diff --git a/src/runtime/c/gu/utf8.c b/src/runtime/c/gu/utf8.c
index 8f22e5823..cd198a83d 100644
--- a/src/runtime/c/gu/utf8.c
+++ b/src/runtime/c/gu/utf8.c
@@ -33,20 +33,21 @@ gu_in_utf8_(GuIn* in, GuExn* err)
 	if (!gu_ok(err)) {
 		return 0;
 	}
-	int len = (c < 0x80 ? 0 : 
-		   c < 0xc2 ? -1 :
-		   c < 0xe0 ? 1 :
-		   c < 0xf0 ? 2 :
-		   c < 0xf5 ? 3 :
-		   -1);
-	if (len < 0) {
-	 	goto fail;
-	} else if (len == 0) {
+	if (c < 0x80) {
 		return c;
+	} 
+	if (c < 0xc2) {
+		goto fail;
 	}
-	static const uint8_t mask[4] = { 0x7f, 0x1f, 0x0f, 0x07 };
-	uint32_t u = c & mask[len];
-	uint8_t buf[3];
+	int len = (c < 0xe0 ? 1 :
+	           c < 0xf0 ? 2 :
+	           c < 0xf8 ? 3 :
+	           c < 0xfc ? 4 :
+	                      5
+	          );
+	uint64_t mask = 0x0103070F1f7f;
+	uint32_t u = c & (mask >> (len * 8));
+	uint8_t buf[5];
 	// If reading the extra bytes causes EOF, it is an encoding
 	// error, not a legitimate end of character stream.
 	gu_in_bytes(in, buf, len, err);
@@ -78,55 +79,6 @@ fail:
 extern inline GuUCS
 gu_in_utf8(GuIn* in, GuExn* err);
 
