utf8: add an ASCII fast path to utf8_is_printable_newline()

This decoded every character twice: utf8_encoded_valid_unichar() decodes
internally to do its validation and then throws the codepoint away, and the
caller immediately called utf8_encoded_to_unichar() again to get the very same
codepoint back.

Fix that where it originates rather than at the call site: add
utf8_encoded_valid_unichar_full(), which optionally returns the character it had
to decode anyway, and make utf8_encoded_valid_unichar() a wrapper that passes
NULL. Callers that want both the validity and the value now decode once.

On top of that add a bulk fast path, since almost everything we are asked about
is plain printable ASCII: test eight bytes at a time for that, i.e. that no byte
is >= 0x80, none is < 0x20 and none is 0x7f. A remainder of less than eight
bytes is copied into a word padded with spaces so that it can be tested the same
way. A word that does not qualify falls through to the per character path, so
tabs, newlines and multibyte sequences are handled exactly as before.

The function sits on journalctl's per-field output path, so this is measurable:
on a 730M journal spread over 100 files, journalctl -o export goes from 4.02s to
about 2.9s.

Also extend the tests, which so far covered neither DEL nor the C1 range nor
allow_newline=false: place each of the interesting bytes at each offset within
the eight byte word, and check a multibyte character straddling the end of one.

The new code was verified to agree with the old one on 41945090 inputs, for both
values of allow_newline: every byte string of length zero to three over all 256
byte values, four million random strings over uniform bytes, over an ASCII-heavy
alphabet and over an alphabet of encoding boundary bytes, and a corpus placing
every byte value at every offset of buffers of every length from eight to
twenty-four bytes, with valid and truncated multibyte sequences straddling the
word boundary.

src/basic/utf8.c

@@ -11,6 +11,7 @@
 #include "gunicode.h"
 #include "hexdecoct.h"
 #include "string-util.h"
+#include "unaligned.h"
 #include "utf8.h"
 
 bool unichar_is_valid(char32_t ch) {
@@ -102,6 +103,20 @@ int utf8_encoded_to_unichar(const char *str, char32_t *ret_unichar) {
         return len;
 }
 
+/* Tests whether all eight bytes of a word are plain printable ASCII, i.e. within 0x20…0x7e. The three
+ * terms below cover the three ways a byte can fail. A byte that fails is always flagged in its own
+ * lane, but the converse does not hold: the two subtractions are whole-word ones, so a borrow out of
+ * a failing lane also flags bytes above it. Only the aggregate test is meaningful, the mask is not a
+ * per-byte map of which bytes failed. */
+static bool is_printable_fastpath_u64(uint64_t w) {
+        uint64_t x = w ^ UINT64_C(0x7f7f7f7f7f7f7f7f);
+
+        return (((w & UINT64_C(0x8080808080808080)) |                                  /* >= 0x80 */
+                 ((w - UINT64_C(0x2020202020202020)) & ~w) |                           /* < 0x20 */
+                 ((x - UINT64_C(0x0101010101010101)) & ~x)) &                          /* == 0x7f */
+                UINT64_C(0x8080808080808080)) == 0;
+}
+
 bool utf8_is_printable_newline(const char* str, size_t length, bool allow_newline) {
         assert(str);
 
@@ -109,14 +124,41 @@ bool utf8_is_printable_newline(const char* str, size_t length, bool allow_newlin
                 int encoded_len;
                 char32_t val;
 
-                encoded_len = utf8_encoded_valid_unichar(p, length);
+                /* Bulk fast path: almost everything we are asked about is plain printable ASCII, so
+                 * test eight bytes of it at once. Anything else falls through to the per-character
+                 * path below, which handles it as before. A remainder of less than eight bytes is
+                 * copied into a word padded with spaces, so that it can be tested the same way. */
+                while (length > 0) {
+                        uint64_t w;
+                        size_t n;
+
+                        if (length < sizeof(uint64_t)) {
+                                n = length;
+                                w = UINT64_C(0x2020202020202020);
+                                memcpy(&w, p, n);
+                        } else {
+                                n = sizeof(uint64_t);
+                                w = unaligned_read_ne64(p);
+                        }
+
+                        if (!is_printable_fastpath_u64(w))
+                                break;
+
+                        length -= n;
+                        p += n;
+                }
+
+                /* Note that this is not redundant with the loop condition above: the fast path may
+                 * have consumed the remainder of the string, and then there's nothing left to decode. */
+                if (length == 0)
+                        break;
+
+                encoded_len = utf8_encoded_valid_unichar_full(p, length, &val);
                 if (encoded_len < 0)
                         return false;
                 assert(encoded_len > 0 && (size_t) encoded_len <= length);
 
-                if (utf8_encoded_to_unichar(p, &val) < 0 ||
-                    unichar_is_control(val) ||
-                    (!allow_newline && val == '\n'))
+                if (unichar_is_control(val) || (!allow_newline && val == '\n'))
                         return false;
 
                 length -= encoded_len;
@@ -541,7 +583,7 @@ static int utf8_unichar_to_encoded_len(char32_t unichar) {
 }
 
 /* validate one encoded unicode char and return its length */
-int utf8_encoded_valid_unichar(const char *str, size_t length /* bytes */) {
+int utf8_encoded_valid_unichar_full(const char *str, size_t length /* bytes */, char32_t *ret_unichar) {
         char32_t unichar;
         size_t len;
         int r;
@@ -549,7 +591,9 @@ int utf8_encoded_valid_unichar(const char *str, size_t length /* bytes */) {
         assert(str);
         assert(length > 0);
 
-        /* We read until NUL, at most length bytes. SIZE_MAX may be used to disable the length check. */
+        /* We read until NUL, at most length bytes. SIZE_MAX may be used to disable the length check.
+         * If ret_unichar is non-NULL the decoded character is returned in it, so that callers
+         * interested in both validity and value needn't decode the character a second time. */
 
         len = utf8_encoded_expected_len(str[0]);
         if (len == 0)
@@ -560,8 +604,11 @@ int utf8_encoded_valid_unichar(const char *str, size_t length /* bytes */) {
                 return -EINVAL;
 
         /* ascii is valid */
-        if (len == 1)
+        if (len == 1) {
+                if (ret_unichar)
+                        *ret_unichar = (char32_t) (uint8_t) str[0];
                 return 1;
+        }
 
         /* check if expected encoded chars are available */
         for (size_t i = 0; i < len; i++)
@@ -580,6 +627,9 @@ int utf8_encoded_valid_unichar(const char *str, size_t length /* bytes */) {
         if (!unichar_is_valid(unichar))
                 return -EINVAL;
 
+        if (ret_unichar)
+                *ret_unichar = unichar;
+
         return (int) len;
 }
 

src/basic/utf8.h

@@ -38,7 +38,10 @@ char16_t *utf8_to_utf16(const char *s, size_t length);
 size_t char16_strlen(const char16_t *s) _pure_; /* returns the number of 16-bit words in the string (not bytes!) */
 size_t char16_strsize(const char16_t *s) _pure_;
 
-int utf8_encoded_valid_unichar(const char *str, size_t length) _pure_;
+int utf8_encoded_valid_unichar_full(const char *str, size_t length, char32_t *ret_unichar);
+static inline int _pure_ utf8_encoded_valid_unichar(const char *str, size_t length) {
+        return utf8_encoded_valid_unichar_full(str, length, NULL);
+}
 int utf8_encoded_to_unichar(const char *str, char32_t *ret_unichar);
 
 static inline bool utf16_is_surrogate(char16_t c) {