[C2y] Support WG14 N3457, the __COUNTER__ macro (llvm#162662)

AaronBallman · web-flow · commit df1d786c460e · 2025-11-03T07:50:25.000-05:00
This implements the parts of https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3457.htm which were adopted at the recent meeting in Brno. Clang already implemented `__COUNTER__`, but needed some changes for conformance. Specifically, we now diagnose when the macro is expanded more than 2147483647 times. Additionally, we now give the expected extension and pre-compat warnings for the feature. To support testing the limits, this also adds a -cc1-only option, `-finitial-counter-value=`, which lets you specify the initial value the `__COUNTER__` macro should expand to.
diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
@@ -385,7 +385,9 @@ Builtin Macros
 
 ``__COUNTER__``
   Defined to an integer value that starts at zero and is incremented each time
-  the ``__COUNTER__`` macro is expanded.
+  the ``__COUNTER__`` macro is expanded. This is a standard feature in C2y but
+  is an extension in earlier language modes and in C++. This macro can only be
+  expanded 2147483647 times at most.
 
 ``__INCLUDE_LEVEL__``
   Defined to an integral value that is the include depth of the file currently
@@ -1821,6 +1823,7 @@ Octal literals prefixed with ``0o`` or ``0O``                                  C
 ``_Countof`` (N3369, N3469)                                                    C2y           C89
 ``_Generic`` with a type operand (N3260)                                       C2y           C89, C++
 ``++``/``--`` on ``_Complex`` value (N3259)                                    C2y           C89, C++
+``__COUNTER__`` (N3457)                                                        C2y           C89, C++
 ============================================= ================================ ============= =============
 
 Builtin type aliases
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
@@ -196,6 +196,11 @@ C2y Feature Support
   function or variable within an extern inline function is no longer a
   constraint per `WG14 N3622 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3622.txt>`_.
 - Clang now supports `N3355 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3355.htm>`_ Named Loops.
+- Clang's implementation of ``__COUNTER__`` was updated to conform to
+  `WG14 N3457 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3457.htm>`_.
+  This includes adding pedantic warnings for the feature being an extension in
+  other language modes as well as an error when the counter is expanded more
+  than 2147483647 times.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -90,6 +90,14 @@ def err_unterminated___pragma : Error<"missing terminating ')' character">;
 
 def err_conflict_marker : Error<"version control conflict marker in file">;
 
+def err_counter_overflow : Error<
+  "'__COUNTER__' value cannot exceed 2'147'483'647">;
+def ext_counter : Extension<
+  "'__COUNTER__' is a C2y extension">, InGroup<C2y>;
+def warn_counter : Warning<
+  "'__COUNTER__' is incompatible with standards before C2y">,
+  InGroup<CPre2yCompat>, DefaultIgnore;
+
 def err_raw_delim_too_long : Error<
   "raw string delimiter longer than 16 characters"
   "; use PREFIX( )PREFIX to delimit raw string">;
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
@@ -8445,6 +8445,10 @@ def aligned_alloc_unavailable : Flag<["-"], "faligned-alloc-unavailable">,
   MarshallingInfoFlag<LangOpts<"AlignedAllocationUnavailable">>,
   ShouldParseIf<faligned_allocation.KeyPath>;
 
+def finitial_counter_value_EQ : Joined<["-"], "finitial-counter-value=">,
+  HelpText<"Sets the initial value for __COUNTER__, defaults to 0.">,
+  MarshallingInfoInt<PreprocessorOpts<"InitialCounterValue">, "0">;
+
 } // let Visibility = [CC1Option]
 
 //===----------------------------------------------------------------------===//
diff --git a/clang/include/clang/Lex/Preprocessor.h b/clang/include/clang/Lex/Preprocessor.h
@@ -226,7 +226,7 @@ class Preprocessor {
       LangOptions::FPEvalMethodKind::FEM_UnsetOnCommandLine;
 
   // Next __COUNTER__ value, starts at 0.
-  unsigned CounterValue = 0;
+  uint32_t CounterValue = 0;
 
   enum {
     /// Maximum depth of \#includes.
@@ -2421,8 +2421,8 @@ class Preprocessor {
   bool SawDateOrTime() const {
     return DATELoc != SourceLocation() || TIMELoc != SourceLocation();
   }
-  unsigned getCounterValue() const { return CounterValue; }
-  void setCounterValue(unsigned V) { CounterValue = V; }
+  uint32_t getCounterValue() const { return CounterValue; }
+  void setCounterValue(uint32_t V) { CounterValue = V; }
 
