[flang-commits] [clang] [flang] [Flang][Driver] Deprecate Ofast (PR #101701)

[Kiran Chandramohan via flang-commits]

https://github.com/kiranchandramohan created https://github.com/llvm/llvm-project/pull/101701

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

>From 60bd3cf870a078433cfa104bc872ad3083ff1e1c Mon Sep 17 00:00:00 2001
From: Kiran Chandramohan <kiran.chandramohan at arm.com>
Date: Fri, 19 Jul 2024 12:46:08 +0000
Subject: [PATCH] [Flang][Driver] Deprecate Ofast

---
 clang/include/clang/Basic/DiagnosticDriverKinds.td | 4 ++++
 clang/include/clang/Driver/Options.td              | 5 ++++-
 clang/lib/Driver/ToolChains/Flang.cpp              | 1 +
 flang/test/Driver/fast-math.f90                    | 5 +++++
 4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/clang/include/clang/Basic/DiagnosticDriverKinds.td b/clang/include/clang/Basic/DiagnosticDriverKinds.td
index 3d8240f8357b4..e04e74de07435 100644
--- a/clang/include/clang/Basic/DiagnosticDriverKinds.td
+++ b/clang/include/clang/Basic/DiagnosticDriverKinds.td
@@ -448,6 +448,10 @@ 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<
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index f690467bb82cd..def5a6db5e6ce 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -933,7 +933,10 @@ def O_flag : Flag<["-"], "O">, Visibility<[ClangOption, CC1Option, FC1Option]>,
 def Ofast : Joined<["-"], "Ofast">, Group<O_Group>,
   Visibility<[ClangOption, CC1Option, FlangOption]>,
   HelpText<"Deprecated; use '-O3 -ffast-math' for the same behavior,"
-  " or '-O3' to enable only conforming optimizations">;
+  " or '-O3' to enable only conforming optimizations">,
+  HelpTextForVariants<[FlangOption],
+    "Deprecated; use '-O3 -ffast-math -fstack-arrays' for the same behavior,"
+    " or '-O3 -fstack-arrays' to enable only conforming optimizations">;
 def P : Flag<["-"], "P">,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
   Group<Preprocessor_Group>,
diff --git a/clang/lib/Driver/ToolChains/Flang.cpp b/clang/lib/Driver/ToolChains/Flang.cpp
index 492d5a3021d9b..c5ec3c26aed9a 100644
--- a/clang/lib/Driver/ToolChains/Flang.cpp
+++ b/clang/lib/Driver/ToolChains/Flang.cpp
@@ -867,6 +867,7 @@ 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);
     }
diff --git a/flang/test/Driver/fast-math.f90 b/flang/test/Driver/fast-math.f90
index 47175488b98bc..e677432bc04fa 100644
--- a/flang/test/Driver/fast-math.f90
+++ b/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