[Ada] Compiler speedup with inlining across units

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This change is aimed at speeding up the inlining across units done by
the Ada compiler when -gnatn is specified and in the presence of units
instantiating a lot of generic packages.

The current implementation is as follows: when a generic package is
being instantiated, the compiler scans its spec for the presence of
subprograms with an aspect/pragma Inline and, upon finding one,
schedules the instantiation of its body.  That's not very efficient
because the compiler doesn't know yet if one of those inlined
subprograms will eventually be called from the main unit.

The new implementation arranges for the compiler to instantiate the body
on demand, i.e. when it encounters a call to one of the inlined
subprograms.  That's still not optimal because, at this point, the
compiler has not yet computed whether the call itself is reachable from
the main unit (it will do this computation at the very end of the
processing, just before sending the inlined units to the code generator)
but that's nevertheless a net progress.

The patch also enhances the -gnatd.j option to make it output the list
of instances "inlined" this way.  The following package is a simple
example:

with Q;

procedure P is
begin
  Q.Proc;
end;

package Q is

  procedure Proc;
  pragma Inline (Proc);

end Q;

with G;

package body Q is

  package My_G is new G (1);

  procedure Proc is
    Val : constant Integer := My_G.Func;
  begin
    if Val /= 1 then
      raise Program_Error;
    end if;
  end;

end Q;

generic

  Value : Integer;

package G is

  function Func return Integer;
  pragma Inline (Func);

end G;

package body G is

  function Func return Integer is
  begin
    return Value;
  end;

end G;

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-08-14  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (Is_Called): Document new usage on E_Package
	entities.
	* einfo.adb (Is_Called): Accept E_Package entities.
	(Set_Is_Called): Likewise.
	* exp_ch6.adb (Expand_Call_Helper): Move code dealing with
	instances for back-end inlining to Add_Inlined_Body.
	* inline.ads: Remove with clauses for Alloc and Table.
	(Pending_Instantiations): Move to...
	* inline.adb: Add with clauses for Alloc, Uintp, Table and
	GNAT.HTable.
	(Backend_Instances): New variable.
	(Pending_Instantiations): ...here.
	(Called_Pending_Instantiations): New table.
	(Node_Table_Size): New constant.
	(Node_Header_Num): New subtype.
	(Node_Hash): New function.
	(To_Pending_Instantiations): New hash table.
	(Add_Inlined_Body): Bail out early for subprograms in the main
	unit or subunit.  Likewise if the Is_Called flag is set.  If the
	subprogram is an instance, invoke Add_Inlined_Instance.  Call
	Set_Is_Called earlier.  If the subrogram is within an instance,
	invoke Add_Inlined_Instance.  Also deal with the case where the
	call itself is within an instance.
	(Add_Inlined_Instance): New procedure.
	(Add_Inlined_Subprogram): Remove conditions always fulfilled.
	(Add_Pending_Instantiation): Move the defence against ludicruous
	number of instantiations to here. When back-end inlining is
	enabled, associate an instantiation with its index in table and
	mark a few selected kinds of instantiations as always needed.
	(Initialize): Set Backend_Instances to No_Elist.
	(Instantiate_Body): New procedure doing the work extracted
	from...
	(Instantiate_Bodies): ...here.  When back-end inlining is
	enabled, loop over Called_Pending_Instantiations instead of
	Pending_Instantiations.
	(Is_Nested): Minor tweak.
	(List_Inlining_Info): Also list the contents of
	Backend_Instances.
	* sem_ch12.adb (Might_Inline_Subp): Return early if Is_Inlined
	is set and otherwise set it before returning true.
	(Analyze_Package_Instantiation): Remove the defence against
	ludicruous number of instantiations.  Invoke
	Remove_Dead_Instance instead of doing the removal manually if
	there is a guaranteed ABE.

Attachment: patch.diff.gz
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