/* Implements exception handling. Copyright (C) 1989, 92-95, 1996 Free Software Foundation, Inc. Contributed by Mike Stump . This file is part of GNU CC. GNU CC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU CC is distributed in the hope that it will be useful, but WITHOUT 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 along with GNU CC; see the file COPYING. If not, write to the Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ /* An exception is an event that can be signaled from within a function. This event can then be "caught" or "trapped" by the callers of this function. This potentially allows program flow to be transferred to any arbitrary code assocated with a function call several levels up the stack. The intended use for this mechanism is for signaling "exceptional events" in an out-of-band fashion, hence its name. The C++ language (and many other OO-styled or functional languages) practically requires such a mechanism, as otherwise it becomes very difficult or even impossible to signal failure conditions in complex situations. The traditional C++ example is when an error occurs in the process of constructing an object; without such a mechanism, it is impossible to signal that the error occurs without adding global state variables and error checks around every object construction. The act of causing this event to occur is referred to as "throwing an exception". (Alternate terms include "raising an exception" or "signaling an exception".) The term "throw" is used because control is returned to the callers of the function that is signaling the exception, and thus there is the concept of "throwing" the exception up the call stack. It is appropriate to speak of the "context of a throw". This context refers to the address where the exception is thrown from, and is used to determine which exception region will handle the exception. Regions of code within a function can be marked such that if it contains the context of a throw, control will be passed to a designated "exception handler". These areas are known as "exception regions". Exception regions cannot overlap, but they can be nested to any arbitrary depth. Also, exception regions cannot cross function boundaries. Exception handlers can either be specified by the user (which we will call a "user-defined handler") or generated by the compiler (which we will designate as a "cleanup"). Cleanups are used to perform tasks such as destruction of objects allocated on the stack. In the current implementaion, cleanups are handled by allocating an exception region for the area that the cleanup is designated for, and the handler for the region performs the cleanup and then rethrows the exception to the outer exception region. From the standpoint of the current implementation, there is little distinction made between a cleanup and a user-defined handler, and the phrase "exception handler" can be used to refer to either one equally well. (The section "Future Directions" below discusses how this will change). Each object file that is compiled with exception handling contains a static array of exception handlers named __EXCEPTION_TABLE__. Each entry contains the starting and ending addresses of the exception region, and the address of the handler designated for that region. At program startup each object file invokes a function named __register_exceptions with the address of its local __EXCEPTION_TABLE__. __register_exceptions is defined in libgcc2.c, and is responsible for recording all of the exception regions into one list (which is kept in a static variable named exception_table_list). The function __throw () is actually responsible for doing the throw. In the C++ frontend, __throw () is generated on a per-object-file basis for each source file compiled with -fexceptions. Before __throw () is invoked, the current context of the throw needs to be placed in the global variable __eh_pc. __throw () attempts to find the appropriate exception handler for the PC value stored in __eh_pc by calling __find_first_exception_table_match (which is defined in libgcc2.c). If __find_first_exception_table_match finds a relevant handler, __throw jumps directly to it. If a handler for the context being thrown from can't be found, __throw is responsible for unwinding the stack, determining the address of the caller of the current function (which will be used as the new context to throw from), and then restarting the process of searching for a handler for the new context. __throw may also call abort () if it is unable to unwind the stack, and can also call an external library function named __terminate if it reaches the top of the stack without finding an appropriate handler. (By default __terminate () invokes abort (), but this behavior can be changed by the user to perform some sort of cleanup behavior before exiting). Internal implementation details: To associate a user-defined handler with a block of statements, the function expand_start_try_stmts () is used to mark the start of the block of statements with which the handler is to be associated (which is known as a "try block"). All statements that appear afterwards will be associated with the try block. A call to expand_start_all_catch () marks the end of the try block, and also marks the start of the "catch block" (the user-defined handler) associated with the try block. This user-defined handler will be invoked for *every* exception thrown with the context of the try block. It is up to the handler to decide whether or not it wishes to handle any given exception, as there is currently no mechanism in this implementation for doing this. (There are plans for conditionally processing an exception based on its "type", which will provide a language-independent mechanism). If the handler chooses