improve strerror speed

change the current O(n) lookup to O(1) based on the machinery
described in "How To Write Shared Libraries" (Appendix B).

src/errno/__strerror.h

-/* This file is sorted such that 'errors' which represent exceptional
- * conditions under which a correct program may fail come first, followed
- * by messages that indicate an incorrect program or system failure. The
- * macro E() along with double-inclusion is used to ensure that ordering
- * of the strings remains synchronized. */
+/* The first entry is a catch-all for codes not enumerated here.
+ * This file is included multiple times to declare and define a structure
+ * with these messages, and then to define a lookup table translating
+ * error codes to offsets of corresponding fields in the structure. */
+
+E(0,            "No error information")
 
 E(EILSEQ,       "Illegal byte sequence")
 E(EDOM,         "Domain error")
@@ -101,5 +102,3 @@ E(EDQUOT,       "Quota exceeded")
 E(ENOMEDIUM,    "No medium found")
 E(EMEDIUMTYPE,  "Wrong medium type")
 E(EMULTIHOP,    "Multihop attempted")
-
-E(0,            "No error information")

src/errno/strerror.c

 #include <errno.h>
+#include <stddef.h>
 #include <string.h>
 #include "locale_impl.h"
 
-#define E(a,b) ((unsigned char)a),
-static const unsigned char errid[] = {
+/* mips has one error code outside of the 8-bit range due to a
+ * historical typo, so we just remap it. */
+#if EDQUOT==1133
+#define EDQUOT_ORIG 1133
+#undef  EDQUOT
+#define EDQUOT 109
+#endif
+
+static const struct errmsgstr_t {
+#define E(n, s) char str##n[sizeof(s)];
+#include "__strerror.h"
+#undef E
+} errmsgstr = {
+#define E(n, s) s,
 #include "__strerror.h"
+#undef E
 };
 
-#undef E
-#define E(a,b) b "\0"
-static const char errmsg[] =
+static const unsigned short errmsgidx[] = {
+#define E(n, s) [n] = offsetof(struct errmsgstr_t, str##n),
 #include "__strerror.h"
-;
+#undef E
+};
 
 char *__strerror_l(int e, locale_t loc)
 {
 	const char *s;
-	int i;
-	/* mips has one error code outside of the 8-bit range due to a
-	 * historical typo, so we just remap it. */
-	if (EDQUOT==1133) {
-		if (e==109) e=-1;
-		else if (e==EDQUOT) e=109;
-	}
-	for (i=0; errid[i] && errid[i] != e; i++);
-	for (s=errmsg; i; s++, i--) for (; *s; s++);
+#ifdef EDQUOT_ORIG
+	if (e==EDQUOT) e=0;
+	else if (e==EDQUOT_ORIG) e=EDQUOT;
+#endif
+	if (e >= sizeof errmsgidx / sizeof *errmsgidx) e = 0;
+	s = (char *)&errmsgstr + errmsgidx[e];
 	return (char *)LCTRANS(s, LC_MESSAGES, loc);
 }