[clang] [Attributes] Support Attributes being declared as only supporting late parsing when passing an experimental feature flag (PR #88596)

Dan Liew via cfe-commits cfe-commits at lists.llvm.org
Fri Apr 19 17:44:53 PDT 2024


https://github.com/delcypher updated https://github.com/llvm/llvm-project/pull/88596

>From 24ea6b9306e0da81b956718590bff3c3cfb9124c Mon Sep 17 00:00:00 2001
From: Dan Liew <dan at su-root.co.uk>
Date: Fri, 12 Apr 2024 17:36:19 -0700
Subject: [PATCH] [Attributes] Support Attributes being declared as supporting
 an "extended" late parsing mode

This patch changes the `LateParsed` field of `Attr` in `Attr.td` to be
an instantiation of the new `LateAttrParseKind` class. The instation can be one of the following:

* `LateAttrParsingNever` - Corresponds with the false value of `LateParsed` prior to this patch (the default for an attribute).
* `LateAttrParseStandard` - Corresponds with the true value of `LateParsed` prior to this patch.
* `LateAttrParseExperimentalExt` - A new mode described below.

`LateAttrParseExperimentalExt` is an experimental extension to
`LateAttrParseStandard`. Essentially this allows
`Parser::ParseGNUAttributes(...)` to distinguish between these cases:

1. Only `LateAttrParseExperimentalExt` attributes should be late parsed.
2. Both `LateAttrParseExperimentalExt` and `LateAttrParseStandard`
  attributes should be late parsed.

Callers (and indirect callers) of `Parser::ParseGNUAttributes(...)`
indicate the desired behavior by setting a flag in the
`LateParsedAttrList` object that is passed to the function.

In addition to the above, a new driver and frontend flag
(`-fexperimental-late-parse-attributes`) with a corresponding LangOpt
(`ExperimentalLateParseAttributes`) is added that changes how
`LateAttrParseExperimentalExt` attributes are parsed.

* When the flag is disabled (default), in cases where only
  `LateAttrParsingExperimentalOnly` late parsing is requested, the
  attribute will be parsed immediately (i.e. **NOT** late parsed). This
  allows the attribute to act just like a `LateAttrParseStandard`
  attribute when the flag is disabled.

* When the flag is enabled, in cases where only
  `LateAttrParsingExperimentalOnly` late parsing is requested, the
  attribute will be late parsed.

The motivation behind this change is to allow the new `counted_by`
attribute (part of `-fbounds-safety`) to support late parsing but
**only** when `-fexperimental-late-parse-attributes` is enabled. This
attribute needs to support late parsing to allow it to refer to fields
later in a struct definition (or function parameters declared later).
However, there isn't a precedent for supporting late attribute parsing
in C so this flag allows the new behavior to exist in Clang but not be
on by default. This behavior was requested as part of the
`-fbounds-safety` RFC process
(https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854/68).

This patch doesn't introduce any uses of `LateAttrParseExperimentalExt`.
This will be added for the `counted_by` attribute in a future patch
(https://github.com/llvm/llvm-project/pull/87596). A consequence is the
new behavior added in this patch is not yet testable. Hence, the lack of
tests covering the new behavior.

rdar://125400257
---
 clang/include/clang/Basic/Attr.td             |  78 +++++++---
 clang/include/clang/Basic/LangOptions.def     |   1 +
 clang/include/clang/Driver/Options.td         |   7 +
 clang/include/clang/Parse/Parser.h            |  15 +-
 clang/lib/Driver/ToolChains/Clang.cpp         |   3 +
 clang/lib/Parse/ParseDecl.cpp                 |  43 +++++-
 .../experimental-late-parse-attributes.c      |  12 ++
 clang/utils/TableGen/ClangAttrEmitter.cpp     | 136 +++++++++++++++---
 8 files changed, 252 insertions(+), 43 deletions(-)
 create mode 100644 clang/test/Driver/experimental-late-parse-attributes.c

diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index dc87a8c6f022dc..0df80118286c89 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -592,6 +592,48 @@ class AttrSubjectMatcherAggregateRule<AttrSubject subject> {
 
 def SubjectMatcherForNamed : AttrSubjectMatcherAggregateRule<Named>;
 
