Implement data formatters for LibStdC++ std::variant (#68012)

This patch implements the data formatters for LibStdC++ `std::variant`.

---------

Co-authored-by: jeffreytan81 <jeffreytan@fb.com>

lldb/examples/synthetic/gnu_libstdcpp.py

@@ -892,3 +892,87 @@ class StdDequeSynthProvider:
         except:
             pass
         return False
+
+
+def VariantSummaryProvider(valobj, dict):
+    raw_obj = valobj.GetNonSyntheticValue()
+    index_obj = raw_obj.GetChildMemberWithName("_M_index")
+    data_obj = raw_obj.GetChildMemberWithName("_M_u")
+    if not (index_obj and index_obj.IsValid() and data_obj and data_obj.IsValid()):
+        return "<Can't find _M_index or _M_u>"
+
+    def get_variant_npos_value(index_byte_size):
+        if index_byte_size == 1:
+            return 0xFF
+        elif index_byte_size == 2:
+            return 0xFFFF
+        else:
+            return 0xFFFFFFFF
+
+    npos_value = get_variant_npos_value(index_obj.GetByteSize())
+    index = index_obj.GetValueAsUnsigned(0)
+    if index == npos_value:
+        return " No Value"
+
+    active_type = data_obj.GetType().GetTemplateArgumentType(index)
+    return f" Active Type = {active_type.GetDisplayTypeName()} "
+
+
+class VariantSynthProvider:
+    def __init__(self, valobj, dict):
+        self.raw_obj = valobj.GetNonSyntheticValue()
+        self.is_valid = False
+        self.index = None
+        self.data_obj = None
+
+    def update(self):
+        try:
+            self.index = self.raw_obj.GetChildMemberWithName(
+                "_M_index"
+            ).GetValueAsSigned(-1)
+            self.is_valid = self.index != -1
+            self.data_obj = self.raw_obj.GetChildMemberWithName("_M_u")
+        except:
+            self.is_valid = False
+        return False
+
+    def has_children(self):
+        return True
+
+    def num_children(self):
+        return 1 if self.is_valid else 0
+
+    def get_child_index(self, name):
+        return 0
+
+    def get_child_at_index(self, index):
+        if not self.is_valid:
+            return None
+        cur = 0
+        node = self.data_obj
+        while cur < self.index:
+            node = node.GetChildMemberWithName("_M_rest")
+            cur += 1
+
+        # _M_storage's type depends on variant field's type "_Type".
+        #  1. if '_Type' is literal type: _Type _M_storage.
+        #  2. otherwise, __gnu_cxx::__aligned_membuf<_Type> _M_storage.
+        #
+        # For 2. we have to cast it to underlying template _Type.
+
+        value = node.GetChildMemberWithName("_M_first").GetChildMemberWithName(
+            "_M_storage"
+        )
+        template_type = value.GetType().GetTemplateArgumentType(0)
+
+        # Literal type will return None for GetTemplateArgumentType(0)
+        if (
+            template_type
+            and "__gnu_cxx::__aligned_membuf" in value.GetType().GetDisplayTypeName()
+            and template_type.IsValid()
+        ):
+            value = value.Cast(template_type)
+
+        if value.IsValid():
+            return value.Clone("Value")
+        return None

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -332,14 +332,12 @@ bool CPlusPlusLanguage::MethodName::ContainsPath(llvm::StringRef path) {
   // If we can't parse the incoming name, then just check that it contains path.
   if (m_parse_error)
     return m_full.GetStringRef().contains(path);
-    
+
   llvm::StringRef identifier;
   llvm::StringRef context;
   std::string path_str = path.str();
-  bool success 
-      = CPlusPlusLanguage::ExtractContextAndIdentifier(path_str.c_str(),
-                                                       context,
-                                                       identifier);
+  bool success = CPlusPlusLanguage::ExtractContextAndIdentifier(
+      path_str.c_str(), context, identifier);
   if (!success)
     return m_full.GetStringRef().contains(path);
 
@@ -372,7 +370,7 @@ bool CPlusPlusLanguage::MethodName::ContainsPath(llvm::StringRef path) {
     return false;
   if (haystack.empty() || !isalnum(haystack.back()))
     return true;
-    
+
   return false;
 }
 
@@ -388,7 +386,7 @@ bool CPlusPlusLanguage::IsCPPMangledName(llvm::StringRef name) {
   return true;
 }
 
-bool CPlusPlusLanguage::DemangledNameContainsPath(llvm::StringRef path, 
+bool CPlusPlusLanguage::DemangledNameContainsPath(llvm::StringRef path,
                                                   ConstString demangled) const {
   MethodName demangled_name(demangled);
   return demangled_name.ContainsPath(path);
@@ -1104,6 +1102,11 @@ static void LoadLibStdcppFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
       SyntheticChildrenSP(new ScriptedSyntheticChildren(
           stl_synth_flags,
           "lldb.formatters.cpp.gnu_libstdcpp.StdForwardListSynthProvider")));
+  cpp_category_sp->AddTypeSynthetic(
+      "^std::variant<.+>$", eFormatterMatchRegex,
+      SyntheticChildrenSP(new ScriptedSyntheticChildren(
+          stl_synth_flags,
+          "lldb.formatters.cpp.gnu_libstdcpp.VariantSynthProvider")));
 
