Improve attribute documentation to list which spellings are used in which syntaxes.

[rsmith]

Summary

Instead of listing all the spellings (including attribute namespaces) in
the section heading, only list the actual attribute names there, and
list the spellings in the supported syntaxes table.

This allows us to properly describe things like [[fallthrough]], for
which we allow a clang:: prefix in C++ but not in C, and AlwaysInline,
which has one spelling as a GNU attribute and a different spelling as a
keyword, without needing to repeat the syntax description in the
documentation text.

Sample rendering: https://pste.eu/p/T1ZV.html

[aaron.ballman]

LGTM with a commenting nit. Thank you for this, I like this exposition better!

utils/TableGen/ClangAttrEmitter.cpp

3881 ↗

(On Diff #163252)

We don't actually support Microsoft's attribute spellings currently and have no attributes there to document. I think the fixme should probably read "TODO: support documenting Microsoft spellings" or something more concrete.

[rsmith]

utils/TableGen/ClangAttrEmitter.cpp

3881 ↗

(On Diff #163252)

Done. (I accidentally pushed the old version, so this is done in r341100.)

For what it's worth, we have one Microsoft spelling listed in the .td file already (but I assume this has no effect):

def Uuid : InheritableAttr {
  let Spellings = [Declspec<"uuid">, Microsoft<"uuid">];

cfe/trunk/include/clang/Basic/AttrDocs.td

def GlobalDocumentation {
   -------------------------------------------------------------------
 
 ===================
 Attributes in Clang
 ===================
 .. contents::
    :local:
 
+.. |br| raw:: html
+
+  <br/>
+
 Introduction
 ============
 
 This page lists the attributes currently supported by Clang.
 }];
 }
 
 def SectionDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
 The ``section`` attribute allows you to specify a specific section a
 global variable or function should be in after translation.
   }];
-  let Heading = "section (gnu::section, __declspec(allocate))";
+  let Heading = "section, __declspec(allocate)";
 }
 
 def InitSegDocs : Documentation {
   let Category = DocCatVariable;
   let Content = [{
 The attribute applied by ``pragma init_seg()`` controls the section into
 which global initialization function pointers are emitted.  It is only
 available with ``-fms-extensions``.  Typically, this function pointer is

 A function declared as ``[[noreturn]]`` shall not return to its caller. The
 compiler will generate a diagnostic for a function declared as ``[[noreturn]]``
 that appears to be capable of returning to its caller.
   }];
 }
 
 def AssertCapabilityDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "assert_capability (assert_shared_capability, clang::assert_capability, clang::assert_shared_capability)";
+  let Heading = "assert_capability, assert_shared_capability";
   let Content = [{
 Marks a function that dynamically tests whether a capability is held, and halts
 the program if it is not held.
   }];
 }
 
 def AcquireCapabilityDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "acquire_capability (acquire_shared_capability, clang::acquire_capability, clang::acquire_shared_capability)";
+  let Heading = "acquire_capability, acquire_shared_capability";
   let Content = [{
 Marks a function as acquiring a capability.
   }];
 }
 
 def TryAcquireCapabilityDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "try_acquire_capability (try_acquire_shared_capability, clang::try_acquire_capability, clang::try_acquire_shared_capability)";
+  let Heading = "try_acquire_capability, try_acquire_shared_capability";
   let Content = [{
 Marks a function that attempts to acquire a capability. This function may fail to
 actually acquire the capability; they accept a Boolean value determining
 whether acquiring the capability means success (true), or failing to acquire
 the capability means success (false).
   }];
 }
 
 def ReleaseCapabilityDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "release_capability (release_shared_capability, clang::release_capability, clang::release_shared_capability)";
+  let Heading = "release_capability, release_shared_capability";
   let Content = [{
 Marks a function as releasing a capability.
   }];
 }
 
 def AssumeAlignedDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
.. code-block:: c++
   SAFE_STATIC T x = {42}; // Initialization OK. Doesn't check destructor.
   SAFE_STATIC T y = 42; // error: variable does not have a constant initializer
   // copy initialization is not a constant expression on a non-literal type.
   }];
 }
 
 def WarnMaybeUnusedDocs : Documentation {
   let Category = DocCatVariable;
-  let Heading = "maybe_unused, unused, gnu::unused";
+  let Heading = "maybe_unused, unused";
   let Content = [{
 When passing the ``-Wunused`` flag to Clang, entities that are unused by the
 program may be diagnosed. The ``[[maybe_unused]]`` (or
 ``__attribute__((unused))``) attribute can be used to silence such diagnostics
 when the entity cannot be removed. For instance, a local variable may exist
 solely for use in an ``assert()`` statement, which makes the local variable
 unused when ``NDEBUG`` is defined.
 
