This is an archive of the discontinued LLVM Phabricator instance.

Differential D120160

[Clang] Add `-funstable` flag to enable unstable and experimental features
ClosedPublic

Authored by egorzhdan on Feb 18 2022, 1:11 PM.

Details

Reviewers
ldionne
zoecarver
var-const
Commits
rG3cdc1c155b40: [Clang] Add `-funstable` flag to enable unstable and experimental features
Summary

This new flag enables __has_feature(cxx_unstable) that would replace libc++ macros for individual unstable/experimental features, e.g. _LIBCPP_HAS_NO_INCOMPLETE_RANGES or _LIBCPP_HAS_NO_INCOMPLETE_FORMAT.

This would make it easier and more convenient to opt-in into all libc++ unstable features at once.

Diff Detail

Event Timeline

egorzhdan created this revision.Feb 18 2022, 1:11 PM
Herald added subscribers: dexonsmith, dang. · View Herald TranscriptFeb 18 2022, 1:11 PM
egorzhdan requested review of this revision.Feb 18 2022, 1:11 PM
Herald added a project: Restricted Project. · View Herald TranscriptFeb 18 2022, 1:11 PM
Herald added a subscriber: cfe-commits. · View Herald Transcript
egorzhdan added reviewers: ldionne, zoecarver, var-const.Feb 18 2022, 1:12 PM
Harbormaster completed remote builds in B150479: Diff 410008.Feb 18 2022, 2:00 PM
zoecarver accepted this revision.Feb 21 2022, 9:23 AM

This looks great, thanks Egor!

This revision is now accepted and ready to land.Feb 21 2022, 9:23 AM
var-const accepted this revision.Feb 22 2022, 5:53 PM
This revision was landed with ongoing or failed builds.Mar 1 2022, 4:35 AM
Closed by commit rG3cdc1c155b40: [Clang] Add `-funstable` flag to enable unstable and experimental features (authored by egorzhdan). · Explain Why
This revision was automatically updated to reflect the committed changes.
egorzhdan added a commit: rG3cdc1c155b40: [Clang] Add `-funstable` flag to enable unstable and experimental features.
ldionne added subscribers: Restricted Project, Restricted Project, rsmith, CaseyCarter.Mar 1 2022, 11:12 AM

Oops, this seems to have happened while I was on vacation. I wanted to have a wider discussion here before shipping the patch. Let's have it now and we can make post-commit changes depending on where the discussion leads.

So, what we're trying to achieve here is to provide a user-controlled flag to enable things that are not "production ready". This includes:

  • TSes (since we remove them after shipping the "real thing" for two releases)
  • Standardized features that we implement only in part (for example <ranges>, where we don't yet have a fully coherent thing to ship, so we'd rather not pretend that we ship it in a stable way yet)
  • Standardized features that we know for a fact are about to be broken by the Standard itself (this happened with <ranges> and <format> recently)

The problem this solves is that in the current state of things, we end up simply not shipping those features at all by fear that users are going to start depending on them, and then we'll break them. We want to be able to ship these features without actually committing to their stability.

We considered a few alternative directions for this patch, which we rejected:

  • -fexperimental: We decided not to go with this because we wanted to convey the fact that said features were unstable, and hence dangerous to use in production code
  • -std=c++latest: This would be close to what MSVC does. We decided not to go down this route because we felt like it didn't properly convey the notion of instability. Instead, we felt that -std=c++latest could/should be a simple alias for whatever the latest known standard is (which is not part of this patch).

Does anyone have thoughts?

clang/include/clang/Basic/LangOptions.def
153

The documentation of this option should be way more beefy. Something like:

Enable unstable and experimental language and library features.

This option enables various language and library features that are either experimental (also known as TSes), or have been standardized but are not stable yet. This option should not be used in production code, since neither ABI nor API stability are guaranteed.

We should also provide documentation inside clang/docs/ClangCommandLineReference.rst.

clang/lib/Driver/ToolChains/Clang.cpp
5771

We should also be adding -lc++experimental if the standard library is libc++.

Mordante added a subscriber: Mordante.Mar 1 2022, 11:56 AM

Thanks for implementing this @egorzhdan! Thanks @ldionne for the notification.