   LangOptions::FPEvalMethodKind getCurrentFPEvalMethod() const {
     assert(CurrentFPEvalMethod != LangOptions::FEM_UnsetOnCommandLine &&
diff --git a/clang/include/clang/Lex/PreprocessorOptions.h b/clang/include/clang/Lex/PreprocessorOptions.h
@@ -198,6 +198,10 @@ class PreprocessorOptions {
   /// If set, the UNIX timestamp specified by SOURCE_DATE_EPOCH.
   std::optional<uint64_t> SourceDateEpoch;
 
+  /// The initial value for __COUNTER__; typically is zero but can be set via a
+  /// -cc1 flag for testing purposes.
+  uint32_t InitialCounterValue = 0;
+
 public:
   PreprocessorOptions() : PrecompiledPreambleBytes(0, false) {}
 
diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
@@ -220,8 +220,8 @@ class ASTReaderListener {
   }
 
   /// Receives __COUNTER__ value.
-  virtual void ReadCounter(const serialization::ModuleFile &M,
-                           unsigned Value) {}
+  virtual void ReadCounter(const serialization::ModuleFile &M, uint32_t Value) {
+  }
 
   /// This is called for each AST file loaded.
   virtual void visitModuleFile(StringRef Filename,
@@ -312,7 +312,7 @@ class ChainedASTReaderListener : public ASTReaderListener {
                                bool Complain,
                                std::string &SuggestedPredefines) override;
 
-  void ReadCounter(const serialization::ModuleFile &M, unsigned Value) override;
+  void ReadCounter(const serialization::ModuleFile &M, uint32_t Value) override;
   bool needsInputFileVisitation() override;
   bool needsSystemInputFileVisitation() override;
   void visitModuleFile(StringRef Filename,
@@ -352,7 +352,7 @@ class PCHValidator : public ASTReaderListener {
                                StringRef ModuleFilename,
                                StringRef SpecificModuleCachePath,
                                bool Complain) override;
-  void ReadCounter(const serialization::ModuleFile &M, unsigned Value) override;
+  void ReadCounter(const serialization::ModuleFile &M, uint32_t Value) override;
 };
 
 /// ASTReaderListenter implementation to set SuggestedPredefines of
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
@@ -518,14 +518,14 @@ class ASTInfoCollector : public ASTReaderListener {
   LangOptions &LangOpts;
   CodeGenOptions &CodeGenOpts;
   TargetOptions &TargetOpts;
-  unsigned &Counter;
+  uint32_t &Counter;
 
 public:
   ASTInfoCollector(HeaderSearchOptions &HSOpts,
                    std::string &SpecificModuleCachePath,
                    PreprocessorOptions &PPOpts, LangOptions &LangOpts,
                    CodeGenOptions &CodeGenOpts, TargetOptions &TargetOpts,
-                   unsigned &Counter)
+                   uint32_t &Counter)
       : HSOpts(HSOpts), SpecificModuleCachePath(SpecificModuleCachePath),
         PPOpts(PPOpts), LangOpts(LangOpts), CodeGenOpts(CodeGenOpts),
         TargetOpts(TargetOpts), Counter(Counter) {}
@@ -577,7 +577,7 @@ class ASTInfoCollector : public ASTReaderListener {
   }
 
   void ReadCounter(const serialization::ModuleFile &M,
-                   unsigned NewCounter) override {
+                   uint32_t NewCounter) override {
     Counter = NewCounter;
   }
 };
diff --git a/clang/lib/Frontend/InitPreprocessor.cpp b/clang/lib/Frontend/InitPreprocessor.cpp
@@ -1542,6 +1542,9 @@ void clang::InitializePreprocessor(Preprocessor &PP,
   llvm::raw_string_ostream Predefines(PredefineBuffer);
   MacroBuilder Builder(Predefines);
 
