[clang] [flang] [flang][Driver] Warn on -fbuiltin and -fno-builtin (PR #164879)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu updated https://github.com/llvm/llvm-project/pull/164879

>From 3ba83978ab065af3207c0fc1de8ee8f52465259d Mon Sep 17 00:00:00 2001
From: Tarun Prabhu <tarun at lanl.gov>
Date: Thu, 23 Oct 2025 13:24:20 -0600
Subject: [PATCH] [flang][Driver] Warn on -fbuiltin and -fno-builtin

The options -fbuiltin and -fno-builtin are not valid for Fortran. However, they
are accepted by gfortran which emits a warning message but continues to compile
the code. Both -fbuiltin and -fno-builtin have been enabled for flang.
Specifying either will result in a warning message being shown but no other
effects. Compilation will proceed normally after these warnings are shown. This
brings flang's behavior in line with gfortran for these options.

Fixes #164766
---
 clang/include/clang/Basic/DiagnosticDriverKinds.td |  3 +++
 clang/include/clang/Driver/Options.td              |  4 ++--
 clang/lib/Driver/ToolChains/Flang.cpp              |  7 +++++++
 flang/test/Driver/flang-f-opts.f90                 | 13 +++++++++++++
 4 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index 0581bf353d936..83980e3ac35b7 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -133,6 +133,9 @@ def warn_drv_unsupported_option_for_offload_arch_req_feature : Warning<
 def warn_drv_unsupported_option_for_target : Warning<
   "ignoring '%0' option as it is not currently supported for target '%1'">,
   InGroup<OptionIgnored>;
+def warn_drv_invalid_argument_for_flang : Warning<
+  "'%0' is not valid for Fortran">,
+  InGroup<OptionIgnored>;
 def warn_drv_unsupported_option_for_flang : Warning<
   "the argument '%0' is not supported for option '%1'. Mapping to '%1%2'">,
   InGroup<OptionIgnored>;
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index ef1c8758705f4..bca8b26bc3d30 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -1955,7 +1955,7 @@ defm borland_extensions : BoolFOption<"borland-extensions",
           "Accept non-standard constructs supported by the Borland compiler">,
   NegFlag<SetFalse>>;
 def fbuiltin : Flag<["-"], "fbuiltin">, Group<f_Group>,
-  Visibility<[ClangOption, CLOption, DXCOption]>;
+  Visibility<[ClangOption, CLOption, DXCOption, FlangOption, FC1Option]>;
 def fbuiltin_module_map : Flag <["-"], "fbuiltin-module-map">, Group<f_Group>,
   Flags<[]>, HelpText<"Load the clang builtins module map file.">;
 defm caret_diagnostics : BoolFOption<"caret-diagnostics",
@@ -3563,7 +3563,7 @@ def fno_assume_sane_operator_new : Flag<["-"], "fno-assume-sane-operator-new">,
   Visibility<[ClangOption, CC1Option]>,
   MarshallingInfoNegativeFlag<CodeGenOpts<"AssumeSaneOperatorNew">>;
 def fno_builtin : Flag<["-"], "fno-builtin">, Group<f_Group>,
-  Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>,
+  Visibility<[ClangOption, CC1Option, CLOption, DXCOption, FlangOption, FC1Option]>,
   HelpText<"Disable implicit builtin knowledge of functions">;
 def fno_builtin_ : Joined<["-"], "fno-builtin-">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>,
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index a56fa41c49d34..88bce181d40d2 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -945,6 +945,13 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
     assert(false && "Unexpected action class for Flang tool.");
   }
 
+  // We support some options that are invalid for Fortran and have no effect.
+  // These are solely for compatibility with other compilers. Emit a warning if
+  // any such options are provided, then proceed normally.
+  for (options::ID Opt : {options::OPT_fbuiltin, options::OPT_fno_builtin})
+    if (const Arg *A = Args.getLastArg(Opt))
+      D.Diag(diag::warn_drv_invalid_argument_for_flang) << A->getSpelling();
+
   const InputInfo &Input = Inputs[0];
   types::ID InputType = Input.getType();
 
diff --git a/flang/test/Driver/flang-f-opts.f90 b/flang/test/Driver/flang-f-opts.f90
index b972b9b7b2a59..77bb4d7aa8a91 100644
--- a/flang/test/Driver/flang-f-opts.f90
+++ b/flang/test/Driver/flang-f-opts.f90
@@ -13,3 +13,16 @@
 ! CHECK-PROFILE-GENERATE-LLVM: "-fprofile-generate"
 ! RUN: %flang -### -S -fprofile-use=%S %s 2>&1 | FileCheck -check-prefix=CHECK-PROFILE-USE-DIR %s
 ! CHECK-PROFILE-USE-DIR: "-fprofile-use={{.*}}"
+
+! RUN: %flang -### -fbuiltin %s 2>&1 \
+! RUN:     | FileCheck %s -check-prefix=WARN-BUILTIN
+! WARN-BUILTIN: warning: '-fbuiltin' is not valid for Fortran
+
+! RUN: %flang -### -fno-builtin %s 2>&1 \
+! RUN:     | FileCheck %s -check-prefix=WARN-NO-BUILTIN
+! WARN-NO-BUILTIN: warning: '-fno-builtin' is not valid for Fortran
+
+! RUN: %flang -### -fbuiltin -fno-builtin %s 2>&1 \
+! RUN:     | FileCheck %s -check-prefix=WARN-BUILTIN-MULTIPLE
+! WARN-BUILTIN-MULTIPLE: warning: '-fbuiltin' is not valid for Fortran
+! WARN-BUILTIN-MULTIPLE: warning: '-fno-builtin' is not valid for Fortran