On Thu, Mar 11, 2021 at 12:05:42PM -0500, Taylor Blau wrote: > From: Jeff King <peff@xxxxxxxx> > > There is a lot of pointer dereferencing in the pre-image version of > 'midx_pack_order_cmp()', which this patch gets rid of. > > Instead of comparing the pack preferred-ness and then the pack id, both > of these checks are done at the same time by using the high-order bit of > the pack id to represent whether it's preferred. Then the pack id and > offset are compared as usual. > > This produces the same result so long as there are less than 2^31 packs, > which seems like a likely assumption to make in practice. Obviously this patch is brilliant. ;) Did we record any numbers to show the improvement here? I don't think it can be demonstrated with this series (since most of the code is dead), but I recall that this was motivated by a noticeable slowdown. I briefly wondered whether the complicated midx_pack_order_cmp() in pack-revindex.c, which is used for the bsearch() there, could benefit from the same speedup. It's only log(n), of course, instead of n*log(n), but one might imagine making "n" calls to it. I don't think it makes sense, though. The pointer dereferencing there is into the midx mmap itself. Creating an auxiliary array would defeat the purpose. -Peff
