This patch modifies the management of secondary stack within transient blocks
by allowing "relaxed management" to be the default. In this mode a block will
not manage the secondary stack if there exists an enclosing scope which already
does that. "Strict management" can be enforced by means of switch -gnatd.s. No
need for a test as Q111-032 already performs all necessary checks.
Tested on x86_64-pc-linux-gnu, committed on trunk
2017-04-27 Hristian Kirtchev <kirtc...@adacore.com>
* debug.adb Change the documentation of switch -gnatd.s.
* exp_ch7.adb (Make_Transient_Block): Transient blocks do not need
to manage the secondary stack when an enclosing scope already
performs this functionality (aka relaxed management). Switch
-gnatd.s may be used to force strict management in which case
the block will manage the secondary stack unconditionally. Add
a guard to stop the traversal when encountering a package or a
subprogram scope.
Index: exp_ch7.adb
===================================================================
--- exp_ch7.adb (revision 247295)
+++ exp_ch7.adb (working copy)
@@ -8275,31 +8275,27 @@
function Manages_Sec_Stack (Id : Entity_Id) return Boolean is
begin
- -- An exception handler with a choice parameter utilizes a dummy
- -- block to provide a declarative region. Such a block should not be
- -- considered because it never manifests in the tree and can never
- -- release the secondary stack.
+ case Ekind (Id) is
- if Ekind (Id) = E_Block
- and then Uses_Sec_Stack (Id)
- and then not Is_Exception_Handler (Id)
- then
- return True;
+ -- An exception handler with a choice parameter utilizes a dummy
+ -- block to provide a declarative region. Such a block should not
+ -- be considered because it never manifests in the tree and can
+ -- never release the secondary stack.
- -- Loops are intentionally excluded because they undergo special
- -- treatment, see Establish_Transient_Scope.
+ when E_Block =>
+ return
+ Uses_Sec_Stack (Id) and then not Is_Exception_Handler (Id);
- elsif Ekind_In (Id, E_Entry,
- E_Entry_Family,
- E_Function,
- E_Procedure)
- and then Uses_Sec_Stack (Id)
- then
- return True;
+ when E_Entry
+ | E_Entry_Family
+ | E_Function
+ | E_Procedure
+ =>
+ return Uses_Sec_Stack (Id);
- else
- return False;
- end if;
+ when others =>
+ return False;
+ end case;
end Manages_Sec_Stack;
-- Local variables
@@ -8326,14 +8322,11 @@
Scop := Scope (Trans_Id);
while Present (Scop) loop
- if Scop = Standard_Standard then
- exit;
- -- The transient block must manage the secondary stack when the
- -- block appears within a loop in order to reclaim the memory at
- -- each iteration.
+ -- It should not be possible to reach Standard without hitting one
+ -- of the other cases first unless Standard was manually pushed.
- elsif Ekind (Scop) = E_Loop then
+ if Scop = Standard_Standard then
exit;
-- The transient block is within a function which returns on the
@@ -8351,15 +8344,36 @@
Set_Uses_Sec_Stack (Trans_Id, False);
exit;
- -- When requested, the transient block does not need to manage the
- -- secondary stack when there exists an enclosing block, entry,
- -- entry family, function, or a procedure which already does that.
+ -- The transient block must manage the secondary stack when the
+ -- block appears within a loop in order to reclaim the memory at
+ -- each iteration.
+
+ elsif Ekind (Scop) = E_Loop then
+ exit;
+
+ -- The transient block does not need to manage the secondary stack
+ -- when there is an enclosing construct which already does that.
-- This optimization saves on SS_Mark and SS_Release calls but may
-- allow objects to live a little longer than required.
- elsif Debug_Flag_Dot_S and then Manages_Sec_Stack (Scop) then
+ -- The transient block must manage the secondary stack when switch
+ -- -gnatd.s (strict management) is in effect.
+
+ elsif Manages_Sec_Stack (Scop) and then not Debug_Flag_Dot_S then
Set_Uses_Sec_Stack (Trans_Id, False);
exit;
+
+ -- Prevent the search from going too far because transient blocks
+ -- are bounded by packages and subprogram scopes.
+
+ elsif Ekind_In (Scop, E_Entry,
+ E_Entry_Family,
+ E_Function,
+ E_Package,
+ E_Procedure,
+ E_Subprogram_Body)
+ then
+ exit;
end if;
Scop := Scope (Scop);
Index: debug.adb
===================================================================
--- debug.adb (revision 247295)
+++ debug.adb (working copy)
@@ -109,7 +109,7 @@
-- d.p Use original Ada 95 semantics for Bit_Order (disable AI95-0133)
-- d.q Suppress optimizations on imported 'in'
-- d.r Enable OK_To_Reorder_Components in non-variant records
- -- d.s Minimize secondary stack Mark and Release calls
+ -- d.s Strict secondary stack management
-- d.t Disable static allocation of library level dispatch tables
-- d.u Enable Modify_Tree_For_C (update tree for c)
-- d.v Enable OK_To_Reorder_Components in variant records
@@ -572,6 +572,11 @@
-- d.r Forces the flag OK_To_Reorder_Components to be set in all record
-- base types that have no discriminants.
+ -- d.s The compiler no longer attempts to optimize the calls to secondary
+ -- stack management routines SS_Mark and SS_Release. As a result, each
+ -- transient block tasked with secondary stack management will fulfill
+ -- its role unconditionally.
+
-- d.s The compiler does not generate calls to secondary stack management
-- routines SS_Mark and SS_Release for a transient block when there is
-- an enclosing scoping construct which already manages the secondary
