On Tue, 21 Jul 2009, Junio C Hamano wrote:
>
> What's that cmovne?
It's "conditional move if not equal".
So the source code is
/* We cannot squash it with earlier one */
for (lline = sline->lost_head;
lline;
lline = lline->next)
if (lline->parent_map & this_mask)
last_one = lline;
adn the compiler has generated this:
loop:
test %r14,0x10(%rdx)
cmovne %rdx,%rcx
mov (%rdx),%rdx
test %rdx,%rdx
jne loop
from it. In the above, '%r14' contains this_mask, and '%rcx' contains
'last_line' and '%rdx' contains 'lline'.
So:
- test %r14,0x10(%rdx)
test "this_mask & lline->parent_map"
- cmovne %rdx,%rcx
"if the test above was non-zero, then last_one = lline"
- mov (%rdx),%rdx
"lline = lline->next"
- test %rdx,%rdx
"is lline NULL"
- jne loop
no, continue.
> The function append_lost() is the meat of combining. When you have seen
> a hunk like this:
>
> @@ -l,k +m,n @@
> -lost line
> -another lost line
> common line
> +added line
>
> We queue the lost lines in front of a surviving line (that is sline that
> points at "common line"). "lost line" and "another lost line" are stored
> in lline (lost line) and they are queued to sline->lost_head.
Right. And "sline->lost_head" is going to have over a _million_ entries.
So each time you add one, we'll traverse that loop a million times. For
each new entry. End result: that loop gets executed on the order of a
million million times. It doesn't help that the compiler made it be very
efficient code ;(
Linus
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