This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
speed up (non)overloaded derefs
Consider a class that has some minimal overloading added - e.g. to give
pretty stringification of objects - but which *doesn't* overload
dereference methods such as '@[]'. '%[]' etc.
In this case, simple dereferencing, such as $obj->[0] or $obj->{foo}
becomes much slower than if the object was blessed into a non-overloaded
class.
This is because every time a dereferencing is performed in pp_rv2av for
example, the "normal" code path has to go through the full checking of:
* is the stash into which the referent is blessed overloaded? If so,
* retrieve the overload magic from the stash;
* check whether the overload method cache has been invalidated and if so
rebuild it;
* check whether we are in the scope of 'no overloading', and if so
is the current method disabled in this scope?
* Is there a '@{}' or whatever (or 'nomethod') method in the cache?
If not, then process the ref as normal.
That's a lot of extra overhead to decide that an overloaded method doesn't
in fact need to be called.
This commit adds a new flag to the newish xhv_aux_flags field,
HvAUXf_NO_DEREF, which signals that the overloading of this stash
contains no deref (nor 'nomethod') overloaded methods. Thus a quick check
for this flag in the common case allows us to short-circuit all the above
checks except the first one.
Before this commit, a simple $obj->[0] was about 40-50% slower if the
class it was blessed into was overloaded (but didn't have deref methods);
after the commit, the slowdown is 0-10%. (These timings are very
approximate, given the vagaries of nano benchmarks.)