gxxint.texi

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Unlike the others, this ambiguity is not recognized by the Working Paper.

@node  Copying Objects, Exception Handling, Parser, Top
@section Copying Objects

The generated copy assignment operator in g++ does not currently do the
right thing for multiple inheritance involving virtual bases; it just
calls the copy assignment operators for its direct bases.  What it
should probably do is:

1) Split up the copy assignment operator for all classes that have
vbases into "copy my vbases" and "copy everything else" parts.  Or do
the trickiness that the constructors do to ensure that vbases don't get
initialized by intermediate bases.

2) Wander through the class lattice, find all vbases for which no
intermediate base has a user-defined copy assignment operator, and call
their "copy everything else" routines.  If not all of my vbases satisfy
this criterion, warn, because this may be surprising behavior.

3) Call the "copy everything else" routine for my direct bases.

If we only have one direct base, we can just foist everything off onto
them.

This issue is currently under discussion in the core reflector
(2/28/94).

@node  Exception Handling, Free Store, Copying Objects, Top
@section Exception Handling

Note, exception handling in g++ is still under development.  

This section describes the mapping of C++ exceptions in the C++
front-end, into the back-end exception handling framework.

The basic mechanism of exception handling in the back-end is
unwind-protect a la elisp.  This is a general, robust, and language
independent representation for exceptions.

The C++ front-end exceptions are mapping into the unwind-protect
semantics by the C++ front-end.  The mapping is describe below.

When -frtti is used, rtti is used to do exception object type checking,
when it isn't used, the encoded name for the type of the object being
thrown is used instead.  All code that originates exceptions, even code
that throws exceptions as a side effect, like dynamic casting, and all
code that catches exceptions must be compiled with either -frtti, or
-fno-rtti.  It is not possible to mix rtti base exception handling
objects with code that doesn't use rtti.  The exceptions to this, are
code that doesn't catch or throw exceptions, catch (...), and code that
just rethrows an exception.

Currently we use the normal mangling used in building functions names
(int's are "i", const char * is PCc) to build the non-rtti base type
descriptors for exception handling.  These descriptors are just plain
NULL terminated strings, and internally they are passed around as char
*.

In C++, all cleanups should be protected by exception regions.  The
region starts just after the reason why the cleanup is created has
ended.  For example, with an automatic variable, that has a constructor,
it would be right after the constructor is run.  The region ends just
before the finalization is expanded.  Since the backend may expand the
cleanup multiple times along different paths, once for normal end of the
region, once for non-local gotos, once for returns, etc, the backend
must take special care to protect the finalization expansion, if the
expansion is for any other reason than normal region end, and it is
`inline' (it is inside the exception region).  The backend can either
choose to move them out of line, or it can created an exception region
over the finalization to protect it, and in the handler associated with
it, it would not run the finalization as it otherwise would have, but
rather just rethrow to the outer handler, careful to skip the normal
handler for the original region.

In Ada, they will use the more runtime intensive approach of having
fewer regions, but at the cost of additional work at run time, to keep a
list of things that need cleanups.  When a variable has finished
construction, they add the cleanup to the list, when the come to the end
of the lifetime of the variable, the run the list down.  If the take a
hit before the section finishes normally, they examine the list for
actions to perform.  I hope they add this logic into the back-end, as it
would be nice to get that alternative approach in C++.

On an rs6000, xlC stores exception objects on that stack, under the try
block.  When is unwinds down into a handler, the frame pointer is
adjusted back to the normal value for the frame in which the handler
resides, and the stack pointer is left unchanged from the time at which
the object was thrown.  This is so that there is always someplace for
the exception object, and nothing can overwrite it, once we start
throwing.  The only bad part, is that the stack remains large.

The below points out some things that work in g++'s exception handling.

All completely constructed temps and local variables are cleaned up in
all unwinded scopes.  Completely constructed parts of partially
constructed objects are cleaned up.  This includes partially built
arrays.  Exception specifications are now handled.  Thrown objects are
now cleaned up all the time.  We can now tell if we have an active
exception being thrown or not (__eh_type != 0).  We use this to call
terminate if someone does a throw; without there being an active
exception object.  uncaught_exception () works.  Exception handling
should work right if you optimize.  Exception handling should work with
-fpic or -fPIC.

The below points out some flaws in g++'s exception handling, as it now
stands.

