Comment #3 on issue 1065 by [email protected]: try catch incurs an
unexpected performance penalty
http://code.google.com/p/v8/issues/detail?id=1065
V8 does unwind the stack (can you think of any other way to implement
throw/catch?). The real performance problem is that functions containing
try/catch are not optimized.
* I think catch-bound variables are allocated in the heap mainly for
historical reasons. They originally used the same implementation machinery
as 'with'. This was changed in 2011. But when it changed, V8 kept the
heap allocation as part of the old implementation. I can't think of a
super compelling reason not to stack allocate them, other than the
implementation effort and the relatively low priority. They do take up
extra space in the stack frame and they do require initialization with some
value that is safe for GC.
* You should definitely profile before you conclude this is a performance
problem. For reads, the code contains only an extra register-register move
in most cases. You'd have to be reading the catch variable in a loop to
even observe the difference. For heap writes, there is a write barrier for
the garbage collector. But who writes to a catch-bound variable anyway?
If you're doing that in a loop there is a simple workaround --- don't.
* V8 optimizes at the granularity of a whole function. It is not simple to
have a function which is partly optimized and partly not (with a stack
frame that is a mix of optimized/unoptimized) --- that's the principle at
play when a programmer abstracts out part of a body into a separate
function. I would estimate that supporting such behavior in the optimizing
compiler would be more difficult than just supporting try/catch in the
first place.
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