+  // Ensure that the initial value of __COUNTER__ is hooked up.
+  PP.setCounterValue(InitOpts.InitialCounterValue);
+
   // Emit line markers for various builtin sections of the file. The 3 here
   // marks <built-in> as being a system header, which suppresses warnings when
   // the same macro is defined multiple times.
diff --git a/clang/lib/Lex/PPMacroExpansion.cpp b/clang/lib/Lex/PPMacroExpansion.cpp
@@ -1735,7 +1735,19 @@ void Preprocessor::ExpandBuiltinMacro(Token &Tok) {
       Diag(getLastFPEvalPragmaLocation(), diag::note_pragma_entered_here);
     }
   } else if (II == Ident__COUNTER__) {
-    // __COUNTER__ expands to a simple numeric value.
+    Diag(Tok.getLocation(),
+         getLangOpts().C2y ? diag::warn_counter : diag::ext_counter);
+    // __COUNTER__ expands to a simple numeric value that must be less than
+    // 2147483647.
+    constexpr uint32_t MaxPosValue = std::numeric_limits<int32_t>::max();
+    if (CounterValue > MaxPosValue) {
+      Diag(Tok.getLocation(), diag::err_counter_overflow);
+      // Retain the maximal value so we don't issue conversion-related
+      // diagnostics by overflowing into a long long. While this does produce
+      // a duplicate value, there's no way to ignore this error so there's no
+      // translation anyway.
+      CounterValue = MaxPosValue;
+    }
     OS << CounterValue++;
     Tok.setKind(tok::numeric_constant);
   } else if (II == Ident__has_feature) {
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
@@ -225,7 +225,7 @@ bool ChainedASTReaderListener::ReadPreprocessorOptions(
 }
 
 void ChainedASTReaderListener::ReadCounter(const serialization::ModuleFile &M,
-                                           unsigned Value) {
+                                           uint32_t Value) {
   First->ReadCounter(M, Value);
   Second->ReadCounter(M, Value);
 }
@@ -973,7 +973,7 @@ bool PCHValidator::ReadHeaderSearchOptions(const HeaderSearchOptions &HSOpts,
       PP.getPreprocessorOpts());
 }
 
-void PCHValidator::ReadCounter(const ModuleFile &M, unsigned Value) {
+void PCHValidator::ReadCounter(const ModuleFile &M, uint32_t Value) {
   PP.setCounterValue(Value);
 }
 
