------------------------------------------------------------------------------ -- -- -- GNAT COMPILER COMPONENTS -- -- -- -- A D A . E X C E P T I O N S . E X C E P T I O N _ P R O P A G A T I O N -- -- -- -- B o d y -- -- -- -- Copyright (C) 1992-2004 Free Software Foundation, Inc. -- -- -- -- GNAT is free software; you can redistribute it and/or modify it under -- -- terms of the GNU General Public License as published by the Free Soft- -- -- ware Foundation; either version 2, or (at your option) any later ver- -- -- sion. GNAT is distributed in the hope that it will be useful, but WITH- -- -- OUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY -- -- or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License -- -- for more details. You should have received a copy of the GNU General -- -- Public License distributed with GNAT; see file COPYING. If not, write -- -- to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, -- -- MA 02111-1307, USA. -- -- -- -- As a special exception, if other files instantiate generics from this -- -- unit, or you link this unit with other files to produce an executable, -- -- this unit does not by itself cause the resulting executable to be -- -- covered by the GNU General Public License. This exception does not -- -- however invalidate any other reasons why the executable file might be -- -- covered by the GNU Public License. -- -- -- -- GNAT was originally developed by the GNAT team at New York University. -- -- Extensive contributions were provided by Ada Core Technologies Inc. -- -- -- ------------------------------------------------------------------------------ with Interfaces; with Ada.Unchecked_Conversion; with Ada.Unchecked_Deallocation; pragma Warnings (Off); -- Since several constructs give warnings in 3.14a1, including unreferenced -- variables and pragma Unreferenced itself. separate (Ada.Exceptions) package body Exception_Propagation is ------------------------------------------------ -- Entities to interface with the GCC runtime -- ------------------------------------------------ -- These come from "C++ ABI for Itanium: Exception handling", which is -- the reference for GCC. They are used only when we are relying on -- back-end tables for exception propagation, which in turn is currenly -- only the case for Zero_Cost_Exceptions in GNAT5. -- Return codes from the GCC runtime functions used to propagate -- an exception. type Unwind_Reason_Code is (URC_NO_REASON, URC_FOREIGN_EXCEPTION_CAUGHT, URC_PHASE2_ERROR, URC_PHASE1_ERROR, URC_NORMAL_STOP, URC_END_OF_STACK, URC_HANDLER_FOUND, URC_INSTALL_CONTEXT, URC_CONTINUE_UNWIND); pragma Unreferenced (URC_FOREIGN_EXCEPTION_CAUGHT, URC_PHASE2_ERROR, URC_PHASE1_ERROR, URC_NORMAL_STOP, URC_END_OF_STACK, URC_HANDLER_FOUND, URC_INSTALL_CONTEXT, URC_CONTINUE_UNWIND); pragma Convention (C, Unwind_Reason_Code); -- Phase identifiers type Unwind_Action is (UA_SEARCH_PHASE, UA_CLEANUP_PHASE, UA_HANDLER_FRAME, UA_FORCE_UNWIND); for Unwind_Action use (UA_SEARCH_PHASE => 1, UA_CLEANUP_PHASE => 2, UA_HANDLER_FRAME => 4, UA_FORCE_UNWIND => 8); pragma Convention (C, Unwind_Action); -- Mandatory common header for any exception object handled by the -- GCC unwinding runtime. subtype Exception_Class is Interfaces.Unsigned_64; GNAT_Exception_Class : constant Exception_Class := 16#474e552d41646100#; -- "GNU-Ada\0" type Unwind_Word is mod 2 ** System.Word_Size; for Unwind_Word'Size use System.Word_Size; -- Map the corresponding C type used in Unwind_Exception below. type Unwind_Exception is record Class : Exception_Class := GNAT_Exception_Class; Cleanup : System.Address := System.Null_Address; Private1 : Unwind_Word; Private2 : Unwind_Word; end record; -- Map the GCC struct used for exception handling. for Unwind_Exception'Alignment use Standard'Maximum_Alignment; -- The C++ ABI mandates the common exception header to be at least -- doubleword aligned, and the libGCC implementation actually makes it -- maximally aligned (see unwind.h). We need to match this because: -- 1/ We pass pointers to such headers down to the underlying -- libGCC unwinder, -- and -- 2/ The GNAT_GCC_Exception record below starts with this common -- common header and has a C counterpart which needs to be laid -- out identically in raise.c. If the alignment of the C and Ada -- common headers mismatch, their size may also differ, and the -- layouts may not match anymore. --------------------------------------------------------------- -- GNAT