[Ada] Adjust growth factor from 1/32 to 1/2 for Unbounded_String

This will reduce significantly the number of allocations done when
doing consecutive append operations.

2018-07-16  Nicolas Roche  <roche@adacore.com>

gcc/ada/

	* libgnat/a-strunb.adb, libgnat/a-strunb__shared.adb: Adjust growth
	factor from 1/32 to 1/2 for Unbounded_String.

From-SVN: r262697

gcc/ada/ChangeLog

+2018-07-16  Nicolas Roche  <roche@adacore.com>
+
+	* libgnat/a-strunb.adb, libgnat/a-strunb__shared.adb: Adjust growth
+	factor from 1/32 to 1/2 for Unbounded_String.
+
 2018-07-13  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gcc-interface/lang.opt (funsigned-char): New option.

gcc/ada/libgnat/a-strunb.adb

@@ -763,13 +763,13 @@ package body Ada.Strings.Unbounded is
      (Source     : in out Unbounded_String;
       Chunk_Size : Natural)
    is
-      Growth_Factor : constant := 32;
+      Growth_Factor : constant := 2;
       --  The growth factor controls how much extra space is allocated when
       --  we have to increase the size of an allocated unbounded string. By
       --  allocating extra space, we avoid the need to reallocate on every
       --  append, particularly important when a string is built up by repeated
       --  append operations of small pieces. This is expressed as a factor so
-      --  32 means add 1/32 of the length of the string as growth space.
+      --  2 means add 1/2 of the length of the string as growth space.
 
       Min_Mul_Alloc : constant := Standard'Maximum_Alignment;
       --  Allocation will be done by a multiple of Min_Mul_Alloc This causes

gcc/ada/libgnat/a-strunb__shared.adb

@@ -36,13 +36,13 @@ package body Ada.Strings.Unbounded is
 
    use Ada.Strings.Maps;
 
-   Growth_Factor : constant := 32;
+   Growth_Factor : constant := 2;
    --  The growth factor controls how much extra space is allocated when
    --  we have to increase the size of an allocated unbounded string. By
    --  allocating extra space, we avoid the need to reallocate on every
    --  append, particularly important when a string is built up by repeated
    --  append operations of small pieces. This is expressed as a factor so
-   --  32 means add 1/32 of the length of the string as growth space.
+   --  2 means add 1/2 of the length of the string as growth space.
 
    Min_Mul_Alloc : constant := Standard'Maximum_Alignment;
    --  Allocation will be done by a multiple of Min_Mul_Alloc. This causes