[[maybe_unused]] void f([[maybe_unused]] bool thing1,
     [[maybe_unused]] bool b = thing1 && thing2;
     assert(b);
   }
   }];
 }
 
 def WarnUnusedResultsDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "nodiscard, warn_unused_result, clang::warn_unused_result, gnu::warn_unused_result";
+  let Heading = "nodiscard, warn_unused_result";
   let Content  = [{
 Clang supports the ability to diagnose when the results of a function call
 expression are discarded under suspicious circumstances. A diagnostic is
 generated when a function or its return type is marked with ``[[nodiscard]]``
 (or ``__attribute__((warn_unused_result))``) and the function call appears as a
 potentially-evaluated discarded-value expression that is not explicitly cast to
 `void`.
 
 .. code-block: c++
   struct [[nodiscard]] error_info { /*...*/ };
   error_info enable_missile_safety_mode();
 
   void launch_missiles();
   void test_missiles() {
     enable_missile_safety_mode(); // diagnoses
     launch_missiles();
   }
   error_info &foo();
   void f() { foo(); } // Does not diagnose, error_info is a reference.
   }];
 }
 
 def FallthroughDocs : Documentation {
   let Category = DocCatStmt;
-  let Heading = "fallthrough, clang::fallthrough";
+  let Heading = "fallthrough";
   let Content = [{
 The ``fallthrough`` (or ``clang::fallthrough``) attribute is used
 to annotate intentional fall-through
 between switch labels.  It can only be applied to a null statement placed at a
 point of execution between any statement and the next switch label.  It is
 common to mark these places with a specific comment, but this attribute is
 meant to replace comments with a more strict annotation, which can be checked
 by the compiler.  This attribute doesn't change semantics of the code and can

 
 These attributes override the `-mmicromips` and `-mno-micromips` options
 on the command line.
   }];
 }
 
 def MipsLongCallStyleDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "long_call (gnu::long_call, gnu::far)";
+  let Heading = "long_call, far";
   let Content = [{
 Clang supports the ``__attribute__((long_call))``, ``__attribute__((far))``,
 and ``__attribute__((near))`` attributes on MIPS targets. These attributes may
 only be added to function declarations and change the code generated
 by the compiler when directly calling the function. The ``near`` attribute
 allows calls to the function to be made using the ``jal`` instruction, which
 requires the function to be located in the same naturally aligned 256MB
 segment as the caller.  The ``long_call`` and ``far`` attributes are synonyms
 and require the use of a different call sequence that works regardless
 of the distance between the functions.
 
 These attributes have no effect for position-independent code.
 
 These attributes take priority over command line switches such
 as ``-mlong-calls`` and ``-mno-long-calls``.
   }];
 }
 
 def MipsShortCallStyleDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "short_call (gnu::short_call, gnu::near)";
+  let Heading = "short_call, near";
   let Content = [{
 Clang supports the ``__attribute__((long_call))``, ``__attribute__((far))``,
 ``__attribute__((short__call))``, and ``__attribute__((near))`` attributes
 on MIPS targets. These attributes may only be added to function declarations
 and change the code generated by the compiler when directly calling
 the function. The ``short_call`` and ``near`` attributes are synonyms and
 allow calls to the function to be made using the ``jal`` instruction, which
 requires the function to be located in the same naturally aligned 256MB segment

 full list of supported sanitizer flags.
   }];
 }
 
 def NoSanitizeAddressDocs : Documentation {
   let Category = DocCatFunction;
   // This function has multiple distinct spellings, and so it requires a custom
   // heading to be specified. The most common spelling is sufficient.
-  let Heading = "no_sanitize_address (no_address_safety_analysis, gnu::no_address_safety_analysis, gnu::no_sanitize_address)";
+  let Heading = "no_sanitize_address, no_address_safety_analysis";
   let Content = [{
 .. _langext-address_sanitizer:
 
 Use ``__attribute__((no_sanitize_address))`` on a function or a global
 variable declaration to specify that address safety instrumentation
 (e.g. AddressSanitizer) should not be applied.
   }];
 }