Only exact type matching or reference matching of throw types works when
-fno-rtti is used.  Only works on a SPARC (like Suns) (both -mflat and
-mno-flat models work), SPARClite, Hitachi SH, i386, arm, rs6000,
PowerPC, Alpha, mips, VAX, m68k and z8k machines.  SPARC v9 may not
work.  HPPA is mostly done, but throwing between a shared library and
user code doesn't yet work.  Some targets have support for data-driven
unwinding.  Partial support is in for all other machines, but a stack
unwinder called __unwind_function has to be written, and added to
libgcc2 for them.  The new EH code doesn't rely upon the
__unwind_function for C++ code, instead it creates per function
unwinders right inside the function, unfortunately, on many platforms
the definition of RETURN_ADDR_RTX in the tm.h file for the machine port
is wrong.  See below for details on __unwind_function.  RTL_EXPRs for EH
cond variables for && and || exprs should probably be wrapped in
UNSAVE_EXPRs, and RTL_EXPRs tweaked so that they can be unsaved.

We only do pointer conversions on exception matching a la 15.3 p2 case
3: `A handler with type T, const T, T&, or const T& is a match for a
throw-expression with an object of type E if [3]T is a pointer type and
E is a pointer type that can be converted to T by a standard pointer
conversion (_conv.ptr_) not involving conversions to pointers to private
or protected base classes.' when -frtti is given.

We don't call delete on new expressions that die because the ctor threw
an exception.  See except/18 for a test case.

15.2 para 13: The exception being handled should be rethrown if control
reaches the end of a handler of the function-try-block of a constructor
or destructor, right now, it is not.

15.2 para 12: If a return statement appears in a handler of
function-try-block of a constructor, the program is ill-formed, but this
isn't diagnosed.

15.2 para 11: If the handlers of a function-try-block contain a jump
into the body of a constructor or destructor, the program is ill-formed,
but this isn't diagnosed.

15.2 para 9: Check that the fully constructed base classes and members
of an object are destroyed before entering the handler of a
function-try-block of a constructor or destructor for that object.

build_exception_variant should sort the incoming list, so that it
implements set compares, not exact list equality.  Type smashing should
smash exception specifications using set union.

Thrown objects are usually allocated on the heap, in the usual way.  If
one runs out of heap space, throwing an object will probably never work.
This could be relaxed some by passing an __in_chrg parameter to track
who has control over the exception object.  Thrown objects are not
allocated on the heap when they are pointer to object types.  We should
extend it so that all small (<4*sizeof(void*)) objects are stored
directly, instead of allocated on the heap.

When the backend returns a value, it can create new exception regions
that need protecting.  The new region should rethrow the object in
context of the last associated cleanup that ran to completion.

The structure of the code that is generated for C++ exception handling
code is shown below:

@example
Ln:					throw value;
        copy value onto heap
        jump throw (Ln, id, address of copy of value on heap)

                                        try @{
+Lstart:	the start of the main EH region
|...						...
+Lend:		the end of the main EH region
                                        @} catch (T o) @{
						...1
                                        @}
Lresume:
        nop	used to make sure there is something before
                the next region ends, if there is one
...                                     ...

        jump Ldone
[
Lmainhandler:    handler for the region Lstart-Lend
	cleanup
] zero or more, depending upon automatic vars with dtors
+Lpartial:
|        jump Lover
+Lhere:
        rethrow (Lhere, same id, same obj);
Lterm:		handler for the region Lpartial-Lhere
        call terminate
Lover:
[
 [
        call throw_type_match
        if (eq) @{
 ] these lines disappear when there is no catch condition
+Lsregion2:
|	...1
|	jump Lresume
|Lhandler:	handler for the region Lsregion2-Leregion2
|	rethrow (Lresume, same id, same obj);
+Leregion2
        @}
] there are zero or more of these sections, depending upon how many
  catch clauses there are
----------------------------- expand_end_all_catch --------------------------
                here we have fallen off the end of all catch
                clauses, so we rethrow to outer
        rethrow (Lresume, same id, same obj);
----------------------------- expand_end_all_catch --------------------------
[
L1:     maybe throw routine
] depending upon if we have expanded it or not
Ldone:
        ret

start_all_catch emits labels: Lresume, 

@end example

The __unwind_function takes a pointer to the throw handler, and is
expected to pop the stack frame that was built to call it, as well as
the frame underneath and then jump to the throw handler.  It must
restore all registers to their proper values as well as all other
machine state as determined by the context in which we are unwinding
into.  The way I normally start is to compile:

        void *g;
        foo(void* a) @{ g = a; @}

with -S, and change the thing that alters the PC (return, or ret
usually) to not alter the PC, making sure to leave all other semantics
(like adjusting the stack pointer, or frame pointers) in.  After that,
replicate the prologue once more at the end, again, changing the PC
altering instructions, and finally, at the very end, jump to `g'.

