[clang] Add deprecation warning for -Ofast driver option

clang/docs/ReleaseNotes.rst

@@ -470,6 +470,13 @@ New Compiler Flags
 Deprecated Compiler Flags
 -------------------------
 
+- The ``-Ofast`` command-line option has been deprecated. This option both
+  enables the ``-O3`` optimization-level, as well as enabling non-standard
+  ``-ffast-math`` behaviors. As such, it is somewhat misleading as an
+  "optimization level". Users are advised to switch to ``-O3 -ffast-math`` if
+  the use of non-standard math behavior is intended, and ``-O3`` otherwise.
+  See `RFC <https://discourse.llvm.org/t/rfc-deprecate-ofast/78687>`_ for details.
+
 Modified Compiler Flags
 -----------------------
 - Added a new diagnostic flag ``-Wreturn-mismatch`` which is grouped under

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -442,6 +442,10 @@ def warn_drv_deprecated_arg : Warning<
 def warn_drv_deprecated_arg_no_relaxed_template_template_args : Warning<
   "argument '-fno-relaxed-template-template-args' is deprecated">,
   InGroup<DeprecatedNoRelaxedTemplateTemplateArgs>;
+def warn_drv_deprecated_arg_ofast : Warning<
+  "argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead,"
+  " or just '-O3' if you do not intend to enable non-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/Basic/DiagnosticGroups.td

@@ -103,6 +103,7 @@ def EnumConversion : DiagGroup<"enum-conversion",
                                 EnumFloatConversion,
                                 EnumCompareConditional]>;
 def DeprecatedNoRelaxedTemplateTemplateArgs : DiagGroup<"deprecated-no-relaxed-template-template-args">;
+def DeprecatedOFast : DiagGroup<"deprecated-ofast">;
 def ObjCSignedCharBoolImplicitIntConversion :
   DiagGroup<"objc-signed-char-bool-implicit-int-conversion">;
 def Shorten64To32 : DiagGroup<"shorten-64-to-32">;
@@ -228,6 +229,7 @@ def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedPragma,
                                           DeprecatedRegister,
                                           DeprecatedNoRelaxedTemplateTemplateArgs,
+                                          DeprecatedOFast,
                                           DeprecatedThisCapture,
                                           DeprecatedType,
                                           DeprecatedVolatile,

clang/include/clang/Driver/Options.td

@@ -931,7 +931,9 @@ 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]>;
+  Visibility<[ClangOption, CC1Option, FlangOption]>,
+  HelpText<"Deprecated; use '-O3 -ffast math' instead,"
+  " or just '-O3' if you don't intend to enable non-conforming optimizations">;
 def P : Flag<["-"], "P">,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
   Group<Preprocessor_Group>,

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5712,6 +5712,8 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
                      options::OPT_fno_zero_initialized_in_bss);
 
   bool OFastEnabled = isOptimizationLevelFast(Args);
+  if (OFastEnabled)
+    D.Diag(diag::warn_drv_deprecated_arg_ofast);
   // If -Ofast is the optimization level, then -fstrict-aliasing should be
   // enabled.  This alias option is being used to simplify the hasFlag logic.
   OptSpecifier StrictAliasingAliasOption =

clang/test/Driver/Ofast.c

@@ -10,31 +10,36 @@
 // RUN: %clang -Ofast -fno-strict-aliasing -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-NO-STRICT-ALIASING %s
 // RUN: %clang -Ofast -fno-vectorize -### %s 2>&1 | FileCheck -check-prefix=CHECK-OFAST-NO-VECTORIZE %s
 
+// CHECK-OFAST: argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead, or just '-O3' if you do not intend to enable non-conforming optimizations
 // CHECK-OFAST: -cc1
 // CHECK-OFAST-NOT: -relaxed-aliasing
 // CHECK-OFAST: -ffast-math
 // CHECK-OFAST: -Ofast
 // CHECK-OFAST: -vectorize-loops
 
+// CHECK-OFAST-O2-NOT: argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead, or just '-O3' if you do not intend to enable non-conforming optimizations
 // CHECK-OFAST-O2: -cc1
 // CHECK-OFAST-O2-ALIASING-NOT: -relaxed-aliasing
 // CHECK-OFAST-O2-ALIASING-MSVC: -relaxed-aliasing
 // CHECK-OFAST-O2-NOT: -ffast-math
 // CHECK-OFAST-O2-NOT: -Ofast
 // CHECK-OFAST-O2: -vectorize-loops
 
+// CHECK-OFAST-NO-FAST-MATH: argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead, or just '-O3' if you do not intend to enable non-conforming optimizations
 // CHECK-OFAST-NO-FAST-MATH: -cc1
 // CHECK-OFAST-NO-FAST-MATH-NOT: -relaxed-aliasing
 // CHECK-OFAST-NO-FAST-MATH-NOT: -ffast-math
 // CHECK-OFAST-NO-FAST-MATH: -Ofast
 // CHECK-OFAST-NO-FAST-MATH: -vectorize-loops
 
+// CHECK-OFAST-NO-STRICT-ALIASING: argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead, or just '-O3' if you do not intend to enable non-conforming optimizations
 // CHECK-OFAST-NO-STRICT-ALIASING: -cc1
 // CHECK-OFAST-NO-STRICT-ALIASING: -relaxed-aliasing
 // CHECK-OFAST-NO-STRICT-ALIASING: -ffast-math
 // CHECK-OFAST-NO-STRICT-ALIASING: -Ofast
 // CHECK-OFAST-NO-STRICT-ALIASING: -vectorize-loops
 
+// CHECK-OFAST-NO-VECTORIZE: argument '-Ofast' is deprecated; use '-O3 -ffast-math' instead, or just '-O3' if you do not intend to enable non-conforming optimizations
 // CHECK-OFAST-NO-VECTORIZE: -cc1
 // CHECK-OFAST-NO-VECTORIZE-NOT: -relaxed-aliasing
 // CHECK-OFAST-NO-VECTORIZE: -ffast-math