 `language extensions
 <http://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations>`_
 for further details including limitations of the unroll hints.
   }];
 }
 
 def OpenCLUnrollHintDocs : Documentation {
   let Category = DocCatStmt;
-  let Heading = "__attribute__((opencl_unroll_hint))";
   let Content = [{
 The opencl_unroll_hint attribute qualifier can be used to specify that a loop
 (for, while and do loops) can be unrolled. This attribute qualifier can be
 used to specify full unrolling or partial unrolling by a specified amount.
 This is a compiler hint and the compiler may ignore this directive. See
 `OpenCL v2.0 <https://www.khronos.org/registry/cl/specs/opencl-2.0.pdf>`_
 s6.11.5 for details.
   }];
 }
 
 def OpenCLIntelReqdSubGroupSizeDocs : Documentation {
   let Category = DocCatStmt;
-  let Heading = "__attribute__((intel_reqd_sub_group_size))";
   let Content = [{
 The optional attribute intel_reqd_sub_group_size can be used to indicate that
 the kernel must be compiled and executed with the specified subgroup size. When
 this attribute is present, get_max_sub_group_size() is guaranteed to return the
 specified integer value. This is important for the correctness of many subgroup
 algorithms, and in some cases may be used by the compiler to generate more optimal
 code. See `cl_intel_required_subgroup_size
 <https://www.khronos.org/registry/OpenCL/extensions/intel/cl_intel_required_subgroup_size.txt>`

 See the RenderScript_ documentation for more information.
 
 .. _RenderScript: https://developer.android.com/guide/topics/renderscript/compute.html
   }];
 }
 
 def XRayDocs : Documentation {
   let Category = DocCatFunction;
-  let Heading = "xray_always_instrument (clang::xray_always_instrument), xray_never_instrument (clang::xray_never_instrument), xray_log_args (clang::xray_log_args)";
+  let Heading = "xray_always_instrument, xray_never_instrument, xray_log_args";
   let Content = [{
 ``__attribute__((xray_always_instrument))`` or ``[[clang::xray_always_instrument]]`` is used to mark member functions (in C++), methods (in Objective C), and free functions (in C, C++, and Objective C) to be instrumented with XRay. This will cause the function to always have space at the beginning and exit points to allow for runtime patching.
 
 Conversely, ``__attribute__((xray_never_instrument))`` or ``[[clang::xray_never_instrument]]`` will inhibit the insertion of these instrumentation points.
 
 If a function has neither of these attributes, they become subject to the XRay heuristics used to determine whether a function should be instrumented or otherwise.
 
 ``__attribute__((xray_log_args(N)))`` or ``[[clang::xray_log_args(N)]]`` is used to preserve N function arguments for the logging function.  Currently, only N==1 is supported.

cfe/trunk/utils/TableGen/ClangAttrEmitter.cpp

void EmitClangAttrParserStringSwitches(RecordKeeper &Records,
   emitClangAttrLateParsedList(Records, OS);
 }
 
 void EmitClangAttrSubjectMatchRulesParserStringSwitches(RecordKeeper &Records,
                                                         raw_ostream &OS) {
   getPragmaAttributeSupport(Records).generateParsingHelpers(OS);
 }
 
+enum class SpellingKind {
+  GNU,
+  CXX11,
+  C2x,
+  Declspec,
+  Microsoft,
+  Keyword,
+  Pragma,
+};
+static const size_t NumSpellingKinds = (size_t)SpellingKind::Pragma + 1;
+
+class SpellingList {
+  std::vector<std::string> Spellings[NumSpellingKinds];
+
+public:
+  ArrayRef<std::string> operator[](SpellingKind K) const {
+    return Spellings[(size_t)K];
+  }
+
+  void add(const Record &Attr, FlattenedSpelling Spelling) {
+    SpellingKind Kind = StringSwitch<SpellingKind>(Spelling.variety())
+                            .Case("GNU", SpellingKind::GNU)
+                            .Case("CXX11", SpellingKind::CXX11)
+                            .Case("C2x", SpellingKind::C2x)
+                            .Case("Declspec", SpellingKind::Declspec)
+                            .Case("Microsoft", SpellingKind::Microsoft)
+                            .Case("Keyword", SpellingKind::Keyword)
+                            .Case("Pragma", SpellingKind::Pragma);
+    std::string Name;
+    if (!Spelling.nameSpace().empty()) {
+      switch (Kind) {
+      case SpellingKind::CXX11:
+      case SpellingKind::C2x:
+        Name = Spelling.nameSpace() + "::";
+        break;
+      case SpellingKind::Pragma:
+        Name = Spelling.nameSpace() + " ";
+        break;
+      default:
+        PrintFatalError(Attr.getLoc(), "Unexpected namespace in spelling");
+      }
+    }
+    Name += Spelling.name();
+
+    Spellings[(size_t)Kind].push_back(Name);
+  }
+};
+
 class DocumentationData {
 public:
   const Record *Documentation;
   const Record *Attribute;
   std::string Heading;
-  unsigned SupportedSpellings;
+  SpellingList SupportedSpellings;
 
