Re: [PATCH 1/2] diffcore-rename: no point trying to find a match better than exact

[Derrick Stolee <stolee@gmail.com>]

On 2/3/2021 12:49 AM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <newren@gmail.com>
> 
> diffcore_rename() had some code to avoid having destination paths that
> already had an exact rename detected from being re-checked for other
> renames.  Source paths, however, were re-checked because we wanted to
> allow the possibility of detecting copies.  But if copy detection isn't
> turned on, then this merely amounts to attempting to find a
> better-than-exact match, which naturally ends up being an expensive
> no-op.  In particular, copy detection is never turned on by the merge
> machinery.
...
> +	num_sources = rename_src_nr;
> +	if (detect_rename != DIFF_DETECT_COPY)
> +		num_sources -= rename_count;

Ok, delete the renamed files from the sources. Using a new variable
because rename_src_nr is actually a static global to diffcore-rename.c,
describing the number of entries in the rename_src table. This is
scary, but I think your new local is a good way to change the local
logic of this method without adjusting that global.

>  
>  	/* All done? */
> -	if (!num_destinations)
> +	if (!num_destinations || !num_sources)
>  		goto cleanup;

And add an extra quit condition which is very possible to hit.
Is it only hit when every "delete" is actually a rename?

> -	switch (too_many_rename_candidates(num_destinations, rename_src_nr,
> +	switch (too_many_rename_candidates(num_destinations, num_sources,
>  					   options)) {

This is all about checking if we need to skip inexact renames. Makes
sense to use the new number.
  
> +			if (one->rename_used &&
> +			    detect_rename != DIFF_DETECT_COPY)
> +				continue;
> +

Have we "consumed" this input? Skip over it. Good. And this is inside
a double-loop:

	for (dst_cnt = i = 0; i < rename_dst_nr; i++) {
		...
		for (j = 0; j < rename_src_nr; j++) {

Keeping rename_src_nr in the inner loop makes sense, but this new
'continue;' gives most of the speedup, I imagine.

This is a nice speedup for such a simple optimization.

Thanks,
-Stolee