String compare optimization

Author: ladd

stdlib/public/runtime/MetadataLookup.cpp

@@ -39,6 +39,7 @@
 #include <vector>
 #include <list>
 #include <new>
+#include <cstring>
 
 using namespace swift;
 using namespace Demangle;
@@ -472,6 +473,13 @@ static bool sameObjCTypeManglings(Demangle::NodePointer node1,
 }
 #endif
 
+/// Optimization for the case where we need to compare a StringRef and a null terminated C string
+/// Not converting s2 to a StringRef avoids the need to call both strlen and memcmp
+static bool stringRefEqualsCString(StringRef s1, const char *s2) {
+  size_t length = s1.size();
+  return strncmp(s1.data(), s2, length) == 0 && s2[length] == '\0';
+}
+
 bool
 swift::_contextDescriptorMatchesMangling(const ContextDescriptor *context,
                                          Demangle::NodePointer node) {
@@ -496,7 +504,7 @@ swift::_contextDescriptorMatchesMangling(const ContextDescriptor *context,
       // Match to a mangled module name.
       if (node->getKind() != Demangle::Node::Kind::Module)
         return false;
-      if (!node->getText().equals(module->Name.get()))
+      if (!stringRefEqualsCString(node->getText(), module->Name.get()))
         return false;
 
       node = nullptr;
@@ -568,7 +576,7 @@ swift::_contextDescriptorMatchesMangling(const ContextDescriptor *context,
         auto nameNode = node->getChild(1);
         if (nameNode->getKind() != Demangle::Node::Kind::Identifier)
           return false;
-        if (nameNode->getText() == proto->Name.get()) {
+        if (stringRefEqualsCString(nameNode->getText(), proto->Name.get())) {
           node = node->getChild(0);
           break;
         }