On 2014-04-09 17:27:55 +0000, "David Nadlinger" <c...@klickverbot.at> said:
On Wednesday, 9 April 2014 at 16:48:54 UTC, Jacob Carlborg wrote:
x86_64 yes, not necessarily only for DMD. I thought if DMD, LDC and GDC
all used the same exception handling model and the same as C++ it would
be easier. Especially for implementing support for Objective-C
exceptions, which is why initially started this thread.
They don't. GDC and LDC use libunwind, whereas DMD uses its own custom
EH implementation.
With GDC and LDC, you'd just need to add your code to handle
Objective-C exceptions to the personality functions. libunwind
exceptions have a "type"/"source language" field, and by default most
implementations either ignore unknown exception types or abort on
encountering them.
Maybe the right course of action would be to just ignore Objective-C
exceptions in 64-bit DMD and finish the rest of D/Objective-C. The day
D/Objective-C is ported to GDC and LDC, it should be much easier to
make exceptions work there.
The funny thing is that within Cocoa exceptions are usually reserved
for logic errors (array out of bound, method calls with unknown
selector, assertion errors, etc.). That's the kind of error we consider
unrecoverable when they happen in D code. So I think people can manage
without Objective-C exceptions for some time.
Given what I wrote above, I'll also argue that it's not a wise move to
support exceptions in D/Objective-C with the legacy runtime even though
I implemented it. Because the legacy runtime uses setjmp/longjmp for
exceptions, try blocks in a function that calls extern(Objective-C)
functions are costly. And the compiler has to implicitly add those
costly try blocks to wrap/unwrap exception objects to prevent unwinding
of the wrong kind from leaving the current function. It was fun to
implement, but it's also the most intrusive changes D/Objective-C makes
to the frontend, and a big parts of the additions to druntime.
If we plan to support mixed unwind mechanisms some day it might be
useful to keep, because the logic for bridging between two or more
unwinding systems is all there. Otherwise I'd probably scrap the whole
exception wrapping/unwrapping thing. I doubt the performance cost is
worth it, and I doubt the maintenance cost for the additional
complexity is worth it. The legacy Objective-C runtime is mostly gone
from Apple's ecosystem anyway.
--
Michel Fortin
michel.for...@michelf.ca
http://michelf.ca
