[Ada] Get rid of linear searches in Lib

[Ada] Get rid of linear searches in Lib

This change is aimed at removing a couple of linear searches in the
units management code that can become problematic performance-wise when
the number of loaded units is in the several hundreds, which can happen
for large files even at -O0 without any inlining.

It introduces an auxiliary hash table to record a mapping between the
name of units and their entry in the units table, and then replaces the
linear searches by lookups in this names table.  This can save up to 2%
of the compilation time spent in the front-end in some cases.

There should be no functional changes, except in the error message
issued for circular unit dependencies in very peculiar and convoluted
cases.

2019-08-20  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* lib.ads: Add with clause for GNAT.HTable.
	Add pragma Inline for Is_Loaded and alphabetize the list.
	(Unit_Name_Table_Size): New constant.
	(Unit_Name_Header_Num): New subtype.
	(Unit_Name_Hash): New function declaration.
	(Unit_Names): New simple hash table.
	(Init_Unit_Name): New procedure declaration.
	* lib.adb (Set_Unit_Name): Unregister the old name in the table,
	if any, and then register the new name.
	(Init_Unit_Name): New procedure.
	(Is_Loaded): Reimplement using a lookup in the names table.
	(Remove_Unit): Unregister the name.
	(Unit_Name_Hash): New function.
	* lib-load.adb (Create_Dummy_Package_Unit): Call Init_Unit_Name.
	(Load_Unit): Use a lookup in the names table to find out whether
	the unit has already been loaded.  Call Init_Unit_Name and then
	Remove_Unit if the loading has failed.
	(Make_Child_Decl_Unit): Call Init_Unit_Name.
	(Make_Instance_Unit): Likewise.
	* lib-writ.adb (Ensure_System_Dependency): Likewise.

From-SVN: r274720