 2011-12-19  Tobias Burnus  <burnus@net-b.de>
 
 	PR fortran/51605
diff --git a/gcc/fortran/dependency.c b/gcc/fortran/dependency.c
index 96c4e5fd990..a2cf21d65f1 100644
--- a/gcc/fortran/dependency.c
+++ b/gcc/fortran/dependency.c
@@ -245,7 +245,9 @@ gfc_dep_compare_functions (gfc_expr *e1, gfc_expr *e2, bool impure_ok)
    * 0 if e1 == e2
    * -1 if e1 < e2
    * -2 if the relationship could not be determined
-   * -3 if e1 /= e2, but we cannot tell which one is larger.  */
+   * -3 if e1 /= e2, but we cannot tell which one is larger.
+   REAL and COMPLEX constants are only compared for equality
+   or inequality; if they are unequal, -2 is returned in all cases.  */
 
 int
 gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
@@ -303,7 +305,7 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
 
   if (e1->expr_type == EXPR_OP && e1->value.op.op == INTRINSIC_PLUS)
     {
-      /* Compare X+C vs. X.  */
+      /* Compare X+C vs. X, for INTEGER only.  */
       if (e1->value.op.op2->expr_type == EXPR_CONSTANT
 	  && e1->value.op.op2->ts.type == BT_INTEGER
 	  && gfc_dep_compare_expr (e1->value.op.op1, e2) == 0)
@@ -342,7 +344,7 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
 	}
     }
 
-  /* Compare X vs. X+C.  */
+  /* Compare X vs. X+C, for INTEGER only.  */
   if (e2->expr_type == EXPR_OP && e2->value.op.op == INTRINSIC_PLUS)
     {
       if (e2->value.op.op2->expr_type == EXPR_CONSTANT
@@ -351,7 +353,7 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
 	return -mpz_sgn (e2->value.op.op2->value.integer);
     }
 
-  /* Compare X-C vs. X.  */
+  /* Compare X-C vs. X, for INTEGER only.  */
   if (e1->expr_type == EXPR_OP && e1->value.op.op == INTRINSIC_MINUS)
     {
       if (e1->value.op.op2->expr_type == EXPR_CONSTANT
@@ -415,7 +417,7 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
 	}
     }
 
-  /* Compare X vs. X-C.  */
+  /* Compare X vs. X-C, for INTEGER only.  */
   if (e2->expr_type == EXPR_OP && e2->value.op.op == INTRINSIC_MINUS)
     {
       if (e2->value.op.op2->expr_type == EXPR_CONSTANT
@@ -434,9 +436,34 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
       if (e1->ts.type == BT_CHARACTER && e2->ts.type == BT_CHARACTER)
 	return gfc_compare_string (e1, e2);
 
+      /* Compare REAL and COMPLEX constants.  Because of the
+	 traps and pitfalls associated with comparing
+	 a + 1.0 with a + 0.5, check for equality only.  */
+      if (e2->expr_type == EXPR_CONSTANT)
+	{
+	  if (e1->ts.type == BT_REAL && e2->ts.type == BT_REAL)
+	    {
+	      if (mpfr_cmp (e1->value.real, e2->value.real) == 0)
+		return 0;
+	      else
+		return -2;
+	    }
+	  else if (e1->ts.type == BT_COMPLEX && e2->ts.type == BT_COMPLEX)
+	    {
+	      if (mpc_cmp (e1->value.complex, e2->value.complex) == 0)
+		return 0;
+	      else
+		return -2;
+	    }
+	}
+
       if (e1->ts.type != BT_INTEGER || e2->ts.type != BT_INTEGER)
 	return -2;
 
+      /* For INTEGER, all cases where e2 is not constant should have
+	 been filtered out above.  */
+      gcc_assert (e2->expr_type == EXPR_CONSTANT);
+
       i = mpz_cmp (e1->value.integer, e2->value.integer);
       if (i == 0)
 	return 0;
@@ -465,7 +492,7 @@ gfc_dep_compare_expr (gfc_expr *e1, gfc_expr *e2)
       else if (e1->value.op.op == INTRINSIC_TIMES
 	       && gfc_dep_compare_expr (e1->value.op.op1, e2->value.op.op2) == 0
 	       && gfc_dep_compare_expr (e1->value.op.op2, e2->value.op.op1) == 0)
-	/* Commutativity of multiplication.  */
+	/* Commutativity of multiplication; addition is handled above.  */
 	return 0;
 
       return -2;
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 732c7d1b18f..306356790a9 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2011-12-29  Thomas Koenig  <tkoenig@gcc.gnu.org>
+
+	* gfortran.dg/function_optimize_9.f90:  New test.
+
 2011-12-27  Fabien Chêne  <fabien@gcc.gnu.org>
 
 	PR c++/23211
diff --git a/gcc/testsuite/gfortran.dg/function_optimize_9.f90 b/gcc/testsuite/gfortran.dg/function_optimize_9.f90
new file mode 100644
index 00000000000..350ce9a8f66
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/function_optimize_9.f90
@@ -0,0 +1,23 @@
+! { dg-do compile }
+! { dg-options "-O -fdump-tree-original" }
+program main
+  integer, parameter :: n=100
+  real, parameter :: pi=4*atan(1.)
+  real, parameter :: tmax=20.
+  real, parameter :: dt = tmax/(2*pi)/real(n)
+  real, parameter :: t0 = dt/30.
+  integer :: i
+  interface
+     pure function purefunc(x)
+       real :: purefunc
+       real, intent(in) :: x
+     end function purefunc
+  end interface
+  real :: a(n)
+  do i=1,n
+     a(i) = purefunc(dt*i + t0) * 3. + 2 * purefunc(t0 + i*dt) 
+  end do
+  print *,a
+end program main
+! { dg-final { scan-tree-dump-times "purefunc" 1 "original" } }
+! { dg-final { cleanup-tree-dump "original" } }