don't do linear allocations, double allocation request each time.
also, remove unneeded casts from MALLOC, etc.

Reference: /n/atom/patch/applied/diffmalloclg
Date: Mon Sep 28 07:11:51 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/diff/diffio.c	Mon Sep 28 07:11:02 2015
+++ /sys/src/cmd/diff/diffio.c	Mon Sep 28 07:11:03 2015
@@ -124,7 +124,7 @@
 prepare(int i, char *arg)
 {
 	struct line *p;
-	int j, h;
+	int j, h, a;
 	Biobuf *bp;
 	char *cp, buf[MAXLINELEN];
 	int nbytes;
@@ -153,9 +153,16 @@
 		}
 		Bseek(bp, 0, 0);
 	}
-	p = MALLOC(struct line, 3);
-	for (j = 0; h = readhash(bp, buf); p[j].value = h)
-		p = REALLOC(p, struct line, (++j+3));
+	a = 1;
+	p = MALLOC(struct line, a+3);
+	for (j = 0; h = readhash(bp, buf); p[j].value = h){
+		if(j > a){
+			a = j<<1;
+			p = REALLOC(p, struct line, a+3);
+		}
+		j++;
+	}
+
 	len[i] = j;
 	file[i] = p;
 	input[i] = bp;			/*fix*/
--- /sys/src/cmd/diff/diffdir.c	Mon Sep 28 07:11:05 2015
+++ /sys/src/cmd/diff/diffdir.c	Mon Sep 28 07:11:06 2015
@@ -19,18 +19,13 @@
 	int nitems;
 	int fd, n;
 
-	if ((fd = open(name, OREAD)) < 0) {
+	if ((fd = open(name, OREAD)) < 0)
 		fprint(2, "%s: can't open %s: %r\n", argv0, name);
-		/* fake an empty directory */
-		cp = MALLOC(char*, 1);
-		cp[0] = 0;
-		return cp;
-	}
 	cp = 0;
 	nitems = 0;
 	if((n = dirreadall(fd, &db)) > 0){
+		cp = MALLOC(char*, n+1);
 		while (n--) {
-			cp = REALLOC(cp, char *, (nitems+1));
 			cp[nitems] = MALLOC(char, strlen((db+n)->name)+1);
 			strcpy(cp[nitems], (db+n)->name);
 			nitems++;
--- /sys/src/cmd/diff/diff.h	Mon Sep 28 07:11:08 2015
+++ /sys/src/cmd/diff/diff.h	Mon Sep 28 07:11:09 2015
@@ -7,9 +7,9 @@
 extern Biobuf	stdout;
 extern int	binary;
 
-#define MALLOC(t, n)		((t *)emalloc((n)*sizeof(t)))
-#define REALLOC(p, t, n)	((t *)erealloc((void *)(p), (n)*sizeof(t)))
-#define FREE(p)			free((void *)(p))
+#define MALLOC(t, n)		(emalloc((n)*sizeof(t)))
+#define REALLOC(p, t, n)	(erealloc((void *)(p), (n)*sizeof(t)))
+#define FREE(p)			free(p)
 
 #define MAXPATHLEN	1024