not to process the exception (perhaps by looking at an "exception type" or some other additional data supplied with the exception), it can fall through to the end of the handler. expand_end_all_catch () and expand_leftover_cleanups () add additional code to the end of each handler to take care of rethrowing to the outer exception handler. The handler also has the option to continue with "normal flow of code", or in other words to resume executing at the statement immediately after the end of the exception region. The variable caught_return_label_stack contains a stack of labels, and jumping to the topmost entry's label via expand_goto () will resume normal flow to the statement immediately after the end of the exception region. If the handler falls through to the end, the exception will be rethrown to the outer exception region. The instructions for the catch block are kept as a separate sequence, and will be emitted at the end of the function along with the handlers specified via expand_eh_region_end (). The end of the catch block is marked with expand_end_all_catch (). Any data associated with the exception must currently be handled by some external mechanism maintained in the frontend. For example, the C++ exception mechanism passes an arbitrary value along with the exception, and this is handled in the C++ frontend by using a global variable to hold the value. (This will be changing in the future.) The mechanism in C++ for handling data associated with the exception is clearly not thread-safe. For a thread-based environment, another mechanism must be used (possibly using a per-thread allocation mechanism if the size of the area that needs to be allocated isn't known at compile time.) Internally-generated exception regions (cleanups) are marked by calling expand_eh_region_start () to mark the start of the region, and expand_eh_region_end (handler) is used to both designate the end of the region and to associate a specified handler/cleanup with the region. The rtl code in HANDLER will be invoked whenever an exception occurs in the region between the calls to expand_eh_region_start and expand_eh_region_end. After HANDLER is executed, additional code is emitted to handle rethrowing the exception to the outer exception handler. The code for HANDLER will be emitted at the end of the function. TARGET_EXPRs can also be used to designate exception regions. A TARGET_EXPR gives an unwind-protect style interface commonly used in functional languages such as LISP. The associated expression is evaluated, and whether or not it (or any of the functions that it calls) throws an exception, the protect expression is always invoked. This implementation takes care of the details of associating an exception table entry with the expression and generating the necessary code (it actually emits the protect expression twice, once for normal flow and once for the exception case). As for the other handlers, the code for the exception case will be emitted at the end of the function. Cleanups can also be specified by using add_partial_entry (handler) and end_protect_partials (). add_partial_entry creates the start of a new exception region; HANDLER will be invoked if an exception is thrown with the context of the region between the calls to add_partial_entry and end_protect_partials. end_protect_partials is used to mark the end of these regions. add_partial_entry can be called as many times as needed before calling end_protect_partials. However, end_protect_partials should only be invoked once for each group of calls to add_partial_entry () as the entries are queued and all of the outstanding entries are processed simultaneously when end_protect_partials is invoked. Similarly to the other handlers, the code for HANDLER will be emitted at the end of the function. The generated RTL for an exception region includes NOTE_INSN_EH_REGION_BEG and NOTE_INSN_EH_REGION_END notes that mark the start and end of the exception region. A unique label is also generated at the start of the exception region, which is available by looking at the ehstack variable. The topmost entry corresponds to the current region. In the current implementation, an exception can only be thrown from a function call (since the mechanism used to actually throw an exception involves calling __throw). If an exception region is created but no function calls occur within that region, the region can be safely optimized away (along with its exception handlers) since no exceptions can ever be caught in that region. Unwinding the stack: The details of unwinding the stack to the next frame can be rather complex. While in many cases a generic __unwind_function () routine can be used by the generated exception handling code to do this, it is often necessary to generate inline code to do the unwinding. Whether or not these inlined unwinders are necessary is target-specific. By default, if the target-specific backend doesn't supply a definition for __unwind_function (), inlined unwinders will be used instead. The main tradeoff here is in text space utilization. Obviously, if inline unwinders have to be generated repeatedly, this uses much more space than if a single routine is used. However, it is simply not possible on some platforms to write a generalized routine for doing stack unwinding without having some form of additional data associated with each function. The current implementation encodes this data in the form of additional machine instructions. This is clearly not desirable, as it is extremely inefficient. The next implementation will provide a set of metadata for each function that will provide the needed information. The backend macro DOESNT_NEED_UNWINDER is used to conditionalize whether or not per-function unwinders are needed. If DOESNT_NEED_UNWINDER