Re: [PATCH v2] unpack-trees: avoid duplicate ODB lookups during checkout

["René Scharfe" <l.s.r@web.de>]

Am 10.04.2017 um 23:26 schrieb Jeff Hostetler:
 > On 4/8/2017 10:06 AM, René Scharfe wrote:
 >> Am 07.04.2017 um 17:53 schrieb git@jeffhostetler.com:
 >>> +            /* implicitly borrow buf[i-1] inside tree_desc[i] */
 >>> +            memcpy(&t[i], &t[i-1], sizeof(struct tree_desc));
 >>
 >> An assignment would be simpler:
 >>
 >>             t[i] = t[i - 1];
 >
 > True, but this might be a coin toss.  Maybe we should
 > see what the generated assembly looks like for each ??

Clang, GCC and ICC inline that memcpy call; the assembly output is the
same for both variants: https://godbolt.org/g/1q0YwK.  I guess you worry
about compilers that are just bad at struct assignments (i.e. worse than
calling memcpy)?  Do you have examples (just curious)?

Assignments are easier on the eye of human readers in any case, and
there is no way to silently get the size wrong.

 > I tried to hit the common cases.  This loop runs a lot
 > and I didn't want to put an O(n^2) thing in there to
 > look for any matching peer.  Most of the time we are
 > in a simple 2 or 3 way effort.  I didn't want to pay
 > for the looping/branching overhead for the obscure [4..8]
 > efforts.

Makes sense, and it's a nice heuristic.  Perhaps it would be a good idea
to document these choices in a comment?

René