[LLVMdev] x86 unwind support[MESSAGE NOT SCANNED]

Mark Shannon marks at dcs.gla.ac.uk
Mon Jul 20 12:17:37 PDT 2009

Jeffrey Yasskin wrote:
> On Mon, Jul 20, 2009 at 7:43 AM, Nick
> Johnson<nicholas.paul.johnson at gmail.com> wrote:
>> On Mon, Jul 20, 2009 at 10:09 AM, Mark Shannon<marks at dcs.gla.ac.uk> wrote:
>>> Andrew Haley wrote:
>>> If you can make your point without any references to any C/C++ specific
>>> features it might be more convincing ;)
>> I did.  Recall my mention of java/c#/ruby/python's finally/ensure
>> blocks, or C#'s using blocks.  For proper implementation, these need
>> multi-level unwinds, as they specify that some code must run, even if
>> an exception would bail-out.
>>> I take it you have never used Python ;)
>>> (Python uses exceptions to terminate loops, so it helps if they aren't
>>> too slow)
>> I have used python, and it is slow (sorry).  In fact, python
>> Exceptions are implemented in python as a second return value, thus
>> EVERY function, even those which don't throw exceptions, must pay the
>> price.  And just because the python community does it, doesn't mean
>> it's good programming practice.
> :)
> Working on Unladen Swallow, we've considered whether to try to port
> Python to use Dwarf unwinding. It's a fairly simple tradeoff: with
> return-value-exceptions you get a small cost in both time (a
> predictable branch) and code size at each call site, whether or not
> the call site is inside a try, but you get pretty cheap exception
> throwing and propagation. With dwarf-exceptions, you have a small
> space cost at each invoke (for the dwarf metadata), no time cost at
> calls that don't throw, but fairly expensive exceptions when you do
> throw them. Because Python throws exceptions for ending loops and
> garbage-collecting generators, we expect return-value-exceptions to be
> cheaper overall. In languages that throw exceptions more rarely (just
> about everyone), dwarf-exceptions should be cheaper. Dwarf-exceptions
> will still be worth investigating for Python eventually; we're just
> not prioritizing it. Anyone who wants us to prioritize it higher
> should bring data. :)
> IronPython's and Jython's experiences are interesting too. IronPython
> has had big problems with the cost of exceptions on the .NET platform.
> Jython has _not_ had problems in the JVM, which indicates that the two
> big virtual machines have made different tradeoffs for this. That
> would make me very hesitant to say that either option is obviously
> wrong.
The JVM is interesting because creating an exception in Java is 
expensive (generating the backtrace) but throwing one is quite cheep
> Jeffrey
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