+// Late Attribute parsing mode enum
+class LateAttrParseKind <int val> {
+  int Kind = val;
+}
+
+// Never late parsed
+def LateAttrParseNever : LateAttrParseKind<0>;
+
+// Standard late attribute parsing
+//
+// This is language dependent. For example:
+//
+// * For C++ enables late parsing of a declaration attributes
+// * For C does not enable late parsing of attributes
+//
+def LateAttrParseStandard: LateAttrParseKind<1>;
+
+// Experimental extension to standard late attribute parsing
+//
+// This extension behaves like `LateAttrParseStandard` but allows
+// late parsing attributes in more contexts.
+//
+// In contexts where `LateAttrParseStandard` attributes are late
+// parsed, `LateAttrParseExperimentalExt` attributes will also
+// be late parsed.
+//
+// In contexts that only late parse `LateAttrParseExperimentalExt` attributes
+// (see `LateParsedAttrList::lateAttrParseExperimentalExtOnly()`)
+//
+// * If `-fexperimental-late-parse-attributes`
+//   (`LangOpts.ExperimentalLateParseAttributes`) is enabled the attribute
+//   will be late parsed.
+// * If `-fexperimental-late-parse-attributes`
+//   (`LangOpts.ExperimentalLateParseAttributes`) is disabled the attribute
+//   will **not** be late parsed (i.e parsed immediately).
+//
+// The following contexts are supported:
+//
+// * TODO: Add contexts here when they are implemented.
+//
+def LateAttrParseExperimentalExt : LateAttrParseKind<2>;
+
 class Attr {
   // The various ways in which an attribute can be spelled in source
   list<Spelling> Spellings;
@@ -603,8 +645,8 @@ class Attr {
   list<Accessor> Accessors = [];
   // Specify targets for spellings.
   list<TargetSpecificSpelling> TargetSpecificSpellings = [];
-  // Set to true for attributes with arguments which require delayed parsing.
-  bit LateParsed = 0;
+  // Specifies the late parsing kind.
+  LateAttrParseKind LateParsed = LateAttrParseNever;
   // Set to false to prevent an attribute from being propagated from a template
   // to the instantiation.
   bit Clone = 1;
@@ -3173,7 +3215,7 @@ def DiagnoseIf : InheritableAttr {
               BoolArgument<"ArgDependent", 0, /*fake*/ 1>,
               DeclArgument<Named, "Parent", 0, /*fake*/ 1>];
   let InheritEvenIfAlreadyPresent = 1;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let AdditionalMembers = [{
     bool isError() const { return diagnosticType == DT_Error; }
     bool isWarning() const { return diagnosticType == DT_Warning; }
@@ -3472,7 +3514,7 @@ def AssertCapability : InheritableAttr {
   let Spellings = [Clang<"assert_capability", 0>,
                    Clang<"assert_shared_capability", 0>];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3488,7 +3530,7 @@ def AcquireCapability : InheritableAttr {
                    GNU<"exclusive_lock_function">,
                    GNU<"shared_lock_function">];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3504,7 +3546,7 @@ def TryAcquireCapability : InheritableAttr {
                    Clang<"try_acquire_shared_capability", 0>];
   let Subjects = SubjectList<[Function],
                              ErrorDiag>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3520,7 +3562,7 @@ def ReleaseCapability : InheritableAttr {
                    Clang<"release_generic_capability", 0>,
                    Clang<"unlock_function", 0>];
   let Subjects = SubjectList<[Function]>;
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3539,7 +3581,7 @@ def RequiresCapability : InheritableAttr {
                    Clang<"requires_shared_capability", 0>,
                    Clang<"shared_locks_required", 0>];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3559,7 +3601,7 @@ def NoThreadSafetyAnalysis : InheritableAttr {
 def GuardedBy : InheritableAttr {
   let Spellings = [GNU<"guarded_by">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3570,7 +3612,7 @@ def GuardedBy : InheritableAttr {
 def PtGuardedBy : InheritableAttr {
   let Spellings = [GNU<"pt_guarded_by">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3581,7 +3623,7 @@ def PtGuardedBy : InheritableAttr {
 def AcquiredAfter : InheritableAttr {
   let Spellings = [GNU<"acquired_after">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3592,7 +3634,7 @@ def AcquiredAfter : InheritableAttr {
 def AcquiredBefore : InheritableAttr {
   let Spellings = [GNU<"acquired_before">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3603,7 +3645,7 @@ def AcquiredBefore : InheritableAttr {
 def AssertExclusiveLock : InheritableAttr {
   let Spellings = [GNU<"assert_exclusive_lock">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3614,7 +3656,7 @@ def AssertExclusiveLock : InheritableAttr {
 def AssertSharedLock : InheritableAttr {
   let Spellings = [GNU<"assert_shared_lock">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3627,7 +3669,7 @@ def AssertSharedLock : InheritableAttr {
 def ExclusiveTrylockFunction : InheritableAttr {
   let Spellings = [GNU<"exclusive_trylock_function">];
   let Args = [ExprArgument<"SuccessValue">, VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3640,7 +3682,7 @@ def ExclusiveTrylockFunction : InheritableAttr {
 def SharedTrylockFunction : InheritableAttr {
   let Spellings = [GNU<"shared_trylock_function">];
   let Args = [ExprArgument<"SuccessValue">, VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
@@ -3651,7 +3693,7 @@ def SharedTrylockFunction : InheritableAttr {
 def LockReturned : InheritableAttr {
   let Spellings = [GNU<"lock_returned">];
   let Args = [ExprArgument<"Arg">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let Subjects = SubjectList<[Function]>;
@@ -3661,7 +3703,7 @@ def LockReturned : InheritableAttr {
 def LocksExcluded : InheritableAttr {
   let Spellings = [GNU<"locks_excluded">];
   let Args = [VariadicExprArgument<"Args">];
-  let LateParsed = 1;
+  let LateParsed = LateAttrParseStandard;
   let TemplateDependent = 1;
   let ParseArgumentsAsUnevaluated = 1;
   let InheritEvenIfAlreadyPresent = 1;
diff --git a/clang/include/clang/Basic/LangOptions.def b/clang/include/clang/Basic/LangOptions.def
index 8ef6700ecdc78e..55c81eab1ec150 100644
--- a/clang/include/clang/Basic/LangOptions.def
+++ b/clang/include/clang/Basic/LangOptions.def
@@ -164,6 +164,7 @@ LANGOPT(ExperimentalLibrary, 1, 0, "enable unstable and experimental library fea
 LANGOPT(PointerAuthIntrinsics, 1, 0, "pointer authentication intrinsics")
 