is defined and has a non-zero value, a per-function unwinder is not emitted for the current function. On some platforms it is possible that neither __unwind_function () nor inlined unwinders are available. For these platforms it is not possible to throw through a function call, and abort () will be invoked instead of performing the throw. Future directions: Currently __throw () makes no differentiation between cleanups and user-defined exception regions. While this makes the implementation simple, it also implies that it is impossible to determine if a user-defined exception handler exists for a given exception without completely unwinding the stack in the process. This is undesirable from the standpoint of debugging, as ideally it would be possible to trap unhandled exceptions in the debugger before the process of unwinding has even started. This problem can be solved by marking user-defined handlers in a special way (probably by adding additional bits to exception_table_list). A two-pass scheme could then be used by __throw () to iterate through the table. The first pass would search for a relevant user-defined handler for the current context of the throw, and if one is found, the second pass would then invoke all needed cleanups before jumping to the user-defined handler. Many languages (including C++ and Ada) make execution of a user-defined handler conditional on the "type" of the exception thrown. (The type of the exception is actually the type of the data that is thrown with the exception.) It will thus be necessary for __throw () to be able to determine if a given user-defined exception handler will actually be executed, given the type of exception. One scheme is to add additional information to exception_table_list as to the types of exceptions accepted by each handler. __throw () can do the type comparisons and then determine if the handler is actually going to be executed. There is currently no significant level of debugging support available, other than to place a breakpoint on __throw (). While this is sufficient in most cases, it would be helpful to be able to know where a given exception was going to be thrown to before it is actually thrown, and to be able to choose between stopping before every exception region (including cleanups), or just user-defined exception regions. This should be possible to do in the two-pass scheme by adding additional labels to __throw () for appropriate breakpoints, and additional debugger commands could be added to query various state variables to determine what actions are to be performed next. Another major problem that is being worked on is the issue with stack unwinding on various platforms. Currently the only platform that has support for __unwind_function () is the Sparc; all other ports require per-function unwinders, which causes large amounts of code bloat. Ideally it would be possible to store a small set of metadata with each function that would then make it possible to write a __unwind_function () for every platform. This would eliminate the need for per-function unwinders. The main reason the data is needed is that on some platforms the order and types of data stored on the stack can vary depending on the type of function, its arguments and returned values, and the compilation options used (optimization versus non-optimization, -fomit-frame-pointer, processor variations, etc). Unfortunately, this also means that throwing through functions that aren't compiled with exception handling support will still not be possible on some platforms. This problem is currently being investigated, but no solutions have been found that do not imply some unacceptable performance penalties. */ #include "config.h" #include #include "rtl.h" #include "tree.h" #include "flags.h" #include "except.h" #include "function.h" #include "insn-flags.h" #include "expr.h" #include "insn-codes.h" #include "regs.h" #include "hard-reg-set.h" #include "insn-config.h" #include "recog.h" #include "output.h" #include "assert.h" /* A list of labels used for exception handlers. Created by find_exception_handler_labels for the optimization passes. */ rtx exception_handler_labels; /* Nonzero means that __throw was invoked. This is used by the C++ frontend to know if code needs to be emitted for __throw or not. */ int throw_used; /* A stack used for keeping track of the currectly active exception handling region. As each exception region is started, an entry describing the region is pushed onto this stack. The current region can be found by looking at the top of the stack, and as we exit regions, the corresponding entries are popped. Entries cannot overlap; they must be nested. So there is only one entry at most that corresponds to the current instruction, and that is the entry on the top of the stack. */ struct eh_stack ehstack; /* A queue used for tracking which exception regions have closed but whose handlers have not yet been expanded. Regions are emitted in groups in an attempt to improve paging performance. As we exit a region, we enqueue a new entry. The entries are then dequeued during expand_leftover_cleanups () and expand_start_all_catch (), We should redo things so that we either take RTL for the handler, or we expand the handler expressed as a tree immediately at region end time. */ struct eh_queue ehqueue; /* Insns for all of the exception handlers for the current function. They are currently emitted by the frontend code. */ rtx catch_clauses; /* A TREE_CHAINed list of handlers for regions that are not yet closed. The TREE_VALUE of each entry contains the handler for the corresponding entry on the ehstack. */ static tree protect_list; /* Stacks to keep track of various labels. */ /* Keeps track of the label to resume to should one want to resume normal control