[Flang][Driver] Deprecate Ofast (#101701)

This is subject to agreement by the Flang community
(https://discourse.llvm.org/t/rfc-deprecate-ofast-in-flang/80243).

Author: kiranchandramohan

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -452,6 +452,10 @@ def warn_drv_deprecated_arg_ofast : Warning<
   "argument '-Ofast' is deprecated; use '-O3 -ffast-math' for the same behavior,"
   " or '-O3' to enable only conforming optimizations">,
   InGroup<DeprecatedOFast>;
+def warn_drv_deprecated_arg_ofast_for_flang : Warning<
+  "argument '-Ofast' is deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
+  " or '-O3 -fstack-arrays' to enable only conforming optimizations">,
+  InGroup<DeprecatedOFast>;
 def warn_drv_deprecated_custom : Warning<
   "argument '%0' is deprecated, %1">, InGroup<Deprecated>;
 def warn_drv_assuming_mfloat_abi_is : Warning<

clang/include/clang/Driver/Options.td

@@ -933,10 +933,12 @@ def O : Joined<["-"], "O">, Group<O_Group>,
 def O_flag : Flag<["-"], "O">, Visibility<[ClangOption, CC1Option, FC1Option]>,
   Alias<O>, AliasArgs<["1"]>;
 def Ofast : Joined<["-"], "Ofast">, Group<O_Group>,
-  Visibility<[ClangOption, CC1Option, FlangOption]>,
-  HelpTextForVariants<[ClangOption, CC1Option],
-                      "Deprecated; use '-O3 -ffast-math' for the same behavior,"
-                      " or '-O3' to enable only conforming optimizations">;
+  Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
+  HelpTextForVariants<[FlangOption, FC1Option],
+    "Deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
+    " or '-O3 -fstack-arrays' to enable only conforming optimizations">,
+  HelpText<"Deprecated; use '-O3 -ffast-math' for the same behavior,"
+           " or '-O3' to enable only conforming optimizations">;
 def P : Flag<["-"], "P">,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
   Group<Preprocessor_Group>,

clang/lib/Driver/ToolChains/Flang.cpp

@@ -937,6 +937,7 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
       D.Diag(diag::warn_O4_is_O3);
     } else if (A->getOption().matches(options::OPT_Ofast)) {
       CmdArgs.push_back("-O3");
+      D.Diag(diag::warn_drv_deprecated_arg_ofast_for_flang);
     } else {
       A->render(Args, CmdArgs);
     }

flang/test/Driver/fast-math.f90

@@ -1,6 +1,11 @@
 ! Test for correct forwarding of fast-math flags from the compiler driver to the
 ! frontend driver
 
+! Check warning message for Ofast deprecation
+! RUN: %flang -Ofast -### %s -o %t 2>&1 | FileCheck %s
+! CHECK: warning: argument '-Ofast' is deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior, or '-O3
+! -fstack-arrays' to enable only conforming optimizations [-Wdeprecated-ofast]
+
 ! -Ofast => -ffast-math -O3 -fstack-arrays
 ! RUN: %flang -Ofast -fsyntax-only -### %s -o %t 2>&1 \
 ! RUN:     | FileCheck --check-prefix=CHECK-OFAST %s