   DocumentationData(const Record &Documentation, const Record &Attribute,
-                    const std::pair<std::string, unsigned> HeadingAndKinds)
+                    std::pair<std::string, SpellingList> HeadingAndSpellings)
       : Documentation(&Documentation), Attribute(&Attribute),
-        Heading(std::move(HeadingAndKinds.first)),
-        SupportedSpellings(HeadingAndKinds.second) {}
+        Heading(std::move(HeadingAndSpellings.first)),
+        SupportedSpellings(std::move(HeadingAndSpellings.second)) {}
 };
 
 static void WriteCategoryHeader(const Record *DocCategory,
                                 raw_ostream &OS) {
   const StringRef Name = DocCategory->getValueAsString("Name");
   OS << Name << "\n" << std::string(Name.size(), '=') << "\n";
 
   // If there is content, print that as well.
   const StringRef ContentStr = DocCategory->getValueAsString("Content");
   // Trim leading and trailing newlines and spaces.
   OS << ContentStr.trim();
 
   OS << "\n\n";
 }
 
-enum SpellingKind {
-  GNU = 1 << 0,
-  CXX11 = 1 << 1,
-  C2x = 1 << 2,
-  Declspec = 1 << 3,
-  Microsoft = 1 << 4,
-  Keyword = 1 << 5,
-  Pragma = 1 << 6
-};
-
-static std::pair<std::string, unsigned>
-GetAttributeHeadingAndSpellingKinds(const Record &Documentation,
-                                    const Record &Attribute) {
+static std::pair<std::string, SpellingList>
+GetAttributeHeadingAndSpellings(const Record &Documentation,
   // FIXME: there is no way to have a per-spelling category for the attribute
   // documentation. This may not be a limiting factor since the spellings
   // should generally be consistently applied across the category.
 
   std::vector<FlattenedSpelling> Spellings = GetFlattenedSpellings(Attribute);
+  if (Spellings.empty())
+    PrintFatalError(Attribute.getLoc(),
+                    "Attribute has no supported spellings; cannot be "
+                    "documented");
 
   // Determine the heading to be used for this attribute.
   std::string Heading = Documentation.getValueAsString("Heading");
-  bool CustomHeading = !Heading.empty();
   if (Heading.empty()) {
     // If there's only one spelling, we can simply use that.
     if (Spellings.size() == 1)
       Heading = Spellings.begin()->name();
     else {
       std::set<std::string> Uniques;
       for (auto I = Spellings.begin(), E = Spellings.end();
            I != E && Uniques.size() <= 1; ++I) {
         std::string Spelling = NormalizeNameForSpellingComparison(I->name());
         Uniques.insert(Spelling);
       }
       // If the semantic map has only one spelling, that is sufficient for our
       // needs.
       if (Uniques.size() == 1)
         Heading = *Uniques.begin();
     }
   }
 
   // If the heading is still empty, it is an error.
   if (Heading.empty())
     PrintFatalError(Attribute.getLoc(),
                     "This attribute requires a heading to be specified");
 
-  // Gather a list of unique spellings; this is not the same as the semantic
-  // spelling for the attribute. Variations in underscores and other non-
-  // semantic characters are still acceptable.
-  std::vector<std::string> Names;
+  SpellingList SupportedSpellings;
+  for (const auto &I : Spellings)
+    SupportedSpellings.add(Attribute, I);
 
