[Development] Question about QCoreApplicationData::*_libpaths

Matthew Woehlke mwoehlke.floss at gmail.com
Tue Jan 19 17:43:59 CET 2016


On 2016-01-19 05:51, Marc Mutz wrote:
> So... is realloc actually the optimisation everyone (incl. me) expected it to 
> be?

It really *ought* to be, at least in the right cases.

Let's say I have a type that stores a buffer, which, for whatever
reason, is not movable (i.e. no move-ctor or swap┬╣), but *is*
relocatable. Resizing a vector of such types involves allocating the new
block, doing a single block memcpy, and releasing the old block.

Resizing if they were *not* relocatable involves a copy ctor for each,
which involves allocating a new memory block, copying the data, and
releasing the old block *per item*, plus the two memory operations

If 2*N heap operations plus N block copies of "large" blocks are
comparably expensive to a single block copy of N*sizeof(void*), I would
be very surprised :-).

(┬╣ Keep in mind also that the move/swap quite possibly must also be
noexcept or it can't be used anyway.)

Maybe we're looking at bad examples, or there are fewer such instances
than expected where it is significantly better. (In particular, I
suspect neither string nor QByteArray are "good examples". Try with a
type that always allocates memory?)

-- 
Matthew




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