flow out of a handler (instead of, say, returning to the caller of the current function or exiting the program). Also used as the context of a throw to rethrow an exception to the outer exception region. */ struct label_node *caught_return_label_stack = NULL; /* A random data area for the front end's own use. */ struct label_node *false_label_stack = NULL; /* The rtx and the tree for the saved PC value. */ rtx eh_saved_pc_rtx; tree eh_saved_pc; rtx expand_builtin_return_addr PROTO((enum built_in_function, int, rtx)); /* Various support routines to manipulate the various data structures used by the exception handling code. */ /* Push a label entry onto the given STACK. */ void push_label_entry (stack, rlabel, tlabel) struct label_node **stack; rtx rlabel; tree tlabel; { struct label_node *newnode = (struct label_node *) xmalloc (sizeof (struct label_node)); if (rlabel) newnode->u.rlabel = rlabel; else newnode->u.tlabel = tlabel; newnode->chain = *stack; *stack = newnode; } /* Pop a label entry from the given STACK. */ rtx pop_label_entry (stack) struct label_node **stack; { rtx label; struct label_node *tempnode; if (! *stack) return NULL_RTX; tempnode = *stack; label = tempnode->u.rlabel; *stack = (*stack)->chain; free (tempnode); return label; } /* Return the top element of the given STACK. */ tree top_label_entry (stack) struct label_node **stack; { if (! *stack) return NULL_TREE; return (*stack)->u.tlabel; } /* Make a copy of ENTRY using xmalloc to allocate the space. */ static struct eh_entry * copy_eh_entry (entry) struct eh_entry *entry; { struct eh_entry *newentry; newentry = (struct eh_entry *) xmalloc (sizeof (struct eh_entry)); bcopy ((char *) entry, (char *) newentry, sizeof (struct eh_entry)); return newentry; } /* Push a new eh_node entry onto STACK, and return the start label for the entry. */ static rtx push_eh_entry (stack) struct eh_stack *stack; { struct eh_node *node = (struct eh_node *) xmalloc (sizeof (struct eh_node)); struct eh_entry *entry = (struct eh_entry *) xmalloc (sizeof (struct eh_entry)); entry->start_label = gen_label_rtx (); entry->end_label = gen_label_rtx (); entry->exception_handler_label = gen_label_rtx (); entry->finalization = NULL_TREE; node->entry = entry; node->chain = stack->top; stack->top = node; return entry->start_label; } /* Pop an entry from the given STACK. */ static struct eh_entry * pop_eh_entry (stack) struct eh_stack *stack; { struct eh_node *tempnode; struct eh_entry *tempentry; tempnode = stack->top; tempentry = tempnode->entry; stack->top = stack->top->chain; free (tempnode); return tempentry; } /* Enqueue an ENTRY onto the given QUEUE. */ static void enqueue_eh_entry (queue, entry) struct eh_queue *queue; struct eh_entry *entry; { struct eh_node *node = (struct eh_node *) xmalloc (sizeof (struct eh_node)); node->entry = entry; node->chain = NULL; if (queue->head == NULL) { queue->head = node; } else { queue->tail->chain = node; } queue->tail = node; } /* Dequeue an entry from the given QUEUE. */ static struct eh_entry * dequeue_eh_entry (queue) struct eh_queue *queue; { struct eh_node *tempnode; struct eh_entry *tempentry; if (queue->head == NULL) return NULL; tempnode = queue->head; queue->head = queue->head->chain; tempentry = tempnode->entry; free (tempnode); return tempentry; } /* Routine to see if exception exception handling is turned on. DO_WARN is non-zero if we want to inform the user that exception handling is turned off. This is used to ensure that -fexceptions has been specified if the compiler tries to use any exception-specific functions. */ int doing_eh (do_warn) int do_warn; { if (! flag_exceptions) { static int warned = 0; if (! warned && do_warn) { error ("exception handling disabled, use -fexceptions to enable"); warned = 1; } return 0; } return 1; } /* Given a return address in ADDR, determine the address we should use to find the corresponding EH region. */ rtx eh_outer_context (addr) rtx addr; { /* First mask out any unwanted bits. */ #ifdef MASK_RETURN_ADDR emit_insn (gen_rtx (SET, Pmode, addr, gen_rtx (AND, Pmode, addr, MASK_RETURN_ADDR))); #endif /* Then subtract out enough to get into the appropriate region. If this is defined, assume we don't need to subtract anything as it is already within the correct region. */ #if ! defined (RETURN_ADDR_OFFSET) addr = plus_constant (addr, -1); #endif return addr; } /* Start a new exception region and push the HANDLER for the region onto protect_list. All of the regions created with add_partial_entry will be ended when end_protect_partials () is invoked. */ void add_partial_entry (handler) tree handler; { expand_eh_region_start (); /* Make sure the entry is on the correct obstack. */ push_obstacks_nochange (); resume_temporary_allocation (); protect_list = tree_cons (NULL_TREE, handler, protect_list); pop_obstacks (); } /* Output a note marking the start of an exception handling region. All instructions emitted after this point are considered to be part of the region until expand_eh_region_end () is invoked. */ void expand_eh_region_start () { rtx note; /* This is the old code. */ if (! doing_eh (0)) return; #if 0 /* Maybe do this to prevent jumping in and so on... */ pushlevel (0); #endif note = emit_note (NULL_PTR, NOTE_INSN_EH_REGION_BEG); emit_label (push_eh_entry (&ehstack)); NOTE_BLOCK_NUMBER (note) = CODE_LABEL_NUMBER (ehstack.top->entry->exception_handler_label); } /* Output a note marking the end of the exception handling region on the top of ehstack. HANDLER is either the cleanup for the exception region, or if we're marking the end of a try block, HANDLER is integer_zero_node. HANDLER will be transformed to rtl when expand_leftover_cleanups () is invoked. */ void expand_eh_region_end (handler) tree handler; { rtx note; struct eh_entry *entry; if (! doing_eh (0)) return; entry = pop_eh_entry (&ehstack); note = emit_note (NULL_PTR, NOTE_INSN_EH_REGION_END); NOTE_BLOCK_NUMBER (note) = CODE_LABEL_NUMBER (entry->exception_handler_label); /* Emit a label marking the end of this exception region. */ emit_label (entry->end_label); /* Put in something that takes up space, as otherwise the end address for this EH region could have the exact same address as its outer region. This would cause us to miss the fact that resuming exception handling with this PC value would be inside the outer region. */ emit_insn (gen_nop ()); entry->finalization = handler; enqueue_eh_entry (&ehqueue, entry); #if 0 /* Maybe do this to prevent jumping in and so on... */ poplevel (1, 0, 0); #endif } /* Emit a call to __throw and note that we threw something, so we know we need to generate the necessary code for __throw. Before invoking throw, the __eh_pc variable must have been set up to contain the PC being thrown from. This address is used by __throw () to determine which exception region (if any) is responsible for handling the exception. */ static void emit_throw () { #ifdef JUMP_TO_THROW emit_indirect_jump (throw_libfunc); #else SYMBOL_REF_USED (throw_libfunc) = 1; emit_library_call (throw_libfunc, 0, VOIDmode, 0); #endif throw_used = 1; emit_barrier (); } /* An internal throw with an indirect CONTEXT we want to throw from. CONTEXT evaluates to the context of the throw. */ static void expand_internal_throw_indirect (context) rtx context; { assemble_external (eh_saved_pc); emit_move_insn (eh_saved_pc_rtx, context); emit_throw (); } /* An internal throw with a direct CONTEXT we want to throw from. CONTEXT must be a label; its address will be used as the context of the throw. */ void expand_internal_throw (context) rtx context; { expand_internal_throw_indirect (gen_rtx (LABEL_REF, Pmode, context)); } /* Called from expand_exception_blocks and expand_end_catch_block to emit any pending handlers/cleanups queued from expand_eh_region_end (). */ void expand_leftover_cleanups () { struct eh_entry *entry; while ((entry = dequeue_eh_entry (&ehqueue)) != 0) { rtx prev; /* A leftover try block. Shouldn't be one here. */ if (entry->finalization == integer_zero_node) abort (); /* Output the label for the start of the exception handler. */ emit_label (entry->exception_handler_label); /* And now generate the insns for the handler. */ expand_expr (entry->finalization, const0_rtx, VOIDmode, 0); prev = get_last_insn (); if (! (prev && GET_CODE (prev) == BARRIER)) { /* The below can be optimized away, and we could just fall into the next EH handler, if we are certain they are nested. */ /* Emit code to throw to the outer context if we fall off the end of the handler. */ expand_internal_throw (entry->end_label); } free (entry); } } /* Called at the start of a block of try statements. */ void expand_start_try_stmts () { if (! doing_eh (1)) return; expand_eh_region_start (); } /* Generate RTL for the start of a group of catch clauses. It is responsible for starting a new instruction sequence for the instructions in the catch block, and expanding the handlers for the internally-generated exception regions nested within the try block corresponding to this catch block. */ void expand_start_all_catch () { struct eh_entry *entry; tree label; if (! doing_eh (1)) return; /* End the try block. */ expand_eh_region_end (integer_zero_node); emit_line_note (input_filename, lineno); label = build_decl (LABEL_DECL, NULL_TREE, NULL_TREE); /* The label for the exception handling block that we will save. This is Lresume in the documention. */ expand_label (label); /* Put in something that takes up space, as otherwise the end address for the EH region could have the exact same address as the outer region, causing us to miss the fact that resuming exception handling with this PC value would be inside the outer region. */ emit_insn (gen_nop ()); /* Push the label that points to where normal flow is resumed onto the top of the label stack. */ push_label_entry (&caught_return_label_stack, NULL_RTX, label); /* Start a new sequence for all the catch blocks. We will add this to the global sequence catch_clauses when we have completed all the handlers in this handler-seq. */ start_sequence (); while (1) { rtx prev; entry = dequeue_eh_entry (&ehqueue); /* Emit the label for the exception handler for this region, and expand the code for the handler. Note that a catch region is handled as a side-effect here; for a try block, entry->finalization will contain integer_zero_node, so no code will be generated in the expand_expr call below. But, the label for the handler will still be emitted, so any code emitted after this point will end up being the handler. */ emit_label (entry->exception_handler_label); expand_expr (entry->finalization, const0_rtx, VOIDmode, 0); /* When we get down to the matching entry for this try block, stop. */ if (entry->finalization == integer_zero_node) { /* Don't forget to free this entry. */ free (entry); break; } prev = get_last_insn (); if (prev == NULL || GET_CODE (prev) != BARRIER) { /* Code to throw out to outer context when we fall off end of the handler. We can't do this here for catch blocks, so it's done in expand_end_all_catch () instead. The below can be optimized away (and we could just fall into the next EH handler) if we are certain they are nested. */ expand_internal_throw (entry->end_label); } free (entry); } } /* Finish up the catch block. At this point all the insns for the catch clauses have already been generated, so we only have to add them to the catch_clauses list. We also want to make sure that if we fall off the end of the catch clauses that we rethrow to the outer EH region. */ void expand_end_all_catch () { rtx new_catch_clause; if (! doing_eh (1)) return; /* Code