   stl_summary_flags.SetDontShowChildren(false);
   stl_summary_flags.SetSkipPointers(false);
@@ -1148,6 +1151,11 @@ static void LoadLibStdcppFormatters(lldb::TypeCategoryImplSP cpp_category_sp) {
       TypeSummaryImplSP(new ScriptSummaryFormat(
           stl_summary_flags,
           "lldb.formatters.cpp.gnu_libstdcpp.ForwardListSummaryProvider")));
+  cpp_category_sp->AddTypeSummary(
+      "^std::variant<.+>$", eFormatterMatchRegex,
+      TypeSummaryImplSP(new ScriptSummaryFormat(
+          stl_summary_flags,
+          "lldb.formatters.cpp.gnu_libstdcpp.VariantSummaryProvider")));
 
   AddCXXSynthetic(
       cpp_category_sp,

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/Makefile

@@ -0,0 +1,5 @@
+CXX_SOURCES := main.cpp
+
+USE_LIBSTDCPP := 1
+CXXFLAGS_EXTRAS := -std=c++17
+include Makefile.rules

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/TestDataFormatterLibStdcxxVariant.py

@@ -0,0 +1,69 @@
+"""
+Test lldb data formatter for LibStdC++ std::variant.
+"""
+
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class LibStdcxxVariantDataFormatterTestCase(TestBase):
+    @add_test_categories(["libstdcxx"])
+    def test_with_run_command(self):
+        """Test LibStdC++ std::variant data formatter works correctly."""
+        self.build()
+
+        (self.target, self.process, _, bkpt) = lldbutil.run_to_source_breakpoint(
+            self, "// break here", lldb.SBFileSpec("main.cpp", False)
+        )
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect(
+            "frame variable v1",
+            substrs=["v1 =  Active Type = int  {", "Value = 12", "}"],
+        )
+
+        self.expect(
+            "frame variable v1_ref",
+            substrs=["v1_ref =  Active Type = int : {", "Value = 12", "}"],
+        )
+
+        self.expect(
+            "frame variable v_v1",
+            substrs=[
+                "v_v1 =  Active Type = std::variant<int, double, char>  {",
+                "Value =  Active Type = int  {",
+                "Value = 12",
+                "}",
+                "}",
+            ],
+        )
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect(
+            "frame variable v1",
+            substrs=["v1 =  Active Type = double  {", "Value = 2", "}"],
+        )
+
+        lldbutil.continue_to_breakpoint(self.process, bkpt)
+
+        self.expect(
+            "frame variable v2",
+            substrs=["v2 =  Active Type = double  {", "Value = 2", "}"],
+        )
+
+        self.expect(
+            "frame variable v3",
+            substrs=["v3 =  Active Type = char  {", "Value = 'A'", "}"],
+        )
+
+        self.expect("frame variable v_no_value", substrs=["v_no_value =  No Value"])
+
+        self.expect(
+            "frame variable v_many_types_no_value",
+            substrs=["v_many_types_no_value =  No Value"],
+        )

lldb/test/API/functionalities/data-formatter/data-formatter-stl/libstdcpp/variant/main.cpp

@@ -0,0 +1,79 @@
+#include <cstdio>
+#include <string>
+#include <variant>
+#include <vector>
+
+struct S {
+  operator int() { throw 42; }
+};
+
+int main() {
+  bool has_variant = true;
+
+  printf("%d\n", has_variant); // break here
+
+  std::variant<int, double, char> v1;
+  std::variant<int, double, char> &v1_ref = v1;
+  std::variant<int, double, char> v2;
+  std::variant<int, double, char> v3;
+  std::variant<std::variant<int, double, char>> v_v1;
+  std::variant<int, double, char> v_no_value;
+  // The next variant has many types, meaning the type index does not fit in
+  // a byte and must be `unsigned short` instead of `unsigned char` when
+  // using the unstable libc++ ABI. With stable libc++ ABI, the type index
+  // is always just `unsigned int`.
+  std::variant<
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int, int, int, int,
+      int, int, int, int, int, int, int, int, int, int, int, int>
+      v_many_types_no_value;
+
+  v1 = 12; // v contains int
+  v_v1 = v1;
+  int i = std::get<int>(v1);
+  printf("%d\n", i); // break here
+
+  v2 = 2.0;
+  double d = std::get<double>(v2);
+  printf("%f\n", d);
+
+  v3 = 'A';
+  char c = std::get<char>(v3);
+  printf("%d\n", c);
+
+  // Checking v1 above and here to make sure we done maintain the incorrect
+  // state when we change its value.
+  v1 = 2.0;
+  d = std::get<double>(v1);
+  printf("%f\n", d); // break here
+
+  try {
+    v_no_value.emplace<0>(S());
+  } catch (...) {
+  }
+
+  printf("%zu\n", v_no_value.index());
+
+  try {
+    v_many_types_no_value.emplace<0>(S());
+  } catch (...) {
+  }
+
+  printf("%zu\n", v_many_types_no_value.index());
+
+  return 0; // break here
+}