sfeed_plain: optimize utf8-decoding and column position calculation

Optimize for the common-case: assuming ASCII.
The input is assumed to be valid UTF-8 input (output of sfeed).

This saves 2 function calls for determining the width of a single ASCII
character, which of course is 1.

Ranges:
< 32 are control-characters and are skipped.
< 127 is typical ASCII and is 1 column wide.
>= 127 is the normal path (control-character and multi-byte UTF-8).

Tested on OpenBSD and Linux with various compilers (clang, gcc, pcc and tcc).
On OpenBSD and Linux glibc much improvement. On Linux musl (almost) no change.

In a common-case upto 40% performance improvement.
In the worst-case negligible performance degration (<1%).

1 files changed, 8 insertions, 3 deletions

diff --git a/util.c b/util.c
index 6b155dc..51f7bd8 100644
--- a/util.c
+++ b/util.c
@@ -247,10 +247,15 @@ printutf8pad(FILE *fp, const char *s, size_t len, int pad)
 
 	slen = strlen(s);
 	for (i = 0; i < slen; i += rl) {
-		if ((rl = mbtowc(&wc, s + i, slen - i < 4 ? slen - i : 4)) <= 0)
-			break;
-		if ((w = wcwidth(wc)) == -1)
+		rl = w = 1;
+		if ((unsigned char)s[i] < 32)
 			continue;
+		if ((unsigned char)s[i] >= 127) {
+			if ((rl = mbtowc(&wc, s + i, slen - i < 4 ? slen - i : 4)) <= 0)
+				break;
+			if ((w = wcwidth(wc)) == -1)
+				continue;
+		}
 		if (col + w > len || (col + w == len && s[i + rl])) {
 			fputs("\xe2\x80\xa6", fp);
 			col++;