 LANGOPT(DoubleSquareBracketAttributes, 1, 0, "'[[]]' attributes extension for all language standard modes")
+LANGOPT(ExperimentalLateParseAttributes, 1, 0, "experimental late parsing of attributes")
 
 COMPATIBLE_LANGOPT(RecoveryAST, 1, 1, "Preserve expressions in AST when encountering errors")
 COMPATIBLE_LANGOPT(RecoveryASTType, 1, 1, "Preserve the type in recovery expressions")
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 9a0b5d3304ca6e..711291c0caa6bb 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1603,6 +1603,13 @@ defm double_square_bracket_attributes : BoolFOption<"double-square-bracket-attri
   LangOpts<"DoubleSquareBracketAttributes">, DefaultTrue, PosFlag<SetTrue>,
  NegFlag<SetFalse>>;
 
+defm experimental_late_parse_attributes : BoolFOption<"experimental-late-parse-attributes",
+  LangOpts<"ExperimentalLateParseAttributes">, DefaultFalse,
+  PosFlag<SetTrue, [], [ClangOption], "Enable">,
+  NegFlag<SetFalse, [], [ClangOption], "Disable">,
+  BothFlags<[], [ClangOption, CC1Option],
+          " experimental late parsing of attributes">>;
+
 defm autolink : BoolFOption<"autolink",
   CodeGenOpts<"Autolink">, DefaultTrue,
   NegFlag<SetFalse, [], [ClangOption, CC1Option],
diff --git a/clang/include/clang/Parse/Parser.h b/clang/include/clang/Parse/Parser.h
index c719218731c35b..2c7e79b73369dd 100644
--- a/clang/include/clang/Parse/Parser.h
+++ b/clang/include/clang/Parse/Parser.h
@@ -1371,14 +1371,23 @@ class Parser : public CodeCompletionHandler {
   };
 
   // A list of late-parsed attributes.  Used by ParseGNUAttributes.
-  class LateParsedAttrList: public SmallVector<LateParsedAttribute *, 2> {
+  class LateParsedAttrList : public SmallVector<LateParsedAttribute *, 2> {
   public:
-    LateParsedAttrList(bool PSoon = false) : ParseSoon(PSoon) { }
+    LateParsedAttrList(bool PSoon = false,
+                       bool LateAttrParseExperimentalExtOnly = false)
+        : ParseSoon(PSoon),
+          LateAttrParseExperimentalExtOnly(LateAttrParseExperimentalExtOnly) {}
 
     bool parseSoon() { return ParseSoon; }
+    /// returns true iff the attribute to be parsed should only be late parsed
+    /// if it is annotated with `LateAttrParseExperimentalExt`
+    bool lateAttrParseExperimentalExtOnly() {
+      return LateAttrParseExperimentalExtOnly;
+    }
 
   private:
-    bool ParseSoon;  // Are we planning to parse these shortly after creation?
+    bool ParseSoon; // Are we planning to parse these shortly after creation?
+    bool LateAttrParseExperimentalExtOnly;
   };
 
