xdiff: avoid unnecessary memory allocations

phillipwood · gitster · commit b82dd3f7f20c · 2021-11-18T22:23:45.000-08:00
rindex and ha are only used by xdl_cleanup_records() which is not
called by the histogram or patience algorithms. The perf test results
show a small reduction in run time but that is probably within the
noise.

Test                                  HEAD^             HEAD
-----------------------------------------------------------------------------
4000.1: log -3000 (baseline)          0.19(0.17+0.02)   0.19(0.12+0.07) +0.0%
4000.2: log --raw -3000 (tree-only)   0.98(0.78+0.20)   0.98(0.81+0.16) +0.0%
4000.3: log -p -3000 (Myers)          4.81(4.15+0.64)   4.81(4.23+0.56) +0.0%
4000.4: log -p -3000 --histogram      5.87(5.19+0.66)   5.83(5.11+0.70) -0.7%
4000.5: log -p -3000 --patience       5.35(4.60+0.73)   5.31(4.61+0.69) -0.7%

Signed-off-by: Phillip Wood &lt;phillip.wood@dunelm.org.uk&gt;
Signed-off-by: Junio C Hamano &lt;gitster@pobox.com&gt;
diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
@@ -213,10 +213,13 @@ static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_
 		goto abort;
 	memset(rchg, 0, (nrec + 2) * sizeof(char));
 
-	if (!(rindex = (long *) xdl_malloc((nrec + 1) * sizeof(long))))
-		goto abort;
-	if (!(ha = (unsigned long *) xdl_malloc((nrec + 1) * sizeof(unsigned long))))
-		goto abort;
+	if ((XDF_DIFF_ALG(xpp->flags) != XDF_PATIENCE_DIFF) &&
+	    (XDF_DIFF_ALG(xpp->flags) != XDF_HISTOGRAM_DIFF)) {
+		if (!(rindex = xdl_malloc((nrec + 1) * sizeof(*rindex))))
+			goto abort;
+		if (!(ha = xdl_malloc((nrec + 1) * sizeof(*ha))))
+			goto abort;
+	}
 
 	xdf->nrec = nrec;
 	xdf->recs = recs;
