gcc-dw2? or fast sjlj-exceptions EH

Eric Blake ebb9@byu.net
Wed Sep 12 00:28:00 GMT 2007


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According to Tatsuro MATSUOKA on 9/11/2007 2:49 PM:
> My question is whether other possibilities to get fast sjlj-exception 
> EH in the cygwin.

I think you are misunderstanding a fundamental concept.  sjlj exception
handling is inherently slower than stack-tracing exception handling,
because it must assume the worst and store the entire stack state (the
'sj' or setjump) prior to any call site where an exception might occur,
whether or not an exception actually happens (the 'lj' or longjump part).
 Dwarf error handling, on the other hand, is a form of stack-tracing,
where there is NO penalty UNLESS there was an exception.  Exception
handling is slower, because it must crawl backwards through the stack to
find all catch points with handlers to run, but more efficient when there
is no exception because you don't waste time saving state that never gets
jumped to.

In other words, the ONLY way to speed up octave is to avoid sjlj
exceptions.  And there are two ways to do that - either switch to
stack-based exceptions (such as dwarf), or avoid exceptions to begin with.
 The octave maintainer has already expressed a reluctance to disable
exceptions, and the current gcc for cygwin, 3.4.4, doesn't have a way to
use anything other than sjlj, so you will just have to wait.

> If there is no way to get the fast sjlj-exception EH, is it possible 
> to handle the 'gcc-dw2'?
> I know there is 'gcc-dw2' for 4.2.1 version of gcc in the mingw.

Have you not been reading this thread?  gcc-dw2 for mingw is the
dwarf-based exception handling build of gcc.  Work is underway for
building a similar gcc for cygwin, but doing it correctly involves
building a shared libgcc, which isn't cleanly supported without gcc 4.3.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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