specific entities to deal with the GCC eh circuitry -- --------------------------------------------------------------- -- A GNAT exception object to be dealt with by the personality routine -- called by the GCC unwinding runtime. This structure shall match the -- one in raise.c and is currently experimental as it might be merged -- with the GNAT runtime definition some day. type GNAT_GCC_Exception is record Header : Unwind_Exception; -- ABI Exception header first. Id : Exception_Id; -- GNAT Exception identifier. This is used by the personality -- routine to determine if the context it examines contains a -- handler for the exception beeing propagated. N_Cleanups_To_Trigger : Integer; -- Number of cleanup only frames encountered in SEARCH phase. -- This is used to control the forced unwinding triggered when -- no handler has been found. Next_Exception : EOA; -- Used to create a linked list of exception occurrences. end record; pragma Convention (C, GNAT_GCC_Exception); type GNAT_GCC_Exception_Access is access all GNAT_GCC_Exception; function To_GNAT_GCC_Exception is new Unchecked_Conversion (System.Address, GNAT_GCC_Exception_Access); procedure Free is new Unchecked_Deallocation (GNAT_GCC_Exception, GNAT_GCC_Exception_Access); procedure Free is new Unchecked_Deallocation (Exception_Occurrence, EOA); function Remove (Top : EOA; Excep : GNAT_GCC_Exception_Access) return Boolean; -- Remove Excep from the stack starting at Top. -- Return True if Excep was found and removed, false otherwise. -- Hooks called when entering/leaving an exception handler for a given -- occurrence, aimed at handling the stack of active occurrences. The -- calls are generated by gigi in tree_transform/N_Exception_Handler. procedure Begin_Handler (GCC_Exception : GNAT_GCC_Exception_Access); pragma Export (C, Begin_Handler, "__gnat_begin_handler"); procedure End_Handler (GCC_Exception : GNAT_GCC_Exception_Access); pragma Export (C, End_Handler, "__gnat_end_handler"); function CleanupUnwind_Handler (UW_Version : Integer; UW_Phases : Unwind_Action; UW_Eclass : Exception_Class; UW_Exception : access GNAT_GCC_Exception; UW_Context : System.Address; UW_Argument : System.Address) return Unwind_Reason_Code; -- Hook called at each step of the forced unwinding we perform to -- trigger cleanups found during the propagation of an unhandled -- exception. -- GCC runtime functions used. These are C non-void functions, actually, -- but we ignore the return values. See raise.c as to why we are using -- __gnat stubs for these. procedure Unwind_RaiseException (UW_Exception : access GNAT_GCC_Exception); pragma Import (C, Unwind_RaiseException, "__gnat_Unwind_RaiseException"); procedure Unwind_ForcedUnwind (UW_Exception : access GNAT_GCC_Exception; UW_Handler : System.Address; UW_Argument : System.Address); pragma Import (C, Unwind_ForcedUnwind, "__gnat_Unwind_ForcedUnwind"); ------------------------------------------------------------ -- Accessors to basic components of a GNAT exception data -- ------------------------------------------------------------ -- As of today, these are only used by the C implementation of the -- propagation personality routine to avoid having to rely on a C -- counterpart of the whole exception_data structure, which is both -- painful and error prone. These subprograms could be moved to a -- more widely visible location if need be. function Is_Handled_By_Others (E : Exception_Data_Ptr) return Boolean; pragma Export (C, Is_Handled_By_Others, "__gnat_is_handled_by_others"); function Language_For (E : Exception_Data_Ptr) return Character; pragma Export (C, Language_For, "__gnat_language_for"); function Import_Code_For (E : Exception_Data_Ptr) return Exception_Code; pragma Export (C, Import_Code_For, "__gnat_import_code_for"); ------------ -- Remove -- ------------ function Remove (Top : EOA; Excep : GNAT_GCC_Exception_Access) return Boolean is Prev : GNAT_GCC_Exception_Access := null; Iter : EOA := Top; GCC_Exception : GNAT_GCC_Exception_Access; begin -- Pop stack loop pragma Assert (Iter.Private_Data /= System.Null_Address); GCC_Exception := To_GNAT_GCC_Exception (Iter.Private_Data); if GCC_Exception = Excep then if Prev = null then -- Special case for the top of the stack: shift the contents -- of the next item to the top, since top is at a fixed -- location and can't be changed. Iter := GCC_Exception.Next_Exception; if Iter = null then -- Stack is now empty Top.Private_Data := System.Null_Address; else Save_Occurrence_And_Private (Top.all, Iter.all); Free (Iter); end if; else Prev.Next_Exception := GCC_Exception.Next_Exception; Free (Iter); end if; Free (GCC_Exception); return True; end if; exit when GCC_Exception.Next_Exception = null; Prev := GCC_Exception; Iter := GCC_Exception.Next_Exception; end loop; return False; end Remove; --------------------------- -- CleanupUnwind_Handler -- --------------------------- function CleanupUnwind_Handler (UW_Version : Integer; UW_Phases : Unwind_Action; UW_Eclass : Exception_Class; UW_Exception : access GNAT_GCC_Exception; UW_Context : System.Address; UW_Argument : System.Address) return Unwind_Reason_Code is begin -- Terminate as soon as we know there is nothing more to run. The -- count is maintained by the personality routine. if UW_Exception.N_Cleanups_To_Trigger = 0 then Unhandled_Exception_Terminate; end if; -- We know there is at least one cleanup further up. Return so that it -- is searched and entered, after which Unwind_Resume will be called -- and this hook will gain control (with an updated count) again. return URC_NO_REASON; end CleanupUnwind_Handler; --------------------- -- Setup_Exception -- --------------------- -- Push the current exception occurrence on the stack before overriding it. procedure Setup_Exception (Excep : EOA; Current : EOA; Reraised : Boolean := False) is Top : constant EOA := Current; Next : EOA; GCC_Exception : GNAT_GCC_Exception_Access; -- Note that we make no use of the Reraised indication at this point. -- The information is still passed around just in case of future needs, -- since we've already switched between using/not-using it a number of -- times. begin -- If the current exception is not live, the stack is empty and there -- is nothing to do. Note that the stack always appears empty for -- mechanisms that do not require one. For the mechanism we implement -- in this unit, the initial Private_Data allocation for an occurrence -- is issued by Propagate_Exception. if Top.Private_Data = System.Null_Address then return; end if; -- Shift the contents of the Top of the stack in a freshly allocated -- entry, which leaves the room in the fixed Top entry available for the -- occurrence about to be propagated. Next := new Exception_Occurrence; Save_Occurrence_And_Private (Next.all, Top.all); -- Allocate Private_Data for the occurrence about to be propagated -- and link everything together. GCC_Exception := new GNAT_GCC_Exception; GCC_Exception.Next_Exception := Next; Top.Private_Data := GCC_Exception.all'Address; end Setup_Exception; ------------------- -- Begin_Handler -- ------------------- procedure Begin_Handler (GCC_Exception : GNAT_GCC_Exception_Access) is begin -- Every necessary operation related to the occurrence stack has -- already been performed by Propagate_Exception. This hook remains for -- potential future necessity in optimizing the overall scheme, as well -- a useful debugging tool. null; end Begin_Handler; ----------------- -- End_Handler -- ----------------- procedure End_Handler (GCC_Exception : GNAT_GCC_Exception_Access) is Removed : Boolean; begin Removed := Remove (Get_Current_Excep.all, GCC_Exception); pragma Assert (Removed); end End_Handler; ------------------------- -- Propagate_Exception -- ------------------------- -- Build an object suitable for the libgcc processing and call -- Unwind_RaiseException to actually throw, taking care of handling -- the two phase scheme it implements. procedure Propagate_Exception (From_Signal_Handler : Boolean) is Excep : EOA := Get_Current_Excep.all; GCC_Exception : GNAT_GCC_Exception_Access; begin if Excep.Private_Data = System.Null_Address then GCC_Exception := new GNAT_GCC_Exception; Excep.Private_Data := GCC_Exception.all'Address; else GCC_Exception := To_GNAT_GCC_Exception (Excep.Private_Data); end if; -- Fill in the useful flags for the personality routine called for each -- frame via Unwind_RaiseException below. GCC_Exception.Id := Excep.Id; GCC_Exception.N_Cleanups_To_Trigger := 0; -- Compute the backtrace for this occurrence if the corresponding -- binder option has been set. Call_Chain takes care of the reraise -- case. -- ??? Using Call_Chain here means we are going to walk up the stack -- once only for backtracing purposes before doing it again for the -- propagation per se. -- The first inspection is much lighter, though, as it only