Re: git-diff-tree inordinately (O(M*N)) slow on files with many changes

From: Junio C Hamano <junkio@cox.net>
Date: Oct 16, 2006, 22:53 UTC
Message-ID: <7vy7rfub36.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <Pine.LNX.4.64.0610161130090.3962@g5.osdl.org>

Linus Torvalds <torvalds@osdl.org> writes:

> Junio, I think this is worthy to go in before a 1.4.3 release. Possibly 
> even back-ported to earlier trees.

Thanks for resolving this quickly; I was (am still somewhat) down sick today and missed all the excitement.

> Quite frankly, I prefer my previous patch more, it just avoids that whole 
> problem, and two shifts and adds (even with a conditional) are often 
> faster than a full 64-bit multiply.

I agree (although I am not sure about the "do it twice for small" bit), and I think Davide agrees with you in his reply:

Davide Libenzi <davidel@xmailserver.org> writes:

Show 6 quoted lines

> On Mon, 16 Oct 2006, Linus Torvalds wrote:
> ...
> I ended up using this one:
>
> #define XDL_HASHLONG(v, b) ((((unsigned long) (v) >> ((CHAR_BIT * sizeof(unsigned long)) - (b))) + \
>                              (unsigned long) (v)) & ((1UL << (b)) - 1))

so I am inclined to apply Davide's version, but I am going to bed again now...