Bug 1542736 - Part 4: Replace JS_UNDEC with js::AsciiDigitToNumber. r=jwalden

js::AsciiDigitToNumber is an optimised version of mozilla::AsciiAlphanumericToNumber
for known ASCII digit-only cases, which avoids the extra comparisons for ASCII
alphabetical characters. This ensures replacing JS_UNDEC with js::AsciiDigitToNumber
still emits the same assembly.

Differential Revision: https://phabricator.services.mozilla.com/D26507

--HG--
extra : moz-landing-system : lando

js/src/builtin/Array.cpp

@@ -227,7 +227,7 @@ static bool StringIsArrayIndexHelper(const CharT* s, uint32_t length,
   }
 
   uint32_t c = 0, previous = 0;
-  uint32_t index = JS7_UNDEC(*s++);
+  uint32_t index = AsciiDigitToNumber(*s++);
 
   /* Don't allow leading zeros. */
   if (index == 0 && s != end) {
@@ -240,7 +240,7 @@ static bool StringIsArrayIndexHelper(const CharT* s, uint32_t length,
     }
 
     previous = index;
-    c = JS7_UNDEC(*s);
+    c = AsciiDigitToNumber(*s);
     index = 10 * index + c;
   }
 

js/src/builtin/RegExp.cpp

@@ -1245,7 +1245,7 @@ static bool InterpretDollar(JSLinearString* matched, JSLinearString* string,
   char16_t c = currentDollar[1];
   if (IsAsciiDigit(c)) {
     /* $n, $nn */
-    unsigned num = JS7_UNDEC(c);
+    unsigned num = AsciiDigitToNumber(c);
     if (num > captures.length()) {
       // The result is implementation-defined, do not substitute.
       return false;
@@ -1255,7 +1255,7 @@ static bool InterpretDollar(JSLinearString* matched, JSLinearString* string,
     if (currentChar < replacementEnd) {
       c = *currentChar;
       if (IsAsciiDigit(c)) {
-        unsigned tmpNum = 10 * num + JS7_UNDEC(c);
+        unsigned tmpNum = 10 * num + AsciiDigitToNumber(c);
         // If num > captures.length(), the result is implementation-defined.
         // Consume next character only if num <= captures.length().
         if (tmpNum <= captures.length()) {

js/src/util/Text.h

@@ -17,6 +17,7 @@
 #include <stdint.h>
 #include <stdio.h>
 #include <string>
+#include <type_traits>
 
 #include "jsutil.h"
 #include "NamespaceImports.h"
@@ -27,13 +28,6 @@
 
 class JSLinearString;
 
-/*
- * Shorthands for ASCII (7-bit) decimal and hex conversion.
- * Manually inline isdigit and isxdigit for performance; MSVC doesn't do this
- * for us.
- */
-#define JS7_UNDEC(c) ((c) - '0')
-
 static MOZ_ALWAYS_INLINE size_t js_strlen(const char16_t* s) {
   return std::char_traits<char16_t>::length(s);
 }
@@ -48,6 +42,13 @@ namespace js {
 
 class StringBuffer;
 
+template <typename CharT>
+constexpr uint8_t AsciiDigitToNumber(CharT c) {
+  using UnsignedCharT = std::make_unsigned_t<CharT>;
+  auto uc = static_cast<UnsignedCharT>(c);
+  return uc - '0';
+}
+
 template <typename Char1, typename Char2>
 inline bool EqualChars(const Char1* s1, const Char2* s2, size_t len) {
   return mozilla::ArrayEqual(s1, s2, len);

js/src/vm/JSContext.cpp

@@ -14,6 +14,7 @@
 #include "mozilla/DebugOnly.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Sprintf.h"
+#include "mozilla/TextUtils.h"
 #include "mozilla/Unused.h"
 
 #include <ctype.h>
@@ -738,7 +739,7 @@ bool ExpandErrorArgumentsHelper(JSContext* cx, JSErrorCallback callback,
         while (*fmt) {
           if (*fmt == '{') {
             if (isdigit(fmt[1])) {
-              int d = JS7_UNDEC(fmt[1]);
+              int d = AsciiDigitToNumber(fmt[1]);
               MOZ_RELEASE_ASSERT(d < args.count());
               strncpy(out, args.args(d), args.lengths(d));
               out += args.lengths(d);

js/src/vm/StringType.cpp

@@ -1095,14 +1095,14 @@ bool JSFlatString::isIndexSlow(const CharT* s, size_t length,
   RangedPtr<const CharT> cp(s, length + 1);
   const RangedPtr<const CharT> end(s + length, s, length + 1);
 
-  uint32_t index = JS7_UNDEC(*cp++);
+  uint32_t index = AsciiDigitToNumber(*cp++);
   uint32_t oldIndex = 0;
   uint32_t c = 0;
 
   if (index != 0) {
     while (IsAsciiDigit(*cp)) {
       oldIndex = index;
-      c = JS7_UNDEC(*cp);
+      c = AsciiDigitToNumber(*cp);
       index = 10 * index + c;
       cp++;
     }

js/src/vm/TypedArrayObject.cpp

@@ -2316,7 +2316,7 @@ bool js::StringIsTypedArrayIndex(const CharT* s, size_t length,
   }
 
   uint64_t index = 0;
-  uint32_t digit = JS7_UNDEC(*s++);
+  uint32_t digit = AsciiDigitToNumber(*s++);
 
   /* Don't allow leading zeros. */
   if (digit == 0 && s != end) {
@@ -2330,7 +2330,7 @@ bool js::StringIsTypedArrayIndex(const CharT* s, size_t length,
       return false;
     }
 
-    digit = JS7_UNDEC(*s);
+    digit = AsciiDigitToNumber(*s);
 
     /* Watch for overflows. */
     if ((UINT64_MAX - digit) / 10 < index) {