requires -- partial unwinding of each frame. Additionally, although we could use -- the personality routine to record the addresses while propagating, -- this method has two drawbacks: -- 1) the trace is incomplete if the exception is handled since we -- don't walk past the frame with the handler, -- and -- 2) we would miss the frames for which our personality routine is not -- called, e.g. if C or C++ calls are on the way. Call_Chain (Excep); -- Perform a standard raise first. If a regular handler is found, it -- will be entered after all the intermediate cleanups have run. If -- there is no regular handler, control will get back to after the -- call, with N_Cleanups_To_Trigger set to the number of frames with -- cleanups found on the way up, and none of these already run. Unwind_RaiseException (GCC_Exception); -- If we get here we know the exception is not handled, as otherwise -- Unwind_RaiseException arranges for the handler to be entered. Take -- the necessary steps to enable the debugger to gain control while the -- stack is still intact. Notify_Unhandled_Exception; -- Now, if cleanups have been found, run a forced unwind to trigger -- them. Control should not resume there, as the unwinding hook calls -- Unhandled_Exception_Terminate as soon as the last cleanup has been -- triggered. if GCC_Exception.N_Cleanups_To_Trigger /= 0 then Unwind_ForcedUnwind (GCC_Exception, CleanupUnwind_Handler'Address, System.Null_Address); end if; -- We get here when there is no handler or cleanup to be run at -- all. The debugger has been notified before the second step above. Unhandled_Exception_Terminate; end Propagate_Exception; --------------------- -- Import_Code_For -- --------------------- function Import_Code_For (E : SSL.Exception_Data_Ptr) return Exception_Code is begin return E.all.Import_Code; end Import_Code_For; -------------------------- -- Is_Handled_By_Others -- -------------------------- function Is_Handled_By_Others (E : SSL.Exception_Data_Ptr) return Boolean is begin return not E.all.Not_Handled_By_Others; end Is_Handled_By_Others; ------------------ -- Language_For -- ------------------ function Language_For (E : SSL.Exception_Data_Ptr) return Character is begin return E.all.Lang; end Language_For; ----------- -- Notes -- ----------- -- The current model implemented for the stack of occurrences is a -- simplification of previous attempts, which all prooved to be flawed or -- would have needed significant additional circuitry to be made to work -- correctly. -- We now represent every propagation by a new entry on the stack, which -- means that an exception occurrence may appear more than once (e.g. when -- it is reraised during the course of its own handler). -- This may seem overcostly compared to the C++ model as implemented in -- the g++ v3 libstd. This is actually understandable when one considers -- the extra variations of possible run-time configurations induced by the -- freedom offered by the Save_Occurrence/Reraise_Occurrence public -- interface. -- The basic point is that arranging for an occurrence to always appear at -- most once on the stack requires a way to determine if a given occurence -- is already there, which is not as easy as it might seem. -- An attempt was made to use the Private_Data pointer for this purpose. -- It did not work because: -- 1/ The Private_Data has to be saved by Save_Occurrence to be usable -- as a key in case of a later reraise, -- 2/ There is no easy way to synchronize End_Handler for an occurrence -- and the data attached to potential copies, so these copies may end -- up pointing to stale data. Moreover ... -- 3/ The same address may be reused for different occurrences, which -- defeats the idea of using it as a key. -- The example below illustrates: -- Saved_CE : Exception_Occurrence; -- -- begin -- raise Constraint_Error; -- exception -- when CE: others => -- Save_Occurrence (Saved_CE, CE); <= Saved_CE.PDA = CE.PDA -- end; -- -- <= Saved_CE.PDA is stale (!) -- -- begin -- raise Program_Error; <= Saved_CE.PDA = PE.PDA (!!) -- exception -- when others => -- Reraise_Occurrence (Saved_CE); -- end; -- Not releasing the Private_Data via End_Handler could be an option, -- but making this to work while still avoiding memory leaks is far -- from trivial. -- The current scheme has the advantage of beeing simple, and induces -- extra costs only in reraise cases which is acceptable. end Exception_Propagation;