-static size_t
-gu_advance_utf8(GuUCS ucs, uint8_t* buf)
-{
-	gu_require(gu_ucs_valid(ucs));
-	if (ucs < 0x80) {
-		buf[0] = (uint8_t) ucs;
-		return 1;
-	} else if (ucs < 0x800) {
-		buf[0] = 0xc0 | (ucs >> 6);
-		buf[1] = 0x80 | (ucs & 0x3f);
-		return 2;
-	} else if (ucs < 0x10000) {
-		buf[0] = 0xe0 | (ucs >> 12);
-		buf[1] = 0x80 | ((ucs >> 6) & 0x3f);
-		buf[2] = 0x80 | (ucs & 0x3f);
-		return 3;
-	} else {
-		buf[0] = 0xf0 | (ucs >> 18);
-		buf[1] = 0x80 | ((ucs >> 12) & 0x3f);
-		buf[2] = 0x80 | ((ucs >> 6) & 0x3f);
-		buf[3] = 0x80 | (ucs & 0x3f);
-		return 4;
-	}
-}
-
-
-void
-gu_out_utf8_(GuUCS ucs, GuOut* out, GuExn* err)
-{
-	uint8_t buf[4];
-	size_t sz = gu_advance_utf8(ucs, buf);
-	switch (sz) {
-	case 2:
-		gu_out_bytes(out, buf, 2, err);
-		break;
-	case 3:
-		gu_out_bytes(out, buf, 3, err);
-		break;
-	case 4:
-		gu_out_bytes(out, buf, 4, err);
-		break;
-	default:
-		gu_impossible();
-	}
-}
-
-extern inline void
-gu_out_utf8(GuUCS ucs, GuOut* out, GuExn* err);
-
 void
 gu_in_utf8_buf(uint8_t** buf, GuIn* in, GuExn* err)
 {
@@ -137,18 +89,21 @@ gu_in_utf8_buf(uint8_t** buf, GuIn* in, GuExn* err)
 		return;
 	}
 	*(p++) = c;
-	int len = (c < 0x80 ? 0 : 
-		   c < 0xc2 ? -1 :
-		   c < 0xe0 ? 1 :
-		   c < 0xf0 ? 2 :
-		   c < 0xf5 ? 3 :
-		   -1);
-	if (len < 0) {
-	 	goto fail;
-	} else if (len == 0) {
+
+	if (c < 0x80) {
 		*buf = p;
 		return;
 	}
+	if (c < 0xc2) {
+		goto fail;
+	}
+
+	int len = (c < 0xe0 ? 1 :
+		       c < 0xf0 ? 2 :
+		       c < 0xf8 ? 3 :
+		       c < 0xfc ? 4 :
+		                  5
+		       );
 	// If reading the extra bytes causes EOF, it is an encoding
 	// error, not a legitimate end of character stream.
 	gu_in_bytes(in, p, len, err);
@@ -166,3 +121,56 @@ fail:
 	gu_raise(err, GuUCSExn);
 	return;
 }
+
+void
+gu_utf8_encode(GuUCS ucs, uint8_t** buf)
+{
+	gu_require(gu_ucs_valid(ucs));
+	uint8_t* p = *buf;
+	if (ucs < 0x80) {
+		p[0] = (uint8_t) ucs;
+		*buf = p+1;
+	} else if (ucs < 0x800) {
+		p[0] = 0xc0 | (ucs >> 6);
+		p[1] = 0x80 | (ucs & 0x3f);
+		*buf = p+2;
+	} else if (ucs < 0x10000) {
+		p[0] = 0xe0 | (ucs >> 12);
+		p[1] = 0x80 | ((ucs >> 6) & 0x3f);
+		p[2] = 0x80 | (ucs & 0x3f);
+		*buf = p+3;
+	} else if (ucs < 0x200000) {
+		p[0] = 0xf0 | (ucs >> 18);
+		p[1] = 0x80 | ((ucs >> 12) & 0x3f);
+		p[2] = 0x80 | ((ucs >> 6) & 0x3f);
+		p[3] = 0x80 | (ucs & 0x3f);
+		*buf = p+4;
+	} else if (ucs < 0x4000000) {
+		p[0] = 0xf8 | (ucs >> 24);
+		p[1] = 0x80 | ((ucs >> 18) & 0x3f);
+		p[2] = 0x80 | ((ucs >> 12) & 0x3f);
+		p[3] = 0x80 | ((ucs >>  6) & 0x3f);
+		p[4] = 0x80 | (ucs & 0x3f);
+		*buf = p+5;
+	} else {
+		p[0] = 0xfc | (ucs >> 30);
+		p[1] = 0x80 | ((ucs >> 24) & 0x3f);
+		p[2] = 0x80 | ((ucs >> 18) & 0x3f);
+		p[3] = 0x80 | ((ucs >> 12) & 0x3f);
+		p[4] = 0x80 | ((ucs >> 6) & 0x3f);
+		p[5] = 0x80 | (ucs & 0x3f);
+		*buf = p+6;
+	}
+}
+
+void
+gu_out_utf8_(GuUCS ucs, GuOut* out, GuExn* err)
+{
+	uint8_t buf[6];
+	uint8_t* p = buf;
+	gu_utf8_encode(ucs, &p);
+	gu_out_bytes(out, buf, p-buf, err);
+}
+
+extern inline void
+gu_out_utf8(GuUCS ucs, GuOut* out, GuExn* err);
diff --git a/src/runtime/c/gu/utf8.h b/src/runtime/c/gu/utf8.h
index db7dccaf9..3ad28946d 100644
--- a/src/runtime/c/gu/utf8.h
+++ b/src/runtime/c/gu/utf8.h
@@ -32,7 +32,10 @@ gu_out_utf8(GuUCS ucs, GuOut* out, GuExn* err)
 // Helper functions used in other modules
 
 GuUCS
-gu_utf8_decode(const uint8_t** utf8);
+gu_utf8_decode(const uint8_t** buf);
+
+void
+gu_utf8_encode(GuUCS ucs, uint8_t** buf);
 
 void
 gu_in_utf8_buf(uint8_t** buf, GuIn* in, GuExn* err);