-  unsigned SupportedSpellings = 0;
-  for (const auto &I : Spellings) {
-    SpellingKind Kind = StringSwitch<SpellingKind>(I.variety())
-                            .Case("GNU", GNU)
-                            .Case("CXX11", CXX11)
-                            .Case("C2x", C2x)
-                            .Case("Declspec", Declspec)
-                            .Case("Microsoft", Microsoft)
-                            .Case("Keyword", Keyword)
-                            .Case("Pragma", Pragma);
-
-    // Mask in the supported spelling.
-    SupportedSpellings |= Kind;
-
-    std::string Name;
-    if ((Kind == CXX11 || Kind == C2x) && !I.nameSpace().empty())
-      Name = I.nameSpace() + "::";
-    Name += I.name();
-
-    // If this name is the same as the heading, do not add it.
-    if (Name != Heading)
-      Names.push_back(Name);
-  }
-
-  // Print out the heading for the attribute. If there are alternate spellings,
-  // then display those after the heading.
-  if (!CustomHeading && !Names.empty()) {
-    Heading += " (";
-    for (auto I = Names.begin(), E = Names.end(); I != E; ++I) {
-      if (I != Names.begin())
-        Heading += ", ";
-      Heading += *I;
-    }
-    Heading += ")";
-  }
-  if (!SupportedSpellings)
-    PrintFatalError(Attribute.getLoc(),
-                    "Attribute has no supported spellings; cannot be "
-                    "documented");
-  return std::make_pair(std::move(Heading), SupportedSpellings);
+  return std::make_pair(std::move(Heading), std::move(SupportedSpellings));
 }
 
 static void WriteDocumentation(RecordKeeper &Records,
                                const DocumentationData &Doc, raw_ostream &OS) {
   OS << Doc.Heading << "\n" << std::string(Doc.Heading.length(), '-') << "\n";
 
   // List what spelling syntaxes the attribute supports.
   OS << ".. csv-table:: Supported Syntaxes\n";
-  OS << "   :header: \"GNU\", \"C++11\", \"C2x\", \"__declspec\", \"Keyword\",";
-  OS << " \"Pragma\", \"Pragma clang attribute\"\n\n";
+  OS << "   :header: \"GNU\", \"C++11\", \"C2x\", \"``__declspec``\",";
+  OS << " \"Keyword\", \"``#pragma``\", \"``#pragma clang attribute``\"\n\n";
   OS << "   \"";
-  if (Doc.SupportedSpellings & GNU) OS << "X";
-  OS << "\",\"";
-  if (Doc.SupportedSpellings & CXX11) OS << "X";
-  OS << "\",\"";
-  if (Doc.SupportedSpellings & C2x) OS << "X";
-  OS << "\",\"";
-  if (Doc.SupportedSpellings & Declspec) OS << "X";
-  OS << "\",\"";
-  if (Doc.SupportedSpellings & Keyword) OS << "X";
-  OS << "\", \"";
-  if (Doc.SupportedSpellings & Pragma) OS << "X";
-  OS << "\", \"";
-  if (getPragmaAttributeSupport(Records).isAttributedSupported(*Doc.Attribute))
-    OS << "X";
+  for (size_t Kind = 0; Kind != NumSpellingKinds; ++Kind) {
+    SpellingKind K = (SpellingKind)Kind;
+    // FIXME: Why are Microsoft spellings not listed?
+    if (K == SpellingKind::Microsoft)
+      continue;
+
+    bool PrintedAny = false;
+    for (StringRef Spelling : Doc.SupportedSpellings[K]) {
+      if (PrintedAny)
+        OS << " |br| ";
+      OS << "``" << Spelling << "``";
+      PrintedAny = true;
+    }
+
+    OS << "\",\"";
+  }
+
+  if (getPragmaAttributeSupport(Records).isAttributedSupported(
+          *Doc.Attribute))
+    OS << "Yes";
   OS << "\"\n\n";
 
   // If the attribute is deprecated, print a message about it, and possibly
   // provide a replacement attribute.
   if (!Doc.Documentation->isValueUnset("Deprecated")) {
     OS << "This attribute has been deprecated, and may be removed in a future "
        << "version of Clang.";
     const Record &Deprecated = *Doc.Documentation->getValueAsDef("Deprecated");
for (const auto *D : Docs) {
       bool Undocumented = Cat == "Undocumented";
       if (Undocumented && Docs.size() > 1)
         PrintFatalError(Doc.getLoc(),
                         "Attribute is \"Undocumented\", but has multiple "
                         "documentation categories");
 
       if (!Undocumented)
         SplitDocs[Category].push_back(DocumentationData(
-            Doc, Attr, GetAttributeHeadingAndSpellingKinds(Doc, Attr)));
+            Doc, Attr, GetAttributeHeadingAndSpellings(Doc, Attr)));
     }
   }
 
   // Having split the attributes out based on what documentation goes where,
   // we can begin to generate sections of documentation.
   for (auto &I : SplitDocs) {
     WriteCategoryHeader(I.first, OS);