Subject: Re: [PATCH 4/4] xdiff: reduce the size of array

From: Junio C Hamano <[email-masked]>
Date: Thu, 02 Apr 2026 12:44:40 -0700

Phillip Wood <[email-masked]> writes:

> From: Phillip Wood <[email-masked]>
>
> When the myers algorithm is selected the input files are pre-processed
> to remove any common prefix and suffix and any lines that appear
> in only one file. This requires a map to be created between the
> lines that are processed by the myers algorithm and the lines in
> the original file. That map does not include the common lines at the
> beginning and end of the files but the array is allocated to be the
> size of the whole file. Move the allocation into xdl_cleanup_records()
> where the map is populated and we know how big it needs to be.
>
> Signed-off-by: Phillip Wood <[email-masked]>
> ---
>  xdiff/xprepare.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
> index cf4ac34f047..c5a3c9cde76 100644
> --- a/xdiff/xprepare.c
> +++ b/xdiff/xprepare.c
> @@ -171,12 +171,6 @@ static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_
>  	if (!XDL_CALLOC_ARRAY(xdf->changed, xdf->nrec + 2))
>  		goto abort;
>  
> -	if ((XDF_DIFF_ALG(xpp->flags) != XDF_PATIENCE_DIFF) &&
> -	    (XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF)) {
> -		if (!XDL_ALLOC_ARRAY(xdf->reference_index, xdf->nrec + 1))
> -			goto abort;
> -	}
> -
>  	xdf->changed += 1;
>  	xdf->nreff = 0;
>  	xdf->dstart = 0;
> @@ -283,7 +277,10 @@ static int xdl_cleanup_records(xdlclassifier_t *cf, xdfile_t *xdf1, xdfile_t *xd
>  	 * changed[i] should remain false, or become true.
>  	 */
>  	if (!XDL_CALLOC_ARRAY(action1, len1) ||
> -	    !XDL_CALLOC_ARRAY(action2, len2)) {
> +	    !XDL_CALLOC_ARRAY(action2, len2) ||
> +	    !XDL_ALLOC_ARRAY(xdf1->reference_index, len1) ||
> +	    !XDL_ALLOC_ARRAY(xdf2->reference_index, len2))
> +	{
>  		ret = -1;
>  		goto cleanup;
>  	}

OK.  In xdl_cleanup_records(), accesses toxdf{1,2}->reference_index[] 
already runs from index 0 (i.e., array element at [0] corresponds to
the xdf1->dstart) even without the previous three patches.  So we
were only wasting the elements near the end in these two arrays.
And the loop that uses the array runs only for len1 times, and the
array may acquire at most one new element per iteration, so len1 is
the reasonable allocation size for xdf1->reference_index[].

Looking good.