Hi, I reported a problem I have with abort to the glibc bug tracker at
<URL:http://sourceware.org/bugzilla/show_bug.cgi?id=6522> which might
provide some reading material.
Anyway, it has been pointed out to me that the requested change would
not accomplish much anyway since GCC has its own builtin notion of the
abort function.
So here is my beef with the current way abort calls get optimized:
basically it boils down to the interaction of -fcrossjumping (enabled
with -O2) and the noreturn attribute of abort. The combination of both
optimizations means that usually only one abort call will actually get
compiled into a compilation unit, and all calls will instead jump there
without bothering to clean the stack (after all, it is a noreturn
function).
This is egregiously bad when using the generated coredump (abort
triggers SIGABRT) for debugging failed assertions or other aborts:
neither the call/return location is correctly associated with the source
code, nor is the stack/register state interpretable using debugging
info.
Given that the distinguishing feature of abort over exit is the creation
of a core dump, it would be good if this core dump would actually be
useful. I know several people who have spent days debugging at the
wrong place, including myself. Currently Emacs contains in etc/DEBUG
the instructions
** When you are trying to analyze failed assertions, it will be
essential to compile Emacs either completely without optimizations or
at least (when using GCC) with the -fno-crossjumping option. Failure
to do so may make the compiler recycle the same abort call for all
assertions in a given function, rendering the stack backtrace useless
for identifying the specific failed assertion.
However, the problem is obviously not restricted to Emacs. If the
"noreturn" attribute for the internal abort were removed, at least only
abort calls with compatible stack frame and the same (tentative)
followup code would get folded. That would avoid the worst
head-scratchers when trying to figure out what went wrong.
Probably better would be to just disable the crossjumping optimization
for calls of abort. Maybe this would warrant a new attribute.
Thanks for caring,
--
David Kastrup
