[clang][auto-init] Deprecate -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang

GCC 12 has been released and contains unconditional support for
-ftrivial-auto-var-init=zero:
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ftrivial-auto-var-init

Maintain compatibility with GCC, and remove the -enable flag for "zero"
mode. The flag is left to generate an "unused" warning, though, to not
break all the existing users. The flag will be fully removed in Clang 17.

Link: llvm#44842

Reviewed By: nickdesaulniers, MaskRay, srhines, xbolva00

Differential Revision: https://reviews.llvm.org/D125142

Author: kees

clang/docs/ReleaseNotes.rst

@@ -277,6 +277,10 @@ New Compiler Flags
 
 Deprecated Compiler Flags
 -------------------------
+- ``-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang``
+  has been deprecated. The flag will be removed in Clang 18.
+  ``-ftrivial-auto-var-init=zero`` is now available unconditionally, to be
+  compatible with GCC.
 
 Modified Compiler Flags
 -----------------------

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -588,11 +588,6 @@ def warn_drv_darwin_sdk_invalid_settings : Warning<
   "SDK settings were ignored as 'SDKSettings.json' could not be parsed">,
   InGroup<DiagGroup<"darwin-sdk-settings">>;
 
-def err_drv_trivial_auto_var_init_zero_disabled : Error<
-  "'-ftrivial-auto-var-init=zero' hasn't been enabled; enable it at your own "
-  "peril for benchmarking purpose only with "
-  "'-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang'">;
-
 def err_drv_trivial_auto_var_init_stop_after_missing_dependency : Error<
   "'-ftrivial-auto-var-init-stop-after=*' is used without "
   "'-ftrivial-auto-var-init=zero' or '-ftrivial-auto-var-init=pattern'">;

clang/include/clang/Driver/Options.td

@@ -247,6 +247,12 @@ def : Flag<["-"], "fno-slp-vectorize-aggressive">, Group<clang_ignored_legacy_op
 def mmpx : Flag<["-"], "mmpx">, Group<clang_ignored_legacy_options_Group>;
 def mno_mpx : Flag<["-"], "mno-mpx">, Group<clang_ignored_legacy_options_Group>;
 
+// Retired with clang-16.0, to provide a deprecation period; it should
+// be removed in Clang 18 or later.
+def enable_trivial_var_init_zero : Flag<["-"], "enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang">,
+  Flags<[CC1Option, CoreOption, NoArgumentUnused]>,
+  Group<clang_ignored_legacy_options_Group>;
+
 // Group that ignores all gcc optimizations that won't be implemented
 def clang_ignored_gcc_optimization_f_Group : OptionGroup<
   "<clang_ignored_gcc_optimization_f_Group>">, Group<f_Group>, Flags<[Ignored]>;
@@ -2791,9 +2797,6 @@ def ftrivial_auto_var_init : Joined<["-"], "ftrivial-auto-var-init=">, Group<f_G
   NormalizedValuesScope<"LangOptions::TrivialAutoVarInitKind">,
   NormalizedValues<["Uninitialized", "Zero", "Pattern"]>,
   MarshallingInfoEnum<LangOpts<"TrivialAutoVarInit">, "Uninitialized">;
-def enable_trivial_var_init_zero : Flag<["-"], "enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang">,
-  Flags<[CC1Option, CoreOption, NoArgumentUnused]>,
-  HelpText<"Trivial automatic variable initialization to zero is only here for benchmarks, it'll eventually be removed, and I'm OK with that because I'm only using it to benchmark">;
 def ftrivial_auto_var_init_stop_after : Joined<["-"], "ftrivial-auto-var-init-stop-after=">, Group<f_Group>,
   Flags<[CC1Option, CoreOption]>, HelpText<"Stop initializing trivial automatic stack variables after the specified number of instances">,
   MarshallingInfoInt<LangOpts<"TrivialAutoVarInitStopAfter">>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3458,8 +3458,6 @@ static void RenderTrivialAutoVarInitOptions(const Driver &D,
     }
 
   if (!TrivialAutoVarInit.empty()) {
-    if (TrivialAutoVarInit == "zero" && !Args.hasArg(options::OPT_enable_trivial_var_init_zero))
-      D.Diag(diag::err_drv_trivial_auto_var_init_zero_disabled);
     CmdArgs.push_back(
         Args.MakeArgString("-ftrivial-auto-var-init=" + TrivialAutoVarInit));
   }

clang/test/Driver/cl-options.c

@@ -719,7 +719,6 @@
 // RUN:     -fimplicit-modules \
 // RUN:     -fno-implicit-modules \
 // RUN:     -ftrivial-auto-var-init=zero \
-// RUN:     -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang \
 // RUN:     --version \
 // RUN:     -Werror /Zs -- %s 2>&1
 

clang/test/Driver/clang_f_opts.c

@@ -569,18 +569,19 @@
 
 // RUN: %clang -### -S -ftrivial-auto-var-init=uninitialized %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-UNINIT %s
 // RUN: %clang -### -S -ftrivial-auto-var-init=pattern %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-PATTERN %s
-// RUN: %clang -### -S -ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-GOOD %s
-// RUN: %clang -### -S -ftrivial-auto-var-init=zero %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-BAD %s
+// RUN: %clang -### -S -ftrivial-auto-var-init=zero %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO %s
+// RUN: %clang -### -S -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang \
+// RUN:   -ftrivial-auto-var-init=zero %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-ENABLE-DEPRECATED %s
 // CHECK-TRIVIAL-UNINIT-NOT: hasn't been enabled
 // CHECK-TRIVIAL-PATTERN-NOT: hasn't been enabled
-// CHECK-TRIVIAL-ZERO-GOOD-NOT: hasn't been enabled
-// CHECK-TRIVIAL-ZERO-BAD: hasn't been enabled
+// CHECK-TRIVIAL-ZERO-NOT: hasn't been enabled
+// CHECK-TRIVIAL-ZERO-ENABLE-DEPRECATED: has been deprecated
 
 // RUN: %clang -### -S -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-stop-after=1 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-PATTERN-STOP-AFTER %s
-// RUN: %clang -### -S -ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang -ftrivial-auto-var-init-stop-after=1 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-STOP-AFTER %s
+// RUN: %clang -### -S -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-stop-after=1 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-STOP-AFTER %s
 // RUN: %clang -### -S -ftrivial-auto-var-init-stop-after=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-STOP-AFTER-MISSING-DEPENDENCY %s
 // RUN: %clang -### -S -ftrivial-auto-var-init=pattern -ftrivial-auto-var-init-stop-after=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-PATTERN-STOP-AFTER-INVALID-VALUE %s
-// RUN: %clang -### -S -ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang -ftrivial-auto-var-init-stop-after=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-STOP-AFTER-INVALID-VALUE %s
+// RUN: %clang -### -S -ftrivial-auto-var-init=zero -ftrivial-auto-var-init-stop-after=0 %s 2>&1 | FileCheck -check-prefix=CHECK-TRIVIAL-ZERO-STOP-AFTER-INVALID-VALUE %s
 // CHECK-TRIVIAL-PATTERN-STOP-AFTER-NOT: is used without '-ftrivial-auto-var-init'
 // CHECK-TRIVIAL-PATTERN-STOP-AFTER-NOT: only accepts positive integers
 // CHECK-TRIVIAL-ZERO-STOP-AFTER-NOT: is used without '-ftrivial-auto-var-init'