   /// Contains the lexed tokens of a member function definition
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 766a9b91e3c0ad..539861601f359f 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -7480,6 +7480,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   Args.addOptInFlag(CmdArgs, options::OPT_fsafe_buffer_usage_suggestions,
                     options::OPT_fno_safe_buffer_usage_suggestions);
 
+  Args.addOptInFlag(CmdArgs, options::OPT_fexperimental_late_parse_attributes,
+                    options::OPT_fno_experimental_late_parse_attributes);
+
   // Setup statistics file output.
   SmallString<128> StatsFile = getStatsFileName(Args, Output, Input, D);
   if (!StatsFile.empty()) {
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 583232f2d610d0..61f85e58eee919 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -89,13 +89,24 @@ static StringRef normalizeAttrName(StringRef Name) {
   return Name;
 }
 
-/// isAttributeLateParsed - Return true if the attribute has arguments that
-/// require late parsing.
-static bool isAttributeLateParsed(const IdentifierInfo &II) {
+/// returns true iff attribute is annotated with `LateAttrParseExperimentalExt`
+/// in `Attr.td`.
+static bool IsAttributeLateParsedExperimentalExt(const IdentifierInfo &II) {
+#define CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST
+    return
+        llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+#include "clang/Parse/AttrParserStringSwitches.inc"
+            .Default(false);
+#undef CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST
+}
+
+/// returns true iff attribute is annotated with `LateAttrParseStandard` in
+/// `Attr.td`.
+static bool isAttributeLateParsedStandard(const IdentifierInfo &II) {
 #define CLANG_ATTR_LATE_PARSED_LIST
-    return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+  return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
 #include "clang/Parse/AttrParserStringSwitches.inc"
-        .Default(false);
+      .Default(false);
 #undef CLANG_ATTR_LATE_PARSED_LIST
 }
 
@@ -220,8 +231,28 @@ void Parser::ParseGNUAttributes(ParsedAttributes &Attrs,
         continue;
       }
 
+      bool LateParse = false;
+      if (!LateAttrs)
+        LateParse = false;
+      else {
+        if (LateAttrs->lateAttrParseExperimentalExtOnly()) {
+          // The caller requested that this attribute **only** be late
+          // parsed for `LateAttrParseExperimentalExt` attributes. This will
+          // only be late parsed if the experimental language option is enabled.
+          LateParse = IsAttributeLateParsedExperimentalExt(*AttrName) &&
+                      getLangOpts().ExperimentalLateParseAttributes;
+        } else {
+          // The caller did not restrict late parsing to only
+          // `LateAttrParseExperimentalExt` attributes so late parse
+          // both `LateAttrParseStandard` and `LateAttrParseExperimentalExt`
+          // attributes.
+          LateParse = IsAttributeLateParsedExperimentalExt(*AttrName) ||
+                      isAttributeLateParsedStandard(*AttrName);
+        }
+      }
+
       // Handle "parameterized" attributes
-      if (!LateAttrs || !isAttributeLateParsed(*AttrName)) {
+      if (!LateParse) {
         ParseGNUAttributeArgs(AttrName, AttrNameLoc, Attrs, &EndLoc, nullptr,
                               SourceLocation(), ParsedAttr::Form::GNU(), D);
         continue;
diff --git a/clang/test/Driver/experimental-late-parse-attributes.c b/clang/test/Driver/experimental-late-parse-attributes.c
new file mode 100644
index 00000000000000..6b54b898afa749
--- /dev/null
+++ b/clang/test/Driver/experimental-late-parse-attributes.c
@@ -0,0 +1,12 @@
+// RUN: %clang %s -c -fexperimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-ON
+// RUN: %clang %s -c -fno-experimental-late-parse-attributes -fexperimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-ON
+
+// CHECK-ON: -cc1
+// CHECK-ON: -fexperimental-late-parse-attributes
+
+// RUN: %clang %s -c 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+// RUN: %clang %s -c -fno-experimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+// RUN: %clang %s -c -fexperimental-late-parse-attributes -fno-experimental-late-parse-attributes 2>&1 -### | FileCheck %s -check-prefix=CHECK-OFF
+
+// CHECK-OFF: -cc1
+// CHECK-OFF-NOT: -fexperimental-late-parse-attributes
diff --git a/clang/utils/TableGen/ClangAttrEmitter.cpp b/clang/utils/TableGen/ClangAttrEmitter.cpp
index 6c56f99f503df4..a6338ba9224de8 100644
--- a/clang/utils/TableGen/ClangAttrEmitter.cpp
+++ b/clang/utils/TableGen/ClangAttrEmitter.cpp
@@ -1822,28 +1822,100 @@ void WriteSemanticSpellingSwitch(const std::string &VarName,
   OS << "  }\n";
 }
 
