[flang] Handle @PROCESS directive

[kkwli0]

Summary

The legacy @PROCESS directive is supported in the IBM XLF compiler to allow specifying compiler option at the source level. This patch is to treat any line that starts with @process as a comment line. This allows code with the directive to be compiled without having syntax errors complaining it.

[klausler]

flang/docs/Extensions.md

366

Please note here that the directive is accepted but ignored.

flang/lib/Parser/prescan.cpp

780

Capitalize, please.

781

This should be static and it can be a char array, As coded, this will be a string potentially requiring dynamic allocation and deallocation on every call.

static const char pAtProc[]{"process"};

784

You really need to have a loop limit here to make sure that you don't run off the end of the string. As coded, this loop can index past the end of the string. Use sizeof pAtProcess as the string size.

790

Just return false; here.

829–833

return IsAtProcess(p) ? p : nullptr;

[kkwli0]

Addressed review comments

[klausler]

flang/lib/Parser/prescan.cpp

782

std::size_t or auto

783

You can now write a more idiomatic for loop here.

for (std::size_t j{0}; j < len; ++j) {
  if (ToLowerCaseLetter(*++p) != pAtProc[j]) {
    return false;
  }
}

794

q is unnecessary.

if (*p == '@' && IsAtProcess(p)) {
  return true;
}

829

q is unnecessary.

[kkwli0]

Address more comments. Thanks.

[klausler]

flang/lib/Parser/prescan.cpp

783

The needless parentheses around ++p are more likely to confuse the reader of the code ("why is this parenthesized? what am I missing?") than to clarify matters.

[kkwli0]

flang/lib/Parser/prescan.cpp

783

I find it easier to read. Anyway, I will remove it. Thanks.

flang/docs/Extensions.md

 * Continuation lines on `INCLUDE` lines
 * `NULL()` actual argument corresponding to an `ALLOCATABLE` dummy data object
 * User (non-intrinsic) `ELEMENTAL` procedures may not be passed as actual
   arguments, in accordance with the standard; some Fortran compilers
   permit such usage.
 * 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
 
 * The preprocessor is always run, whatever the filename extension may be.
 * 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.
 

flang/lib/Parser/prescan.cpp

while (inFixedForm_ && !tabInCurrentLine_ && at_[1] == '\n') {
     }
     CHECK(column_ == 7);
     --at_; // point to column 6 of continuation line
     column_ = 6;
   }
   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))) {
     return true;
   }
   bool anyTabs{false};
   while (true) {
     if (*p == ' ') {
bool Prescanner::IsFixedFormCommentLine(const char *start) const {
   }
   return *p == '\n';
 }
 
 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;
   }
 }
 
 std::optional<std::size_t> Prescanner::IsIncludeLine(const char *start) const {
   const char *p{SkipWhiteSpace(start)};
   for (char ch : "include"s) {
     if (ToLowerCaseLetter(*p++) != ch) {
       return std::nullopt;
     }
   }

flang/test/Parser/at-process.f

+! 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

+! 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
+