[flang] Add __COUNTER__ preprocessor macro (#136827)

This commit adds support for the `__COUNTER__` preprocessor macro, which
works the same as the one found in clang.
It is useful to generate unique names at compile-time.

flang/docs/Extensions.md

@@ -521,6 +521,8 @@ end
 * We respect Fortran comments in macro actual arguments (like GNU, Intel, NAG;
   unlike PGI and XLF) on the principle that macro calls should be treated
   like function references.  Fortran's line continuation methods also work.
+* We implement the `__COUNTER__` preprocessing extension,
+  see [Non-standard Extensions](Preprocessing.md#non-standard-extensions)
 
 ## Standard features not silently accepted
 

flang/docs/Preprocessing.md

@@ -138,6 +138,18 @@ text.
 OpenMP-style directives that look like comments are not addressed by
 this scheme but are obvious extensions.
 
+## Currently implemented built-ins
+
+* `__DATE__`: Date, given as e.g. "Jun 16 1904"
+* `__TIME__`: Time in 24-hour format including seconds, e.g. "09:24:13"
+* `__TIMESTAMP__`: Date, time and year of last modification, given as e.g. "Fri May  9 09:16:17 2025"
+* `__FILE__`: Current file
+* `__LINE__`: Current line
+
+### Non-standard Extensions
+
+* `__COUNTER__`: Replaced by sequential integers on each expansion, starting from 0.
+
 ## Appendix
 `N` in the table below means "not supported"; this doesn't
 mean a bug, it just means that a particular behavior was

flang/include/flang/Parser/preprocessor.h

@@ -122,6 +122,8 @@ private:
   std::list<std::string> names_;
   std::unordered_map<CharBlock, Definition> definitions_;
   std::stack<CanDeadElseAppear> ifStack_;
+
+  unsigned int counterVal_{0};
 };
 } // namespace Fortran::parser
 #endif // FORTRAN_PARSER_PREPROCESSOR_H_

flang/lib/Parser/preprocessor.cpp

@@ -22,6 +22,7 @@
 #include <memory>
 #include <optional>
 #include <set>
+#include <string>
 #include <utility>
 #include <vector>
 
@@ -299,6 +300,7 @@ void Preprocessor::DefineStandardMacros() {
   Define("__FILE__"s, "__FILE__"s);
   Define("__LINE__"s, "__LINE__"s);
   Define("__TIMESTAMP__"s, "__TIMESTAMP__"s);
+  Define("__COUNTER__"s, "__COUNTER__"s);
 }
 
 static const std::string idChars{
@@ -495,6 +497,8 @@ std::optional<TokenSequence> Preprocessor::MacroReplacement(
               repl = "\""s + time + '"';
             }
           }
+        } else if (name == "__COUNTER__") {
+          repl = std::to_string(counterVal_++);
         }
         if (!repl.empty()) {
           ProvenanceRange insert{allSources_.AddCompilerInsertion(repl)};

flang/test/Preprocessing/counter.F90

@@ -0,0 +1,9 @@
+! RUN: %flang -E %s | FileCheck %s
+! CHECK: print *, 0
+! CHECK: print *, 1
+! CHECK: print *, 2
+! Check incremental counter macro
+#define foo bar
+print *, __COUNTER__
+print *, __COUNTER__
+print *, __COUNTER__