diff --git a/clang/test/C/C2y/n3457.c b/clang/test/C/C2y/n3457.c
@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
+// RUN: %clang_cc1 -verify=ext -pedantic -x c++ %s
+// RUN: %clang_cc1 -verify=pre -std=c2y -pedantic -Wpre-c2y-compat %s
+
+/* WG14 N3457: Clang 22
+ * The __COUNTER__ predefined macro
+ *
+ * This predefined macro was supported as an extension in earlier versions of
+ * Clang, but the required diagnostics for the limits were not added until 22.
+ */
+
+// Ensure that __COUNTER__ starts from 0.
+static_assert(__COUNTER__ == 0); /* ext-warning {{'__COUNTER__' is a C2y extension}}
+                                    pre-warning {{'__COUNTER__' is incompatible with standards before C2y}}
+                                  */
+
+// Ensure that the produced value can be used with token concatenation.
+#define CAT_IMPL(a, b) a ## b
+#define CAT(a, b) CAT_IMPL(a, b)
+#define NAME_WITH_COUNTER(a) CAT(a, __COUNTER__)
+void test() {
+  // Because this is the 2nd expansion, this defines test1.
+  int NAME_WITH_COUNTER(test); /* ext-warning {{'__COUNTER__' is a C2y extension}}
+                                  pre-warning {{'__COUNTER__' is incompatible with standards before C2y}}
+                                */
+  int other_test = test1;      // Ok
+}
+
+// Ensure that __COUNTER__ increments each time you mention it.
+static_assert(__COUNTER__ == 2); /* ext-warning {{'__COUNTER__' is a C2y extension}}
+                                    pre-warning {{'__COUNTER__' is incompatible with standards before C2y}}
+                                 */
+static_assert(__COUNTER__ == 3); /* ext-warning {{'__COUNTER__' is a C2y extension}}
+                                    pre-warning {{'__COUNTER__' is incompatible with standards before C2y}}
+                                 */
+static_assert(__COUNTER__ == 4); /* ext-warning {{'__COUNTER__' is a C2y extension}}
+                                    pre-warning {{'__COUNTER__' is incompatible with standards before C2y}}
+                                 */
diff --git a/clang/test/C/C2y/n3457_1.c b/clang/test/C/C2y/n3457_1.c
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -verify -std=c2y -finitial-counter-value=2147483646 %s
+
+// The value produced needs to be a type that's representable with a signed
+// long. However, the actual type it expands to does *not* need to be forced to
+// be signed long because that would generally mean suffixing the value with L,
+// which would be very surprising for folks using this to generate unique ids.
+// We'll test this by ensuring the largest value can be expanded properly and
+// an assertion that signed long is always at least four bytes wide (which is
+// what's required to represent that maximal value).
+//
+// So we set the initial counter value to 2147483646, we'll validate that,
+// increment it once to get to the maximal value and ensure there's no
+// diagnostic, then increment again to ensure we get the constraint violation.
+
+static_assert(__COUNTER__ == 2147483646); // Test and increment
+static_assert(__COUNTER__ == 2147483647); // Test and increment
+
+// This one should fail.
+signed long i = __COUNTER__; // expected-error {{'__COUNTER__' value cannot exceed 2'147'483'647}}
+
diff --git a/clang/test/C/C2y/n3457_2.c b/clang/test/C/C2y/n3457_2.c
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -verify=good -std=c2y -finitial-counter-value=2147483648 %s
+// RUN: %clang_cc1 -verify -std=c2y -finitial-counter-value=2147483648 -DEXPAND_IT %s
+// good-no-diagnostics
+
+// This sets the intial __COUNTER__ value to something that's too big. Setting
+// the value too large is fine. Expanding to a too-large value is not.
+#ifdef EXPAND_IT
+  // This one should fail.
+  signed long i = __COUNTER__; // expected-error {{'__COUNTER__' value cannot exceed 2'147'483'647}}
+#endif
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
@@ -329,7 +329,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>The __COUNTER__ predefined macro</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3457.htm">N3457</a></td>
-      <td class="unknown" align="center">Unknown</td>
+      <td class="unreleased" align="center">Clang 22</td>
 	</tr>
     <tr>
       <td>Chasing Ghosts I: constant expressions v2</td>
diff --git a/third-party/benchmark/include/benchmark/benchmark.h b/third-party/benchmark/include/benchmark/benchmark.h
@@ -250,13 +250,19 @@ BENCHMARK(BM_test)->Unit(benchmark::kMillisecond);
   _Pragma("GCC diagnostic push")             \
   _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
 #define BENCHMARK_RESTORE_DEPRECATED_WARNING _Pragma("GCC diagnostic pop")
+#define BENCHMARK_DISABLE_PEDANTIC_WARNING \
+  _Pragma("GCC diagnostic push")             \
+  _Pragma("GCC diagnostic ignored \"-Wpedantic\"")
+#define BENCHMARK_RESTORE_PEDANTIC_WARNING _Pragma("GCC diagnostic pop")
 #elif defined(__NVCOMPILER)
 #define BENCHMARK_BUILTIN_EXPECT(x, y) __builtin_expect(x, y)
 #define BENCHMARK_DEPRECATED_MSG(msg) __attribute__((deprecated(msg)))
 #define BENCHMARK_DISABLE_DEPRECATED_WARNING \
   _Pragma("diagnostic push") \
   _Pragma("diag_suppress deprecated_entity_with_custom_message")
 #define BENCHMARK_RESTORE_DEPRECATED_WARNING _Pragma("diagnostic pop")
+#define BENCHMARK_DISABLE_PEDANTIC_WARNING
+#define BENCHMARK_RESTORE_PEDANTIC_WARNING
 #else
 #define BENCHMARK_BUILTIN_EXPECT(x, y) x
 #define BENCHMARK_DEPRECATED_MSG(msg)
@@ -265,6 +271,8 @@ BENCHMARK(BM_test)->Unit(benchmark::kMillisecond);
       __LINE__) ") : warning note: " msg))
 #define BENCHMARK_DISABLE_DEPRECATED_WARNING
 #define BENCHMARK_RESTORE_DEPRECATED_WARNING
+#define BENCHMARK_DISABLE_PEDANTIC_WARNING
+#define BENCHMARK_RESTORE_PEDANTIC_WARNING
 #endif
 // clang-format on
 
@@ -1462,11 +1470,13 @@ class Fixture : public internal::Benchmark {
 // Check that __COUNTER__ is defined and that __COUNTER__ increases by 1
 // every time it is expanded. X + 1 == X + 0 is used in case X is defined to be
 // empty. If X is empty the expression becomes (+1 == +0).
+BENCHMARK_DISABLE_PEDANTIC_WARNING
 #if defined(__COUNTER__) && (__COUNTER__ + 1 == __COUNTER__ + 0)
 #define BENCHMARK_PRIVATE_UNIQUE_ID __COUNTER__
 #else
 #define BENCHMARK_PRIVATE_UNIQUE_ID __LINE__
 #endif
+BENCHMARK_RESTORE_PEDANTIC_WARNING
 
 // Helpers for generating unique variable names
 #ifdef BENCHMARK_HAS_CXX11
