[flang] Fixes in preprocessing conditional compilation code when preceded by macros

flang/include/flang/Parser/parsing.h

@@ -19,6 +19,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include <optional>
 #include <string>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -66,6 +67,7 @@ class Parsing {
   void DumpProvenance(llvm::raw_ostream &) const;
   void DumpParsingLog(llvm::raw_ostream &) const;
   void Parse(llvm::raw_ostream &debugOutput);
+  const char *IsCompilerDirectiveSentinel(const char *, std::size_t) const;
   void ClearLog();
 
   void EmitMessage(llvm::raw_ostream &o, const char *at,
@@ -87,6 +89,7 @@ class Parsing {
   std::optional<Program> parseTree_;
   ParsingLog log_;
   Preprocessor preprocessor_{allCooked_.allSources()};
+  std::unordered_set<std::string> compilerDirectiveSentinels_;
 };
 } // namespace Fortran::parser
 #endif // FORTRAN_PARSER_PARSING_H_

flang/lib/Parser/parsing.cpp

@@ -106,6 +106,7 @@ const SourceFile *Parsing::Prescan(const std::string &path, Options options) {
   if (options.showColors) {
     allSources.setShowColors(/*showColors=*/true);
   }
+  compilerDirectiveSentinels_ = prescanner.getCompilerDirectiveSentinels();
   return sourceFile;
 }
 
@@ -153,7 +154,8 @@ void Parsing::EmitPreprocessedSource(
         return ch;
       }};
 
-      if (ch == '!' && lineWasBlankBefore) {
+      if (ch == '!' && lineWasBlankBefore &&
+          IsCompilerDirectiveSentinel((const char *)&atChar + 1, 4)) {
         // Other comment markers (C, *, D) in original fixed form source
         // input card column 1 will have been deleted or normalized to !,
         // which signifies a comment (directive) in both source forms.
@@ -258,6 +260,12 @@ void Parsing::Parse(llvm::raw_ostream &out) {
   finalRestingPlace_ = parseState.GetLocation();
 }
 
+const char *Parsing::IsCompilerDirectiveSentinel(
+    const char *sentinel, std::size_t len) const {
+  const auto iter{compilerDirectiveSentinels_.find(std::string(sentinel, len))};
+  return iter == compilerDirectiveSentinels_.end() ? nullptr : iter->c_str();
+}
+
 void Parsing::ClearLog() { log_.clear(); }
 
 } // namespace Fortran::parser

flang/lib/Parser/prescan.cpp

@@ -511,7 +511,7 @@ bool Prescanner::MustSkipToEndOfLine() const {
   if (inFixedForm_ && column_ > fixedFormColumnLimit_ && !tabInCurrentLine_) {
     return true; // skip over ignored columns in right margin (73:80)
   } else if (*at_ == '!' && !inCharLiteral_) {
-    return !IsCompilerDirectiveSentinel(at_);
+    return !IsCompilerDirectiveSentinel(at_ + 1);
   } else {
     return false;
   }

flang/lib/Parser/prescan.h

@@ -90,6 +90,9 @@ class Prescanner {
   template <typename... A> Message &Say(A &&...a) {
     return messages_.Say(std::forward<A>(a)...);
   }
+  std::unordered_set<std::string> getCompilerDirectiveSentinels() {
+    return compilerDirectiveSentinels_;
+  }
 
 private:
   struct LineClassification {

flang/test/Preprocessing/bug126459.f90

@@ -0,0 +1,8 @@
+! RUN: %flang -fopenmp -E %s 2>&1 | FileCheck %s
+!CHECK: !$ NDIR=0
+program main
+integer NDIR
+#define BLANKMACRO
+
+BLANKMACRO !$ NDIR=0
+end program main