[flang][driver] add negative from of -fsave-main-program (#124110)

Add the `-fno` form for consistency and to make it easy to switch the
default for downstream users.

Author: jeanPerier

clang/include/clang/Driver/Options.td

@@ -6970,8 +6970,11 @@ defm unsigned : OptInFC1FFlag<"unsigned", "Enables UNSIGNED type">;
 def fno_automatic : Flag<["-"], "fno-automatic">, Group<f_Group>,
   HelpText<"Implies the SAVE attribute for non-automatic local objects in subprograms unless RECURSIVE">;
 
-def fsave_main_program : Flag<["-"], "fsave-main-program">, Group<f_Group>,
-  HelpText<"Place all variables from the main program in static memory (otherwise scalars may be placed on the stack)">;
+defm save_main_program : BoolOptionWithoutMarshalling<"f", "save-main-program",
+  PosFlag<SetTrue, [], [],
+    "Place all main program variables in static memory (otherwise scalars may be placed on the stack)">,
+  NegFlag<SetFalse, [], [],
+    "Allow placing main program variables on the stack (default)">>;
 
 defm stack_arrays : BoolOptionWithoutMarshalling<"f", "stack-arrays",
   PosFlag<SetTrue, [], [ClangOption], "Attempt to allocate array temporaries on the stack, no matter their size">,

clang/lib/Driver/ToolChains/Flang.cpp

@@ -58,7 +58,8 @@ void Flang::addFortranDialectOptions(const ArgList &Args,
                             options::OPT_fhermetic_module_files,
                             options::OPT_frealloc_lhs,
                             options::OPT_fno_realloc_lhs,
-                            options::OPT_fsave_main_program});
+                            options::OPT_fsave_main_program,
+                            options::OPT_fno_save_main_program});
 }
 
 void Flang::addPreprocessingOptions(const ArgList &Args,

flang/lib/Frontend/CompilerInvocation.cpp

@@ -770,10 +770,11 @@ static bool parseFrontendArgs(FrontendOptions &opts, llvm::opt::ArgList &args,
     opts.features.Enable(Fortran::common::LanguageFeature::DefaultSave);
   }
 
-  // -fsave-main-program
-  if (args.hasArg(clang::driver::options::OPT_fsave_main_program)) {
-    opts.features.Enable(Fortran::common::LanguageFeature::SaveMainProgram);
-  }
+  // -f{no}-save-main-program
+  opts.features.Enable(
+      Fortran::common::LanguageFeature::SaveMainProgram,
+      args.hasFlag(clang::driver::options::OPT_fsave_main_program,
+                   clang::driver::options::OPT_fno_save_main_program, false));
 
   if (args.hasArg(
           clang::driver::options::OPT_falternative_parameter_statement)) {

flang/test/Driver/fsave-main-program.f90

@@ -1,5 +1,9 @@
 ! Check that the driver passes through -fsave-main-program:
 ! RUN: %flang -### -S -fsave-main-program %s -o - 2>&1 | FileCheck %s
+! CHECK: "-fc1"{{.*}}"-fsave-main-program"
+
+! RUN: %flang -### -S -fno-save-main-program %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK2
+! CHECK2: "-fc1"{{.*}}"-fno-save-main-program"
+
 ! Check that the compiler accepts -fsave-main-program:
 ! RUN: %flang_fc1 -emit-hlfir -fsave-main-program %s -o -
-! CHECK: "-fc1"{{.*}}"-fsave-main-program"

flang/test/Lower/fsave-main-program.f90

@@ -1,6 +1,7 @@
 ! Test -fsave-main-program switch.
 ! RUN: %flang_fc1 -emit-hlfir -o - %s | FileCheck --check-prefix=CHECK-DEFAULT %s
 ! RUN: %flang_fc1 -fsave-main-program -emit-hlfir -o - %s | FileCheck --check-prefix=CHECK-SAVE %s
+! RUN: %flang_fc1 -fsave-main-program -fno-save-main-program -emit-hlfir -o - %s | FileCheck --check-prefix=CHECK-DEFAULT %s
 program test
 integer :: i
 call foo(i)