to throw out to outer context, if we fall off end of catch handlers. This is rethrow (Lresume, same id, same obj) in the documentation. We use Lresume because we know that it will throw to the correct context. In other words, if the catch handler doesn't exit or return, we do a "throw" (using the address of Lresume as the point being thrown from) so that the outer EH region can then try to process the exception. */ expand_internal_throw (DECL_RTL (top_label_entry (&caught_return_label_stack))); /* Now we have the complete catch sequence. */ new_catch_clause = get_insns (); end_sequence (); /* This level of catch blocks is done, so set up the successful catch jump label for the next layer of catch blocks. */ pop_label_entry (&caught_return_label_stack); /* Add the new sequence of catches to the main one for this function. */ push_to_sequence (catch_clauses); emit_insns (new_catch_clause); catch_clauses = get_insns (); end_sequence (); /* Here we fall through into the continuation code. */ } /* End all the pending exception regions on protect_list. The handlers will be emitted when expand_leftover_cleanups () is invoked. */ void end_protect_partials () { while (protect_list) { expand_eh_region_end (TREE_VALUE (protect_list)); protect_list = TREE_CHAIN (protect_list); } } /* The exception table that we build that is used for looking up and dispatching exceptions, the current number of entries, and its maximum size before we have to extend it. The number in eh_table is the code label number of the exception handler for the region. This is added by add_eh_table_entry () and used by output_exception_table_entry (). */ static int *eh_table; static int eh_table_size; static int eh_table_max_size; /* Note the need for an exception table entry for region N. If we don't need to output an explicit exception table, avoid all of the extra work. Called from final_scan_insn when a NOTE_INSN_EH_REGION_BEG is seen. N is the NOTE_BLOCK_NUMBER of the note, which comes from the code label number of the exception handler for the region. */ void add_eh_table_entry (n) int n; { #ifndef OMIT_EH_TABLE if (eh_table_size >= eh_table_max_size) { if (eh_table) { eh_table_max_size += eh_table_max_size>>1; if (eh_table_max_size < 0) abort (); if ((eh_table = (int *) realloc (eh_table, eh_table_max_size * sizeof (int))) == 0) fatal ("virtual memory exhausted"); } else { eh_table_max_size = 252; eh_table = (int *) xmalloc (eh_table_max_size * sizeof (int)); } } eh_table[eh_table_size++] = n; #endif } /* Return a non-zero value if we need to output an exception table. On some platforms, we don't have to output a table explicitly. This routine doesn't mean we don't have one. */ int exception_table_p () { if (eh_table) return 1; return 0; } /* Output the entry of the exception table corresponding to to the exception region numbered N to file FILE. N is the code label number corresponding to the handler of the region. */ static void output_exception_table_entry (file, n) FILE *file; int n; { char buf[256]; rtx sym; ASM_GENERATE_INTERNAL_LABEL (buf, "LEHB", n); sym = gen_rtx (SYMBOL_REF, Pmode, buf); assemble_integer (sym, POINTER_SIZE / BITS_PER_UNIT, 1); ASM_GENERATE_INTERNAL_LABEL (buf, "LEHE", n); sym = gen_rtx (SYMBOL_REF, Pmode, buf); assemble_integer (sym, POINTER_SIZE / BITS_PER_UNIT, 1); ASM_GENERATE_INTERNAL_LABEL (buf, "L", n); sym = gen_rtx (SYMBOL_REF, Pmode, buf); assemble_integer (sym, POINTER_SIZE / BITS_PER_UNIT, 1); putc ('\n', file); /* blank line */ } /* Output the exception table if we have and need one. */ void output_exception_table () { int i; extern FILE *asm_out_file; if (! doing_eh (0)) return; exception_section (); /* Beginning marker for table. */ assemble_align (GET_MODE_ALIGNMENT (ptr_mode)); assemble_label ("__EXCEPTION_TABLE__"); assemble_integer (const0_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); assemble_integer (const0_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); assemble_integer (const0_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); putc ('\n', asm_out_file); /* blank line */ for (i = 0; i < eh_table_size; ++i) output_exception_table_entry (asm_out_file, eh_table[i]); free (eh_table); /* Ending marker for table. */ assemble_label ("__EXCEPTION_END__"); assemble_integer (constm1_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); assemble_integer (constm1_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); assemble_integer (constm1_rtx, POINTER_SIZE / BITS_PER_UNIT, 1); putc ('\n', asm_out_file); /* blank line */ } /* Generate code to initialize the exception table at program startup time. */ void register_exception_table () { emit_library_call (gen_rtx (SYMBOL_REF, Pmode, "__register_exceptions"), 0, VOIDmode, 1, gen_rtx (SYMBOL_REF, Pmode, "__EXCEPTION_TABLE__"), Pmode); } /* Emit the RTL for the start of the per-function unwinder for the current function. See emit_unwinder () for further information. DOESNT_NEED_UNWINDER is a target-specific macro that determines if the current function actually needs a per-function unwinder or not. By default, all functions need one. */ void start_eh_unwinder () { #ifdef DOESNT_NEED_UNWINDER if (DOESNT_NEED_UNWINDER) return; #endif expand_eh_region_start (); } /* Emit insns for the end of the per-function unwinder for the current function. */ void end_eh_unwinder () { tree expr; rtx return_val_rtx, ret_val, label, end, insns; if (! doing_eh (0)) return; #ifdef DOESNT_NEED_UNWINDER if (DOESNT_NEED_UNWINDER) return; #endif assemble_external (eh_saved_pc); expr = make_node (RTL_EXPR); TREE_TYPE (expr) = void_type_node; RTL_EXPR_RTL (expr) = const0_rtx; TREE_SIDE_EFFECTS (expr) = 1; start_sequence_for_rtl_expr (expr); /* ret_val will contain the address of the code where the call to the current function occurred. */ ret_val = expand_builtin_return_addr (BUILT_IN_RETURN_ADDRESS, 0, hard_frame_pointer_rtx); return_val_rtx = copy_to_reg (ret_val); /* Get the address we need to use to determine what exception handler should be invoked, and store it in __eh_pc. */ return_val_rtx = eh_outer_context (return_val_rtx); emit_move_insn (eh_saved_pc_rtx, return_val_rtx); /* Either set things up so we do a return directly to __throw, or we return here instead. */ #ifdef JUMP_TO_THROW emit_move_insn (ret_val, throw_libfunc); #else label = gen_label_rtx (); emit_move_insn (ret_val, gen_rtx (LABEL_REF, Pmode, label)); #endif #ifdef RETURN_ADDR_OFFSET return_val_rtx = plus_constant (ret_val, -RETURN_ADDR_OFFSET); if (return_val_rtx != ret_val) emit_move_insn (ret_val, return_val_rtx); #endif end = gen_label_rtx (); emit_jump (end); RTL_EXPR_SEQUENCE (expr) = get_insns (); end_sequence (); expand_eh_region_end (expr); emit_jump (end); #ifndef JUMP_TO_THROW emit_label (label); emit_throw (); #endif expand_leftover_cleanups (); emit_label (end); } /* If necessary, emit insns for the per function unwinder for the current function. Called after all the code that needs unwind protection is output. The unwinder takes care of catching any exceptions that have not been previously caught within the function, unwinding the stack to the next frame, and rethrowing using the address of the current function's caller as the context of the throw. On some platforms __throw can do this by itself (or with the help of __unwind_function) so the per-function unwinder is unnecessary. We cannot place the unwinder into the function until after we know we are done inlining, as we don't want to have more than one unwinder per non-inlined function. */ void emit_unwinder () { rtx insns, insn; start_sequence (); start_eh_unwinder (); insns = get_insns (); end_sequence (); /* We place the start of the exception region associated with the per function unwinder at the top of the function. */ if (insns) emit_insns_after (insns, get_insns ()); start_sequence (); end_eh_unwinder (); insns = get_insns (); end_sequence (); /* And we place the end of the exception region before the USE and CLOBBER insns that may come at the end of the function. */ if (insns == 0) return; insn = get_last_insn (); while (GET_CODE (insn) == NOTE || (GET_CODE (insn) == INSN && (GET_CODE (PATTERN (insn)) == USE || GET_CODE (PATTERN (insn)) == CLOBBER))) insn = PREV_INSN (insn); if (GET_CODE (insn) == CODE_LABEL && GET_CODE (PREV_INSN (insn)) == BARRIER) { insn = PREV_INSN (insn); } else { rtx label = gen_label_rtx (); emit_label_after (label, insn); insn = emit_jump_insn_after (gen_jump (label), insn); insn = emit_barrier_after (insn); } emit_insns_after (insns, insn); } /* Scan the current insns and build a list of handler labels. The resulting list is placed in the global variable exception_handler_labels. It is called after the last exception handling region is added to the current function (when the rtl is almost all built for the current function) and before the jump optimization pass. */ void find_exception_handler_labels () { rtx insn; int max_labelno = max_label_num (); int min_labelno = get_first_label_num (); rtx *labels; exception_handler_labels = NULL_RTX; /* If we aren't doing exception handling, there isn't much to check. */ if (! doing_eh (0)) return; /* Generate a handy reference to each label. */ labels = (rtx *) alloca ((max_labelno - min_labelno) * sizeof (rtx)); bzero ((char *) labels, (max_labelno - min_labelno) * sizeof (rtx)); /* Arrange for labels to be indexed directly by CODE_LABEL_NUMBER. */ labels -= min_labelno; for (insn = get_insns (); insn; insn = NEXT_INSN (insn)) { if (GET_CODE (insn) == CODE_LABEL) if (CODE_LABEL_NUMBER (insn) >= min_labelno && CODE_LABEL_NUMBER (insn) < max_labelno) labels[CODE_LABEL_NUMBER (insn)] = insn; } /* For each start of a region, add its label to the list. */ for (insn = get_insns (); insn; insn = NEXT_INSN (insn)) { if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_BEG) { rtx label = NULL_RTX; if (NOTE_BLOCK_NUMBER (insn) >= min_labelno && NOTE_BLOCK_NUMBER (insn) < max_labelno) { label = labels[NOTE_BLOCK_NUMBER (insn)]; if (label) exception_handler_labels = gen_rtx (EXPR_LIST, VOIDmode, label, exception_handler_labels); else warning ("didn't find handler for EH region %d", NOTE_BLOCK_NUMBER (insn)); } else warning ("mismatched EH region %d", NOTE_BLOCK_NUMBER (insn)); } } } /* Perform sanity checking on the exception_handler_labels list. Can be called after find_exception_handler_labels is called to build the list of exception handlers for the current function and before we finish processing the current function. */ void check_exception_handler_labels () { rtx insn, handler; /* If we aren't doing exception handling, there isn't much to check. */ if (! doing_eh (0)) return; /* Ensure that the CODE_LABEL_NUMBER for the CODE_LABEL entry point in each handler corresponds to the CODE_LABEL_NUMBER of the handler. */ for (handler = exception_handler_labels; handler; handler = XEXP (handler, 1)) { for (insn = get_insns (); insn; insn = NEXT_INSN (insn)) { if (GET_CODE (insn) == CODE_LABEL) { if (CODE_LABEL_NUMBER (insn) == CODE_LABEL_NUMBER (XEXP (handler, 0))) { if (insn != XEXP (handler, 0)) warning ("mismatched handler %d", CODE_LABEL_NUMBER (insn)); break; } } } if (insn == NULL_RTX) warning ("handler not found %d", CODE_LABEL_NUMBER (XEXP (handler, 0))); } /* Now go through and make sure that for each region there is a corresponding label. */ for (insn = get_insns (); insn; insn = NEXT_INSN (insn)) { if (GET_CODE (insn) == NOTE && (NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_BEG || NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_END)) { for (handler = exception_handler_labels; handler; handler = XEXP (handler, 1)) { if (CODE_LABEL_NUMBER (XEXP (handler, 0)) == NOTE_BLOCK_NUMBER (insn)) break; } if (handler == NULL_RTX) warning ("region exists, no handler %d", NOTE_BLOCK_NUMBER (insn)); } } } /* This group of functions initializes the exception handling data structures at the start of the compilation, initializes the data structures at the start of a function, and saves and restores the exception handling data structures for the start/end of a nested function. */ /* Toplevel initialization for EH things. */ void init_eh () { /* Generate rtl to reference the variable in which the PC of the current context is saved. */ tree type = build_pointer_type (make_node (VOID_TYPE)); eh_saved_pc = build_decl (VAR_DECL, get_identifier ("__eh_pc"), type); DECL_EXTERNAL (eh_saved_pc) = 1; TREE_PUBLIC (eh_saved_pc) = 1; make_decl_rtl (eh_saved_pc, NULL_PTR, 1); eh_saved_pc_rtx = DECL_RTL (eh_saved_pc); } /* Initialize the per-function EH information. */ void init_eh_for_function () { ehstack.top = 0; ehqueue.head = ehqueue.tail = 0; catch_clauses = NULL_RTX; false_label_stack = 0; caught_return_label_stack = 0; protect_list = NULL_TREE; } /* Save some of the per-function EH info into the save area denoted by P. This is currently called from save_stmt_status (). */ void save_eh_status (p) struct function *p; { assert (p != NULL); p->ehstack = ehstack; p->ehqueue = ehqueue; p->catch_clauses = catch_clauses; p->false_label_stack = false_label_stack; p->caught_return_label_stack = caught_return_label_stack; p->protect_list = protect_list; init_eh (); } /* Restore the per-function EH info saved into the area denoted by P. This is currently called from restore_stmt_status. */ void restore_eh_status (p) struct function *p; { assert (p != NULL); protect_list = p->protect_list; caught_return_label_stack = p->caught_return_label_stack; false_label_stack = p->false_label_stack; catch_clauses = p->catch_clauses; ehqueue = p->ehqueue; ehstack = p->ehstack; } /* This section is for the exception handling specific optimization pass. First are the internal routines, and then the main optimization pass. */ /* Determine if the given INSN can throw an exception. */ static int can_throw (insn) rtx insn; { /* Calls can always potentially throw exceptions. */ if (GET_CODE (insn) == CALL_INSN) return 1; #ifdef ASYNCH_EXCEPTIONS /* If we wanted asynchronous exceptions, then everything but NOTEs and CODE_LABELs could throw. */ if (GET_CODE (insn) != NOTE && GET_CODE (insn) != CODE_LABEL) return 1; #endif return 0; } /* Scan a exception region looking for the matching end and then remove it if possible. INSN is the start of the region, N is the region number, and DELETE_OUTER is to note if anything in this region can throw. Regions are removed if they cannot possibly catch an exception. This is determined by invoking can_throw () on each insn within the region; if can_throw returns true for any of the instructions, the region can catch an exception, since there is an insn within the region that is capable of throwing an exception. Returns the NOTE_INSN_EH_REGION_END corresponding to this region, or calls abort () if it can't find one. Can abort if INSN is not a NOTE_INSN_EH_REGION_BEGIN, or if N doesn't correspond to the region number, or if DELETE_OUTER is NULL. */ static rtx scan_region (insn, n, delete_outer) rtx insn; int n; int *delete_outer; { rtx start = insn; /* Assume we can delete the region. */ int delete = 1; assert (insn != NULL_RTX && GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_BEG && NOTE_BLOCK_NUMBER (insn) == n && delete_outer != NULL); insn = NEXT_INSN (insn); /* Look for the matching end. */ while (! (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_END)) { /* If anything can throw, we can't remove the region. */ if (delete && can_throw (insn)) { delete = 0; } /* Watch out for and handle nested regions. */ if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_BEG) { insn = scan_region (insn, NOTE_BLOCK_NUMBER (insn), &delete); } insn = NEXT_INSN (insn); } /* The _BEG/_END NOTEs must match and nest. */ if (NOTE_BLOCK_NUMBER (insn) != n) abort (); /* If anything in this exception region can throw, we can throw. */ if (! delete) *delete_outer = 0; else { /* Delete the start and end of the region. */ delete_insn (start); delete_insn (insn); /* Only do this part if we have built the exception handler labels. */ if (exception_handler_labels) { rtx x, *prev = &exception_handler_labels; /* Find it in the list of handlers. */ for (x = exception_handler_labels; x; x = XEXP (x, 1)) { rtx label = XEXP (x, 0); if (CODE_LABEL_NUMBER (label) == n) { /* If we are the last reference to the handler, delete it. */ if (--LABEL_NUSES (label) == 0) delete_insn (label); if (optimize) { /* Remove it from the list of exception handler labels, if we are optimizing. If we are not, then leave it in the list, as we are not really going to remove the region. */ *prev = XEXP (x, 1); XEXP (x, 1) = 0; XEXP (x, 0) = 0; } break; } prev = &XEXP (x, 1); } } } return insn; } /* Perform various interesting optimizations for exception handling code. We look for empty exception regions and make them go (away). The jump optimization code will remove the handler if nothing else uses it. */ void exception_optimize () { rtx insn, regions = NULL_RTX; int n; /* Remove empty regions. */ for (insn = get_insns (); insn; insn = NEXT_INSN (insn)) { if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) == NOTE_INSN_EH_REGION_BEG) { /* Since scan_region () will return the NOTE_INSN_EH_REGION_END insn, we will indirectly skip through all the insns inbetween. We are also guaranteed that the value of insn returned will be valid, as otherwise scan_region () won't return. */ insn = scan_region (insn, NOTE_BLOCK_NUMBER (insn), &n); } } }