+enum class LateAttrParseKind { Never = 0, Standard = 1, ExperimentalExt = 2 };
+
+static LateAttrParseKind getLateAttrParseKind(const Record *Attr) {
+  // This function basically does
+  // `Attr->getValueAsDef("LateParsed")->getValueAsInt("Mode")` but does
+  // a bunch of sanity checking to ensure that
+  // `LateAttrParseMode` in `Attr.td` is in sync with the `LateAttrParseKind`
+  // enum in this source file.
+
+  static constexpr StringRef LateParsedStr = "LateParsed";
+  static constexpr StringRef LateAttrParseKindStr = "LateAttrParseKind";
+  static constexpr StringRef KindFieldStr = "Kind";
+
+  auto *LAPK = Attr->getValueAsDef(LateParsedStr);
+
+  // Typecheck the `LateParsed` field.
+  SmallVector<Record *, 1> SuperClasses;
+  LAPK->getDirectSuperClasses(SuperClasses);
+  if (SuperClasses.size() != 1)
+    PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) +
+                              "`should only have one super class");
+
+  if (SuperClasses[0]->getName().compare(LateAttrParseKindStr) != 0)
+    PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) +
+                              "`should only have type `" +
+                              llvm::Twine(LateAttrParseKindStr) +
+                              "` but found type `" +
+                              SuperClasses[0]->getName() + "`");
+
+  // Get Kind and verify the enum name matches the name in `Attr.td`.
+  unsigned Kind = LAPK->getValueAsInt(KindFieldStr);
+  switch (LateAttrParseKind(Kind)) {
+#define CASE(X)                                                                \
+  case LateAttrParseKind::X:                                                   \
+    if (LAPK->getName().compare("LateAttrParse" #X) != 0) {                    \
+      PrintFatalError(Attr,                                                    \
+                      "Field `" + llvm::Twine(LateParsedStr) + "` set to `" +  \
+                          LAPK->getName() +                                    \
+                          "` but this converts to `LateAttrParseKind::" +      \
+                          llvm::Twine(#X) + "`");                              \
+    }                                                                          \
+    return LateAttrParseKind::X;
+
+    CASE(Never)
+    CASE(Standard)
+    CASE(ExperimentalExt)
+#undef CASE
+  }
+
+  // The Kind value is completely invalid
+  auto KindValueStr = llvm::utostr(Kind);
+  PrintFatalError(Attr, "Field `" + llvm::Twine(LateParsedStr) + "` set to `" +
+                            LAPK->getName() + "` has unexpected `" +
+                            llvm::Twine(KindFieldStr) + "` value of " +
+                            KindValueStr);
+}
+
 // Emits the LateParsed property for attributes.
-static void emitClangAttrLateParsedList(RecordKeeper &Records, raw_ostream &OS) {
-  OS << "#if defined(CLANG_ATTR_LATE_PARSED_LIST)\n";
-  std::vector<Record*> Attrs = Records.getAllDerivedDefinitions("Attr");
+static void emitClangAttrLateParsedListImpl(RecordKeeper &Records,
+                                            raw_ostream &OS,
+                                            LateAttrParseKind LateParseMode) {
+  std::vector<Record *> Attrs = Records.getAllDerivedDefinitions("Attr");
 
   for (const auto *Attr : Attrs) {
-    bool LateParsed = Attr->getValueAsBit("LateParsed");
+    if (LateAttrParseKind LateParsed = getLateAttrParseKind(Attr);
+        LateParsed != LateParseMode)
+      continue;
 
