I decided to give it a try :)

atoi() is only used in findhistrel(), which in turn is only used if you
enable csh-like history.  findhistrel() is used only once in lex.c.  it
is expected to return the item in the history array to lookup.

Regress still passes (not suprising since csh-history is not tested) and
runtime testing seems to behave just like before.  I do enable and use
csh-style history expansion, but admittedly only in the most basic
forms.

diff /usr/src
commit - cfb8aef91a90e62370ec0c79ff80c4089a8f0cfb
path + /usr/src
blob - c768563496eccde8ced1df4a6bd73ee967a1cfff
file + bin/ksh/history.c
--- bin/ksh/history.c
+++ bin/ksh/history.c
@@ -507,19 +507,19 @@ findhist(int start, int fwd, const char *str, int anch
 int
 findhistrel(const char *str)
 {
+	const char *errstr;
 	int	maxhist = histptr - history;
 	int	start = maxhist - 1;
-	int	rec = atoi(str);
+	int	rec;
 
+	rec = strtonum(str, -maxhist, maxhist, &errstr);
+	if (errstr)
+		return -1;
+
 	if (rec == 0)
 		return -1;
-	if (rec > 0) {
-		if (rec > maxhist)
-			return -1;
+	if (rec > 0)
 		return rec - 1;
-	}
-	if (rec > maxhist)
-		return -1;
 	return start + rec + 1;
 }