Reduce memory reads when looking up MIME type from extension

Since, by construction, the extensions are stored sorted and
sequentially in memory, if we're comparing only the strings found in
the blob generated by mimegen, we can compare only the pointers;
otherwise, fall back to two 64-bit reads, replacing the indirect call
to strncmp().

The extensions are now stored in big-endian rather than in host-endian,
making it possible to compare things properly.  As a result, Brotli
liked the input data a bit more and was able to save a bit over 50
bytes; it's not much, but, hey, savings are savings.

src/bin/tools/mimegen.c

@@ -20,6 +20,7 @@
 #include <assert.h>
 #include <ctype.h>
 #include <errno.h>
+#include <endian.h>
 #include <fcntl.h>
 #include <stdint.h>
 #include <stdio.h>
@@ -67,20 +68,9 @@ output_append_full(struct output *output, const char *str, size_t str_len)
     return 0;
 }
 
-static int output_append_padded(struct output *output, const char *str)
+static int output_append_u64(struct output *output, uint64_t value)
 {
-    size_t str_len = strlen(str);
-
-    assert(str_len <= 8);
-
-    int r = output_append_full(output, str, str_len);
-    if (r < 0)
-        return r;
-
-    if (str_len != 8)
-        return output_append_full(output, "\0\0\0\0\0\0\0\0", 8 - str_len);
-
-    return 0;
+    return output_append_full(output, (char *)&value, 8);
 }
 
 static int output_append(struct output *output, const char *str)
@@ -316,14 +306,18 @@ int main(int argc, char *argv[])
         return 1;
     }
     for (i = 0; i < hash_get_count(ext_mime); i++) {
-        char ext_lower[9] = {0};
+        uint64_t ext_lower = 0;
 
-        strncpy(ext_lower, exts[i], 8);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstringop-truncation"
+        /* See lwan_determine_mime_type_for_file_name() in lwan-tables.c */
+        strncpy((char *)&ext_lower, exts[i], 8);
+#pragma GCC diagnostic pop
 
-        for (char *p = ext_lower; *p; p++)
-            *p &= ~0x20;
+        ext_lower &= ~0x2020202020202020ull;
+        ext_lower = htobe64(ext_lower);
 
-        if (output_append_padded(&output, ext_lower) < 0) {
+        if (output_append_u64(&output, ext_lower) < 0) {
             fprintf(stderr, "Could not append to output\n");
             fclose(fp);
             return 1;

src/lib/lwan-tables.c

@@ -120,13 +120,29 @@ LWAN_SELF_TEST(status_codes)
 #undef ASSERT_STATUS
 }
 
-static int
-compare_mime_entry(const void *a, const void *b)
+static int compare_mime_entry(const void *a, const void *b)
 {
-    const char *exta = (const char *)a;
-    const char *extb = (const char *)b;
+    static const uintptr_t begin = (uintptr_t)uncompressed_mime_entries;
+    static const uintptr_t end = begin + 8 * MIME_ENTRIES;
+    const uintptr_t pa = (uintptr_t)a;
+    const uintptr_t pb = (uintptr_t)b;
+    uint64_t exta;
+    uint64_t extb;
+
+    if (end - pa >= begin && end - pb >= begin) {
+        /* If both keys are within the uncompressed mime entries range, then
+         * we don't need to load from memory, just compare the pointers: they're
+         * all stored sequentially in memory by construction. */
+        exta = pa;
+        extb = pb;
+    } else {
+        /* These are stored in big-endian so the comparison below works
+         * as expected. */
+        exta = string_as_uint64((const char *)a);
+        extb = string_as_uint64((const char *)b);
+    }
 
-    return strncmp(exta, extb, 8);
+    return (exta > extb) - (exta < extb);
 }
 
 const char *
@@ -147,19 +163,19 @@ lwan_determine_mime_type_for_file_name(const char *file_name)
     }
 
     if (LIKELY(*last_dot)) {
-        uint64_t key;
+        uint64_t key = 0;
         const unsigned char *extension;
 
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wstringop-truncation"
         /* Data is stored with NULs on strings up to 7 chars, and no NULs
          * for 8-char strings, because that's implicit.  So truncation is
-         * intentional here: comparison in compare_mime_entry() uses
-         * strncmp(..., 8), so even if NUL isn't present, it'll stop at the
-         * right place.  */
+         * intentional here: comparison in compare_mime_entry() always loads
+         * 8 bytes per extension. */
         strncpy((char *)&key, last_dot + 1, 8);
 #pragma GCC diagnostic pop
         key &= ~0x2020202020202020ull;
+        key = htobe64(key);
 
         extension = bsearch(&key, uncompressed_mime_entries, MIME_ENTRIES, 8,
                             compare_mime_entry);