It takes about a week to write this routine, if someone wants to
volunteer to write this routine for any architecture, exception support
for that architecture will be added to g++.  Please send in those code
donations.  One other thing that needs to be done, is to double check
that __builtin_return_address (0) works.

@subsection Specific Targets

For the alpha, the __unwind_function will be something resembling:

@example
void
__unwind_function(void *ptr)
@{
  /* First frame */
  asm ("ldq $15, 8($30)"); /* get the saved frame ptr; 15 is fp, 30 is sp */
  asm ("bis $15, $15, $30"); /* reload sp with the fp we found */

  /* Second frame */
  asm ("ldq $15, 8($30)"); /* fp */
  asm ("bis $15, $15, $30"); /* reload sp with the fp we found */

  /* Return */
  asm ("ret $31, ($16), 1"); /* return to PTR, stored in a0 */
@}
@end example

@noindent
However, there are a few problems preventing it from working.  First of
all, the gcc-internal function @code{__builtin_return_address} needs to
work given an argument of 0 for the alpha.  As it stands as of August
30th, 1995, the code for @code{BUILT_IN_RETURN_ADDRESS} in @file{expr.c}
will definitely not work on the alpha.  Instead, we need to define
the macros @code{DYNAMIC_CHAIN_ADDRESS} (maybe),
@code{RETURN_ADDR_IN_PREVIOUS_FRAME}, and definitely need a new
definition for @code{RETURN_ADDR_RTX}.

In addition (and more importantly), we need a way to reliably find the
frame pointer on the alpha.  The use of the value 8 above to restore the
frame pointer (register 15) is incorrect.  On many systems, the frame
pointer is consistently offset to a specific point on the stack.  On the
alpha, however, the frame pointer is pushed last.  First the return
address is stored, then any other registers are saved (e.g., @code{s0}),
and finally the frame pointer is put in place.  So @code{fp} could have
an offset of 8, but if the calling function saved any registers at all,
they add to the offset.

The only places the frame size is noted are with the @samp{.frame}
directive, for use by the debugger and the OSF exception handling model
(useless to us), and in the initial computation of the new value for
@code{sp}, the stack pointer.  For example, the function may start with:

@example
lda $30,-32($30)
.frame $15,32,$26,0
@end example 

@noindent
The 32 above is exactly the value we need.  With this, we can be sure
that the frame pointer is stored 8 bytes less---in this case, at 24(sp)).
The drawback is that there is no way that I (Brendan) have found to let
us discover the size of a previous frame @emph{inside} the definition
of @code{__unwind_function}.

So to accomplish exception handling support on the alpha, we need two
things: first, a way to figure out where the frame pointer was stored,
and second, a functional @code{__builtin_return_address} implementation
for except.c to be able to use it.

Or just support DWARF 2 unwind info.

@subsection New Backend Exception Support

This subsection discusses various aspects of the design of the
data-driven model being implemented for the exception handling backend.

The goal is to generate enough data during the compilation of user code,
such that we can dynamically unwind through functions at run time with a
single routine (@code{__throw}) that lives in libgcc.a, built by the
compiler, and dispatch into associated exception handlers.

This information is generated by the DWARF 2 debugging backend, and
includes all of the information __throw needs to unwind an arbitrary
frame.  It specifies where all of the saved registers and the return
address can be found at any point in the function.

Major disadvantages when enabling exceptions are:

@itemize @bullet
@item
Code that uses caller saved registers, can't, when flow can be
transferred into that code from an exception handler.  In high performance
code this should not usually be true, so the effects should be minimal.

@end itemize

@subsection Backend Exception Support

The backend must be extended to fully support exceptions.  Right now
there are a few hooks into the alpha exception handling backend that
resides in the C++ frontend from that backend that allows exception
handling to work in g++.  An exception region is a segment of generated
code that has a handler associated with it.  The exception regions are
denoted in the generated code as address ranges denoted by a starting PC
value and an ending PC value of the region.  Some of the limitations
with this scheme are:

@itemize @bullet
@item
The backend replicates insns for such things as loop unrolling and
function inlining.  Right now, there are no hooks into the frontend's
exception handling backend to handle the replication of insns.  When
replication happens, a new exception region descriptor needs to be
generated for the new region.

@item
The backend expects to be able to rearrange code, for things like jump
optimization.  Any rearranging of the code needs have exception region
descriptors updated appropriately.