Speed up day/month parsing (#12592)

* Speed up day/month parsing

The "DFA" class is quite slow for parsing dates in headers because
it uses regex matching.  I instead pack characters into an integer
and do a simple linear search.

I benchmarked this approach against other methods like Trie, DFA,
SIMD, tree search.  I found this method is among the fastest for
this task, and remains portable, and readable.  It's at least 10x
faster than the previous way on x64/arm CPUs I tested.

Author: moonchen

src/proxy/hdrs/MIME.cc

@@ -25,10 +25,12 @@
 #include "tscore/ink_platform.h"
 #include "tscore/ink_memory.h"
 #include <cassert>
+#include <cctype>
 #include <cstdio>
 #include <cstring>
 #include <cctype>
 #include <algorithm>
+#include <string_view>
 #include "proxy/hdrs/MIME.h"
 #include "proxy/hdrs/HdrHeap.h"
 #include "proxy/hdrs/HdrToken.h"
@@ -51,17 +53,61 @@ using swoc::TextView;
  *                          C O N S T A N T S                          *
  *                                                                     *
  ***********************************************************************/
-static DFA *day_names_dfa   = nullptr;
-static DFA *month_names_dfa = nullptr;
 
-static constexpr const char *day_names[] = {
+namespace
+{
+constexpr std::array<std::string_view, 7> day_names = {
   "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat",
 };
 
-static constexpr const char *month_names[] = {
+constexpr std::array<std::string_view, 12> month_names = {
   "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec",
 };
 
+template <size_t count>
+consteval std::array<uint32_t, count>
+make_packed(const std::array<std::string_view, count> &names)
+{
+  std::array<uint32_t, count> packed{};
+
+  auto tl = [](char c) -> char { return (c >= 'A' && c <= 'Z') ? (c + 32) : c; };
+
+  for (size_t i = 0; i < count; ++i) {
+    const auto    &sv = names[i];
+    const uint32_t c0 = tl(static_cast<unsigned char>(sv[0]));
+    const uint32_t c1 = tl(static_cast<unsigned char>(sv[1]));
+    const uint32_t c2 = tl(static_cast<unsigned char>(sv[2]));
+    packed[i]         = (c0 << 16) | (c1 << 8) | c2;
+  }
+  return packed;
+}
+
+constexpr std::array<uint32_t, day_names.size()>   day_names_packed   = make_packed(day_names);
+constexpr std::array<uint32_t, month_names.size()> month_names_packed = make_packed(month_names);
+
+// Case-insensitive match of first 3 characters of input string against array of names
+// Longer strings will match if their first 3 characters match - this is intentional for
+// matching non-standard day/month names like "Thursday" or "September".
+template <size_t count>
+__attribute__((always_inline)) constexpr int
+match_3char_ci(const std::string_view s, const std::array<uint32_t, count> &names_packed)
+{
+  if (s.size() < 3) {
+    return -1;
+  }
+
+  auto           tl     = [](char c) -> char { return (c >= 'A' && c <= 'Z') ? (c + 32) : c; };
+  const uint32_t packed = (tl(s[0]) << 16) | (tl(s[1]) << 8) | tl(s[2]);
+
+  for (size_t i = 0; i < count; i++) {
+    if (packed == names_packed[i]) {
+      return i;
+    }
+  }
+  return -1;
+}
+} // namespace
+
 struct MDY {
   uint8_t  m;
   uint8_t  d;
@@ -594,11 +640,6 @@ mime_init()
     init = 0;
 
     hdrtoken_init();
-    day_names_dfa = new DFA;
-    day_names_dfa->compile(day_names, SIZEOF(day_names), RE_CASE_INSENSITIVE);
-
-    month_names_dfa = new DFA;
-    month_names_dfa->compile(month_names, SIZEOF(month_names), RE_CASE_INSENSITIVE);
 
     MIME_FIELD_ACCEPT                    = hdrtoken_string_to_wks_sv("Accept");
     MIME_FIELD_ACCEPT_CHARSET            = hdrtoken_string_to_wks_sv("Accept-Charset");
@@ -3123,8 +3164,7 @@ mime_parse_int64(const char *buf, const char *end)
 int
 mime_parse_rfc822_date_fastcase(const char *buf, int length, struct tm *tp)
 {
-  unsigned int     three_char_wday, three_char_mon;
-  std::string_view view{buf, size_t(length)};
+  unsigned int three_char_wday, three_char_mon;
 
   ink_assert(length >= 29);
   ink_assert(!is_ws(buf[0]));
@@ -3155,7 +3195,7 @@ mime_parse_rfc822_date_fastcase(const char *buf, int length, struct tm *tp)
     }
   }
   if (tp->tm_wday < 0) {
-    tp->tm_wday = day_names_dfa->match(view);
+    tp->tm_wday = match_3char_ci({buf, 3}, day_names_packed);
     if (tp->tm_wday < 0) {
       return 0;
     }
@@ -3208,7 +3248,7 @@ mime_parse_rfc822_date_fastcase(const char *buf, int length, struct tm *tp)
     }
   }
   if (tp->tm_mon < 0) {
-    tp->tm_mon = month_names_dfa->match(view);
+    tp->tm_mon = match_3char_ci({buf + 8, 3}, month_names_packed);
     if (tp->tm_mon < 0) {
       return 0;
     }
@@ -3353,7 +3393,7 @@ mime_parse_day(const char *&buf, const char *end, int *day)
     e += 1;
   }
 
-  *day = day_names_dfa->match({buf, size_t(e - buf)});
+  *day = match_3char_ci({buf, static_cast<size_t>(e - buf)}, day_names_packed);
   if (*day < 0) {
     return false;
   } else {
@@ -3376,7 +3416,7 @@ mime_parse_month(const char *&buf, const char *end, int *month)
     e += 1;
   }
 
-  *month = month_names_dfa->match({buf, size_t(e - buf)});
+  *month = match_3char_ci({buf, static_cast<size_t>(e - buf)}, month_names_packed);
   if (*month < 0) {
     return false;
   } else {