Hi Robert;
On 09.01.2014 14:37, Robert Mustacchi wrote:
On 1/9/14 10:45 , Pedro Giffuni wrote:
Hello;
Just thought I'd point out, for those not following toolchain@, that
there are issues with newer compilers using dwarf4 by default[1].
It looks like instead of waiting for the elftoolchain project to catch
up with the dwarf standard we could be using dwarf support from LLVM
instead though [2].
Apparently the linux dtrace port is having similar issues. Another task
for the Dtrace TODO list I guess, but perhaps the Illumos guys following
the list may have some input.
As one of the illumos folk following the list, I can kind of describe
what at least some of us would like to do with CTF. It's worth
mentioning that we're still using gcc4.4 so we're not in a realm where
DWARF4 is a concern for us yet. I'm not familiar with all the
differences yet, so I'm not sure how useful the rest of this e-mail will be.
Great to have you here!
Currently I've been retooling ctfconvert + ctfmerge to be actually
implemented in terms of libctf to make it easier for other things to
actually consume, such as more long term illumos ld(1) and ideally the
compilers themselves. None of this code is finished or in illumos yet.
That really means that I have a slightly more useful merge (and as a
result diff) written, and convert is kind of a work in progress.
As a side note ... completely unrelated to the issue at hand.
I don't know much about how DTrace uses dwarf, but I had some contact
with the Oracle guys doing the linux port and I think they are doing
something drastically different to illumos (and FreeBSD). They are
avoiding the issue of getting DTrace in the kernel and they are using
dwarf2 instead of the stabs-like format supported by CTF. Their stuff is
in a GIT repository somewhere but I would have to dig deep in my private
email for the finer details. They were likely to have problems with
dwarf4 too.
Probably the simplest way forward that we can all leverage is writing a
new ctfconvert in terms of DWARF4 that can exist side by side the old
one. eg. if you have an older compiler with dwarf2 (or you're still
stuck with stabs or some other language has its own exquisite format),
that'll still work. illumos last updated libdwarf in 2011. If there's a
newer libdwarf that has better DWARF4 support or we need to come up with
something else, let's make sure not to duplicate effort.
I recall Illumos is using SGI's libdwarf which we have avoiding using
mainly because it's copyleft but I recall there were also some
implementation details that made it undesirable. I haven't followed the
SGI libdwarf but when I last tried to update the pre-packaged port I
noticed it was becoming more difficult to port to non-linux (that may
have been solved though).
Some FreeBSD developers reimplemented libelf and libdwarf specifically
to use it in DTrace:
http://sourceforge.net/p/elftoolchain/wiki/libdwarf/
Their implementation is very clean and portable and it's basically what
we have been using but we have to update it and I am not sure about
their dwarf4 support.
If illumos and freebsd can use the same libdwarf than that would be
good. OTOH, with llvm having support for dwarf4 it would certainly be
tempting to try to integrate what we can with the toolchain. I don't see
illumos advancing much in the llvm direction though, so this option
would be good for FreeBSD but probably not for illumos.
FreeBSD 10+ has moved to llvm/clang already and, as noted in the
previous posting, FreeBSD 11 is using the latest clang with dwarf4 so
the issue became urgent.
Pedro.
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