C++ Exception handling

Peter Naulls peter at chocky.org
Thu Feb 26 01:15:47 PST 2004

Graham Shaw has found a fix for exception handling issues in GCC 2.95.4.
I don't know if they also apply to 3.3.3, but I've quoted the relevant
parts and the patch:

> The problem is that not enough memory is being allocated for the setjmp
> buffer, and as a result the last word is being overwritten by other code. 
> (The last word contains the program counter, so the consequences of
> overwriting it are invariably rather serious :-)
> AFAIKT the problem is (surprisingly) not the definition of JMP_BUF_SIZE,
> but rather the number of words allocated above and beyond this value (to
> accommodate some other data stored by the exception handling code).  A
> possible fix is attached.
> What I don't understand is why this bug has suddenly appeared now, because
> the code in question can be traced right back to the upstream GCC sources.
> It would only happen on architectures that use setjmp buffers for
> exception handling, and then only if JMP_BUF_SIZE is defined in the
> makefile (rather than using the built-in heuristic), but even then I would
> have expected it to have surfaced by now - if not for other platforms,
> then at least for the 26-bit RISC OS compiler.
> (The code certainly looks wrong.  It says that the jump buffer starts two
> words into the area allocated, but the requested size is
> GET_MODE_SIZE(Pmode)*(size+1).)
> Really this needs to be checked by someone who knows a bit more about the
> internals of GCC than I do.  On the upside, I can't see my patch doing
> much harm (except for slightly increasing run-time memory usage) and it
> does seem to work.

diff -ur gccsdk/gcc/gcc/except.c gccsdk-295/gcc/gcc/except.c
--- gccsdk/gcc/gcc/except.c	Thu Feb 26 08:16:48 2004
+++ gccsdk-295/gcc/gcc/except.c	Thu Feb 26 08:14:25 2004
@@ -1363,7 +1363,7 @@
      but there is no allocation routine that allocates at the level of
      the last binding contour.  */
   arg = assign_stack_local (BLKmode,
-			    GET_MODE_SIZE (Pmode)*(size+2),
+			    GET_MODE_SIZE (Pmode)*(size+1),
   arg = change_address (arg, Pmode, NULL_RTX);

Peter Naulls - peter at chocky.org        | http://www.chocky.org/
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