[flang] Handle @process directive

Treat lines that start with @process as a comment line. The directive
is accepted and ignored.

Differential Revision: https://reviews.llvm.org/D150883

flang/docs/Extensions.md

@@ -363,6 +363,7 @@ end
 * Constraint C1406, which prohibits the same module name from being used
   in a scope for both an intrinsic and a non-intrinsic module, is implemented
   as a portability warning only, not a hard error.
+* IBM @PROCESS directive is accepted but ignored.
 
 ## Preprocessing behavior
 

flang/lib/Parser/prescan.cpp

@@ -777,8 +777,23 @@ bool Prescanner::PadOutCharacterLiteral(TokenSequence &tokens) {
   return false;
 }
 
+static bool IsAtProcess(const char *p) {
+  static const char pAtProc[]{"process"};
+  for (std::size_t i{0}; i < sizeof pAtProc - 1; ++i) {
+    if (ToLowerCaseLetter(*++p) != pAtProc[i])
+      return false;
+  }
+  return true;
+}
+
 bool Prescanner::IsFixedFormCommentLine(const char *start) const {
   const char *p{start};
+
+  // The @process directive must start in column 1.
+  if (*p == '@' && IsAtProcess(p)) {
+    return true;
+  }
+
   if (IsFixedFormCommentChar(*p) || *p == '%' || // VAX %list, %eject, &c.
       ((*p == 'D' || *p == 'd') &&
           !features_.IsEnabled(LanguageFeature::OldDebugLines))) {
@@ -810,6 +825,8 @@ const char *Prescanner::IsFreeFormComment(const char *p) const {
   p = SkipWhiteSpaceAndCComments(p);
   if (*p == '!' || *p == '\n') {
     return p;
+  } else if (*p == '@') {
+    return IsAtProcess(p) ? p : nullptr;
   } else {
     return nullptr;
   }

flang/test/Parser/at-process.f

@@ -0,0 +1,20 @@
+! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+
+! Test ignoring @PROCESS directive in fixed source form
+
+@process opt(3)
+@process	opt(0)
+@process
+@processopt(3)
+      subroutine f()
+c@process
+      end
+
+!CHECK: Character in fixed-form label field must be a digit
+@
+
+!CHECK: Character in fixed-form label field must be a digit
+@proce 
+
+!CHECK: Character in fixed-form label field must be a digit
+@precoss 

flang/test/Parser/at-process.f90

@@ -0,0 +1,23 @@
+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+
+! Test ignoring @PROCESS directive in free source form
+
+@process opt(3)
+@process	opt(0)
+      @process strict
+@processopt(3)
+subroutine f()
+print *, "@process"
+   ! @process
+end subroutine f
+
+!CHECK: error: expected '('
+@p
+
+!CHECK: error: expected '('
+@proce
+
+!CHECK: error: expected '('
+@precoss
+end
+