[Flang][Driver] Deprecate Ofast

[kiranchandramohan]

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

[llvmbot]

@llvm/pr-subscribers-flang-driver
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-driver

Author: Kiran Chandramohan (kiranchandramohan)

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/101701.diff

4 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticDriverKinds.td (+4)
• (modified) clang/include/clang/Driver/Options.td (+4-1)
• (modified) clang/lib/Driver/ToolChains/Flang.cpp (+1)
• (modified) flang/test/Driver/fast-math.f90 (+5)

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

[kiranchandramohan]

I did not see any opposition in https://discourse.llvm.org/t/rfc-deprecate-ofast-in-flang/80243. So would like to land this before llvm 20.

[luporl]

This may be out of the scope of this change, but why isn't -fstack-arrays enabled with -O3?

If -fstack-arrays is not specified are array temporaries always allocated on the heap?

[kiranchandramohan]

This is currently mimicking gfortran (https://gcc.gnu.org/onlinedocs/gfortran/Code-Gen-Options.html), which runs it at Ofast.

We have not seen much benefits because Flang mostly allocates on the stack. @tblah is that right?

[tblah]

I think we saw significant benefits on SNAP but not on SPEC2017 rate. I don't think we experimented with anything else.

[luporl]

Makes sense, thanks for the explanations.

[tarunprabhu]

Mimicking gfortran's behavior for now seems reasonable to me. We can always update the warning message later if needed.

Thanks Kiran!

[mjklemm]

LGTM

[kparzysz]

Thanks!

[DavidTruby]

LGTM, I'm glad we're matching clang's behaviour here. Thanks!