-    if (LateParsed) {
-      std::vector<FlattenedSpelling> Spellings = GetFlattenedSpellings(*Attr);
+    std::vector<FlattenedSpelling> Spellings = GetFlattenedSpellings(*Attr);
 
-      // FIXME: Handle non-GNU attributes
-      for (const auto &I : Spellings) {
-        if (I.variety() != "GNU")
-          continue;
-        OS << ".Case(\"" << I.name() << "\", " << LateParsed << ")\n";
-      }
+    // FIXME: Handle non-GNU attributes
+    for (const auto &I : Spellings) {
+      if (I.variety() != "GNU")
+        continue;
+      OS << ".Case(\"" << I.name() << "\", 1)\n";
     }
   }
+}
+
+static void emitClangAttrLateParsedList(RecordKeeper &Records,
+                                        raw_ostream &OS) {
+  OS << "#if defined(CLANG_ATTR_LATE_PARSED_LIST)\n";
+  emitClangAttrLateParsedListImpl(Records, OS, LateAttrParseKind::Standard);
   OS << "#endif // CLANG_ATTR_LATE_PARSED_LIST\n\n";
 }
 
+static void emitClangAttrLateParsedExperimentalList(RecordKeeper &Records,
+                                                    raw_ostream &OS) {
+  OS << "#if defined(CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST)\n";
+  emitClangAttrLateParsedListImpl(Records, OS,
+                                  LateAttrParseKind::ExperimentalExt);
+  OS << "#endif // CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST\n\n";
+}
+
 static bool hasGNUorCXX11Spelling(const Record &Attribute) {
   std::vector<FlattenedSpelling> Spellings = GetFlattenedSpellings(Attribute);
   for (const auto &I : Spellings) {
@@ -2101,9 +2173,21 @@ bool PragmaClangAttributeSupport::isAttributedSupported(
     return SpecifiedResult;
 
   // Opt-out rules:
-  // An attribute requires delayed parsing (LateParsed is on)
-  if (Attribute.getValueAsBit("LateParsed"))
+
+  // An attribute requires delayed parsing (LateParsed is on).
+  switch (getLateAttrParseKind(&Attribute)) {
+  case LateAttrParseKind::Never:
+    break;
+  case LateAttrParseKind::Standard:
+    return false;
+  case LateAttrParseKind::ExperimentalExt:
+    // FIXME: fix this comment
+    // This is only late parsed when `LangOpts.ExperimentalLateParseAttributes`
+    // is true. Unfortunately `LangOpts` is not available in this method so
+    // just opt this attribute out.
     return false;
+  }
+
   // An attribute has no GNU/CXX11 spelling
   if (!hasGNUorCXX11Spelling(Attribute))
     return false;
@@ -2885,8 +2969,27 @@ static void emitAttributes(RecordKeeper &Records, raw_ostream &OS,
         return;
       }
       OS << "\n  : " << SuperName << "(Ctx, CommonInfo, ";
-      OS << "attr::" << R.getName() << ", "
-         << (R.getValueAsBit("LateParsed") ? "true" : "false");
+      OS << "attr::" << R.getName() << ", ";
+
+      // Handle different late parsing modes.
+      OS << "/*IsLateParsed=*/";
+      switch (getLateAttrParseKind(&R)) {
+      case LateAttrParseKind::Never:
+        OS << "false";
+        break;
+      case LateAttrParseKind::ExperimentalExt:
+        // Currently no clients need to know the distinction between `Standard`
+        // and `ExperimentalExt` so treat `ExperimentalExt` just like
+        // `Standard` for now.
+      case LateAttrParseKind::Standard:
+        // Note: This is misleading. `IsLateParsed` doesn't mean the
+        // attribute was actually late parsed. Instead it means the attribute in
+        // `Attr.td` is marked as being late parsed. Maybe it should be called
+        // `IsLateParseable`?
+        OS << "true";
+        break;
+      }
+
       if (Inheritable) {
         OS << ", "
            << (R.getValueAsBit("InheritEvenIfAlreadyPresent") ? "true"
@@ -4843,6 +4946,7 @@ void EmitClangAttrParserStringSwitches(RecordKeeper &Records, raw_ostream &OS) {
   emitClangAttrAcceptsExprPack(Records, OS);
   emitClangAttrTypeArgList(Records, OS);
   emitClangAttrLateParsedList(Records, OS);
+  emitClangAttrLateParsedExperimentalList(Records, OS);
 }
 
 void EmitClangAttrSubjectMatchRulesParserStringSwitches(RecordKeeper &Records,



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