[Ada] Streamline comparison for equality of 2-element arrays

In the general case, the comparison for equality of array objects is
implemented by a local function that contains, among other things, a
loop running over the elements, comparing them one by one and exiting
as soon as an element is not the same in the two array objects.

For the specific case of constrained 2-element arrays, this is rather
heavy and unnecessarily obfuscates the control flow of the program,
so this change implements a simple conjunction of comparisons for it.

Running these commands:

  gcc -c p.ads -O -gnatD
  grep loop p.ads.dg

On the following sources:

package P is

  type Rec is record
    Re : Float;
    Im : Float;
  end record;

  type Arr is array (1 .. 2) of Rec;

  function Equal (A, B : Arr) return Boolean is (A = B);

end P;

Should execute silently.

2019-09-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch4.adb (Expand_Array_Equality): If optimization is
	enabled, generate a simple conjunction of comparisons for the
	specific case of constrained 1-dimensional 2-element arrays.
	Fix formatting.

From-SVN: r275941

gcc/ada/ChangeLog

+2019-09-19  Eric Botcazou  <ebotcazou@adacore.com>
+
+	* exp_ch4.adb (Expand_Array_Equality): If optimization is
+	enabled, generate a simple conjunction of comparisons for the
+	specific case of constrained 1-dimensional 2-element arrays.
+	Fix formatting.
+
 2019-09-19  Piotr Trojanek  <trojanek@adacore.com>
 
 	* exp_dbug.ads, exp_dbug.adb (Get_Homonym_Number): Remove.

gcc/ada/exp_ch4.adb

@@ -1582,7 +1582,7 @@ package body Exp_Ch4 is
       Index_List1 : constant List_Id    := New_List;
       Index_List2 : constant List_Id    := New_List;
 
-      Actuals   : List_Id;
+      First_Idx : Node_Id;
       Formals   : List_Id;
       Func_Name : Entity_Id;
       Func_Body : Node_Id;
@@ -1594,6 +1594,10 @@ package body Exp_Ch4 is
       Rtyp : Entity_Id;
       --  The parameter types to be used for the formals
 
+      New_Lhs : Node_Id;
+      New_Rhs : Node_Id;
+      --  The LHS and RHS converted to the parameter types
+
       function Arr_Attr
         (Arr : Entity_Id;
          Nam : Name_Id;
@@ -1962,6 +1966,82 @@ package body Exp_Ch4 is
          pragma Assert (Ltyp = Rtyp);
       end if;
 
+      --  If the array type is distinct from the type of the arguments, it
+      --  is the full view of a private type. Apply an unchecked conversion
+      --  to ensure that analysis of the code below succeeds.
+
+      if No (Etype (Lhs))
+        or else Base_Type (Etype (Lhs)) /= Base_Type (Ltyp)
+      then
+         New_Lhs := OK_Convert_To (Ltyp, Lhs);
+      else
+         New_Lhs := Lhs;
+      end if;
+
+      if No (Etype (Rhs))
+        or else Base_Type (Etype (Rhs)) /= Base_Type (Rtyp)
+      then
+         New_Rhs := OK_Convert_To (Rtyp, Rhs);
+      else
+         New_Rhs := Rhs;
+      end if;
+
+      First_Idx := First_Index (Ltyp);
+
+      --  If optimization is enabled and the array boils down to a couple of
+      --  consecutive elements, generate a simple conjunction of comparisons
+      --  which should be easier to optimize by the code generator.
+
+      if Optimization_Level > 0
+        and then Ltyp = Rtyp
+        and then Is_Constrained (Ltyp)
+        and then Number_Dimensions (Ltyp) = 1
+        and then Nkind (First_Idx) = N_Range
+        and then Compile_Time_Known_Value (Low_Bound (First_Idx))
+        and then Compile_Time_Known_Value (High_Bound (First_Idx))
+        and then Expr_Value (High_Bound (First_Idx)) =
+                                         Expr_Value (Low_Bound (First_Idx)) + 1
+      then
+         declare
+            Ctyp         : constant Entity_Id := Component_Type (Ltyp);
+            L, R         : Node_Id;
+            TestL, TestH : Node_Id;
+            Index_List   : List_Id;
+
+         begin
+            Index_List := New_List (New_Copy_Tree (Low_Bound (First_Idx)));
+
+            L :=
+              Make_Indexed_Component (Loc,
+                Prefix      => New_Copy_Tree (New_Lhs),
+                Expressions => Index_List);
+
+            R :=
+              Make_Indexed_Component (Loc,
+                Prefix      => New_Copy_Tree (New_Rhs),
+                Expressions => Index_List);
+
+            TestL := Expand_Composite_Equality (Nod, Ctyp, L, R, Bodies);
+
+            Index_List := New_List (New_Copy_Tree (High_Bound (First_Idx)));
+
+            L :=
+              Make_Indexed_Component (Loc,
+                Prefix      => New_Lhs,
+                Expressions => Index_List);
+
+            R :=
+              Make_Indexed_Component (Loc,
+                Prefix      => New_Rhs,
+                Expressions => Index_List);
+
+            TestH := Expand_Composite_Equality (Nod, Ctyp, L, R, Bodies);
+
+            return
+              Make_And_Then (Loc, Left_Opnd => TestL, Right_Opnd => TestH);
+         end;
+      end if;
+
       --  Build list of formals for function
 
       Formals := New_List (
@@ -2004,46 +2084,20 @@ package body Exp_Ch4 is
                     Make_Simple_Return_Statement (Loc,
                       Expression => New_Occurrence_Of (Standard_False, Loc)))),
 
-                Handle_One_Dimension (1, First_Index (Ltyp)),
+                Handle_One_Dimension (1, First_Idx),
 
                 Make_Simple_Return_Statement (Loc,
                   Expression => New_Occurrence_Of (Standard_True, Loc)))));
 
-         Set_Has_Completion (Func_Name, True);
-         Set_Is_Inlined (Func_Name);
-
-         --  If the array type is distinct from the type of the arguments, it
-         --  is the full view of a private type. Apply an unchecked conversion
-         --  to ensure that analysis of the call succeeds.
-
-         declare
-            L, R : Node_Id;
-
-         begin
-            L := Lhs;
-            R := Rhs;
-
-            if No (Etype (Lhs))
-              or else Base_Type (Etype (Lhs)) /= Base_Type (Ltyp)
-            then
-               L := OK_Convert_To (Ltyp, Lhs);
-            end if;
-
-            if No (Etype (Rhs))
-              or else Base_Type (Etype (Rhs)) /= Base_Type (Rtyp)
-            then
-               R := OK_Convert_To (Rtyp, Rhs);
-            end if;
-
-            Actuals := New_List (L, R);
-         end;
+      Set_Has_Completion (Func_Name, True);
+      Set_Is_Inlined (Func_Name);
 
-         Append_To (Bodies, Func_Body);
+      Append_To (Bodies, Func_Body);
 
-         return
-           Make_Function_Call (Loc,
-             Name                   => New_Occurrence_Of (Func_Name, Loc),
-             Parameter_Associations => Actuals);
+      return
+        Make_Function_Call (Loc,
+          Name                   => New_Occurrence_Of (Func_Name, Loc),
+          Parameter_Associations => New_List (New_Lhs, New_Rhs));
    end Expand_Array_Equality;
 
    -----------------------------