Micro-optimize jsStringWithCache()

https://bugs.webkit.org/show_bug.cgi?id=255106

Reviewed by Geoffrey Garen.

Micro-optimize jsStringWithCache():
- Call JSString::getValueImpl() instead of tryGetValueImpl() on the cached
  JSString. We know we never cache JSRopeStrings.
- Call JSString::create() directly in the slow path instead of jsString()
  to avoid special handling of strings with length 0 and 1. It is unecessary
  since the fast path in jsStringWithCache() already did this special handling.

* Source/JavaScriptCore/runtime/JSString.cpp:
(JSC::jsStringWithCacheSlowCase):
* Source/JavaScriptCore/runtime/JSString.h:
(JSC::jsStringWithCache):

Canonical link: https://commits.webkit.org/262712@main

Source/JavaScriptCore/runtime/JSString.cpp

@@ -397,7 +397,8 @@ bool JSString::getStringPropertyDescriptor(JSGlobalObject* globalObject, Propert
 
 JSString* jsStringWithCacheSlowCase(VM& vm, StringImpl& stringImpl)
 {
-    JSString* string = jsString(vm, String(stringImpl));
+    ASSERT(stringImpl.length() > 1 || (stringImpl.length() == 1 && stringImpl[0] > maxSingleCharacterString));
+    JSString* string = JSString::create(vm, stringImpl);
     vm.lastCachedString.set(vm, string);
     return string;
 }

Source/JavaScriptCore/runtime/JSString.h

@@ -267,6 +267,7 @@ class JSString : public JSCell {
     friend JSString* jsString(JSGlobalObject*, const String&, const String&);
     friend JSString* jsString(JSGlobalObject*, JSString*, JSString*, JSString*);
     friend JSString* jsString(JSGlobalObject*, const String&, const String&, const String&);
+    friend JS_EXPORT_PRIVATE JSString* jsStringWithCacheSlowCase(VM&, StringImpl&);
     friend JSString* jsSingleCharacterString(VM&, UChar);
     friend JSString* jsSingleCharacterString(VM&, LChar);
     friend JSString* jsNontrivialString(VM&, const String&);
@@ -1036,22 +1037,23 @@ inline JSString* jsOwnedString(VM& vm, const String& s)
 
 ALWAYS_INLINE JSString* jsStringWithCache(VM& vm, const String& s)
 {
-    StringImpl* stringImpl = s.impl();
-    if (!stringImpl || !stringImpl->length())
+    unsigned length = s.length();
+    if (!length)
         return jsEmptyString(vm);
 
-    if (stringImpl->length() == 1) {
-        UChar singleCharacter = (*stringImpl)[0u];
-        if (singleCharacter <= maxSingleCharacterString)
-            return vm.smallStrings.singleCharacterString(static_cast<unsigned char>(singleCharacter));
+    auto& stringImpl = *s.impl();
+    if (length == 1) {
+        auto c = stringImpl[0];
+        if (c <= maxSingleCharacterString)
+            return vm.smallStrings.singleCharacterString(c);
     }
 
-    if (JSString* lastCachedString = vm.lastCachedString.get()) {
-        if (lastCachedString->tryGetValueImpl() == stringImpl)
+    if (auto* lastCachedString = vm.lastCachedString.get()) {
+        if (lastCachedString->getValueImpl() == &stringImpl)
             return lastCachedString;
     }
 
-    return jsStringWithCacheSlowCase(vm, *stringImpl);
+    return jsStringWithCacheSlowCase(vm, stringImpl);
 }
 
 ALWAYS_INLINE bool JSString::getStringPropertySlot(JSGlobalObject* globalObject, PropertyName propertyName, PropertySlot& slot)