diff --git a/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
--- a/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
+++ b/lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
@@ -555,6 +555,7 @@
 
 /// Determine the size in bytes of \p valobj (a libc++ std::string object) and
 /// extract its data payload. Return the size + payload pair.
+// TODO: Support big-endian architectures.
 static llvm::Optional<std::pair<uint64_t, ValueObjectSP>>
 ExtractLibcxxStringInfo(ValueObject &valobj) {
   ValueObjectSP D(valobj.GetChildAtIndexPath({0, 0, 0, 0}));
@@ -572,14 +573,27 @@
   ConstString g_size_name("__size_");
   bool short_mode = false; // this means the string is in short-mode and the
                            // data is stored inline
+  bool using_bitmasks = true; // Whether the class uses bitmasks for the mode
+                              // flag (pre-D123580).
+  uint64_t size;
   LibcxxStringLayoutMode layout = (layout_decider->GetName() == g_data_name)
                                       ? eLibcxxStringLayoutModeDSC
                                       : eLibcxxStringLayoutModeCSD;
   uint64_t size_mode_value = 0;
 
-  if (layout == eLibcxxStringLayoutModeDSC) {
+  if (ValueObjectSP is_long = D->GetChildAtNamePath(
+          {ConstString("__s"), ConstString("__is_long_")})) {
+    using_bitmasks = false;
+    short_mode = !is_long->GetValueAsUnsigned(/*fail_value=*/0);
+    if (ValueObjectSP size_member =
+            D->GetChildAtNamePath({ConstString("__s"), ConstString("__size_")}))
+      size = size_member->GetValueAsUnsigned(/*fail_value=*/0);
+    else
+      return {};
+  } else if (layout == eLibcxxStringLayoutModeDSC) {
     llvm::SmallVector<size_t, 3> size_mode_locations[] = {
-        {1, 2}, // Post-c3d0205ee771 layout
+        {1, 2}, // Post-c3d0205ee771 layout. This was in use for only a brief
+                // period, so we can delete it if it becomes a burden.
         {1, 1, 0},
         {1, 1, 1},
     };
@@ -610,9 +624,10 @@
       return {};
     ValueObjectSP location_sp = s->GetChildAtIndex(
         (layout == eLibcxxStringLayoutModeDSC) ? 0 : 1, true);
-    const uint64_t size = (layout == eLibcxxStringLayoutModeDSC)
-                              ? size_mode_value
-                              : ((size_mode_value >> 1) % 256);
+    if (using_bitmasks)
+      size = (layout == eLibcxxStringLayoutModeDSC)
+                 ? size_mode_value
+                 : ((size_mode_value >> 1) % 256);
 
     // When the small-string optimization takes place, the data must fit in the
     // inline string buffer (23 bytes on x86_64/Darwin). If it doesn't, it's
@@ -631,18 +646,18 @@
   if (!l)
     return {};
   // we can use the layout_decider object as the data pointer
-  ValueObjectSP location_sp = (layout == eLibcxxStringLayoutModeDSC)
-                                  ? layout_decider
-                                  : l->GetChildAtIndex(2, true);
-  ValueObjectSP size_vo(l->GetChildAtIndex(1, true));
-  const unsigned capacity_index =
-      (layout == eLibcxxStringLayoutModeDSC) ? 2 : 0;
-  ValueObjectSP capacity_vo(l->GetChildAtIndex(capacity_index, true));
+  ValueObjectSP location_sp =
+      l->GetChildMemberWithName(ConstString("__data_"), /*can_create=*/true);
+  ValueObjectSP size_vo =
+      l->GetChildMemberWithName(ConstString("__size_"), /*can_create=*/true);
+  ValueObjectSP capacity_vo =
+      l->GetChildMemberWithName(ConstString("__cap_"), /*can_create=*/true);
   if (!size_vo || !location_sp || !capacity_vo)
     return {};
-  const uint64_t size = size_vo->GetValueAsUnsigned(LLDB_INVALID_OFFSET);
-  const uint64_t capacity =
-      capacity_vo->GetValueAsUnsigned(LLDB_INVALID_OFFSET);
+  size = size_vo->GetValueAsUnsigned(LLDB_INVALID_OFFSET);
+  uint64_t capacity = capacity_vo->GetValueAsUnsigned(LLDB_INVALID_OFFSET);
+  if (!using_bitmasks && layout == eLibcxxStringLayoutModeCSD)
+    capacity *= 2;
   if (size == LLDB_INVALID_OFFSET || capacity == LLDB_INVALID_OFFSET ||
       capacity < size)
     return {};