-funstable is an option that controls multiple flags therefore I think it would be good to have a separate flag to opt-in for unstable/incomplete libc++ features. That makes it easier to _only_ opt-in to these features. Then -funstable can also enable that flag. Maybe -funstable-libc++ and adding a feature libcxx_unstable that we can use in libc++ to detect that the flag was set.

clang/include/clang/Basic/LangOptions.def
153

+1

clang/lib/Driver/ToolChains/Clang.cpp
5770

Is this option still useful? In LLVM 14 the <coroutine> header is no longer experimental. The experimental header will soon be removed.

egorzhdan mentioned this in D121141: [Clang] Add `-fexperimental-library` flag to enable unstable and experimental features: follow-up fixes.Mar 7 2022, 11:35 AM
egorzhdan added a comment.Mar 7 2022, 11:37 AM

Apologies for landing this too quickly! I'll adjust this logic in a separate patch: https://reviews.llvm.org/D121141

In D120160#3352399, @Mordante wrote:

-funstable is an option that controls multiple flags therefore I think it would be good to have a separate flag to opt-in for unstable/incomplete libc++ features. That makes it easier to _only_ opt-in to these features.

@Mordante could you please elaborate a little bit, are you proposing a separate flag that would be equivalent to -Xcc -funstable (essentially to enable LangOpts.Unstable but not the rest of the flags) and have a different spelling to indicate that it is specific to libc++?

Herald added a project: Restricted Project. · View Herald TranscriptMar 7 2022, 11:37 AM

Revision Contents

PathSize
clang/
include/
clang/
Basic/
1 line
1 line
Driver/
5 lines
lib/
Driver/
ToolChains/
7 lines
test/
Driver/
5 lines
Lexer/
10 lines

Diff 412049

clang/include/clang/Basic/Features.def

Show First 20 Lines • Show All 167 Lines • ▼ Show 20 Lines
FEATURE(cxx_rvalue_references, LangOpts.CPlusPlus11) FEATURE(cxx_rvalue_references, LangOpts.CPlusPlus11)
FEATURE(cxx_strong_enums, LangOpts.CPlusPlus11) FEATURE(cxx_strong_enums, LangOpts.CPlusPlus11)
FEATURE(cxx_static_assert, LangOpts.CPlusPlus11) FEATURE(cxx_static_assert, LangOpts.CPlusPlus11)
FEATURE(cxx_thread_local, FEATURE(cxx_thread_local,
LangOpts.CPlusPlus11 &&PP.getTargetInfo().isTLSSupported()) LangOpts.CPlusPlus11 &&PP.getTargetInfo().isTLSSupported())
FEATURE(cxx_trailing_return, LangOpts.CPlusPlus11) FEATURE(cxx_trailing_return, LangOpts.CPlusPlus11)
FEATURE(cxx_unicode_literals, LangOpts.CPlusPlus11) FEATURE(cxx_unicode_literals, LangOpts.CPlusPlus11)
FEATURE(cxx_unrestricted_unions, LangOpts.CPlusPlus11) FEATURE(cxx_unrestricted_unions, LangOpts.CPlusPlus11)
FEATURE(cxx_unstable, LangOpts.Unstable)
FEATURE(cxx_user_literals, LangOpts.CPlusPlus11) FEATURE(cxx_user_literals, LangOpts.CPlusPlus11)
FEATURE(cxx_variadic_templates, LangOpts.CPlusPlus11) FEATURE(cxx_variadic_templates, LangOpts.CPlusPlus11)
// C++14 features // C++14 features
FEATURE(cxx_aggregate_nsdmi, LangOpts.CPlusPlus14) FEATURE(cxx_aggregate_nsdmi, LangOpts.CPlusPlus14)
FEATURE(cxx_binary_literals, LangOpts.CPlusPlus14) FEATURE(cxx_binary_literals, LangOpts.CPlusPlus14)
FEATURE(cxx_contextual_conversions, LangOpts.CPlusPlus14) FEATURE(cxx_contextual_conversions, LangOpts.CPlusPlus14)
FEATURE(cxx_decltype_auto, LangOpts.CPlusPlus14) FEATURE(cxx_decltype_auto, LangOpts.CPlusPlus14)
FEATURE(cxx_generic_lambdas, LangOpts.CPlusPlus14) FEATURE(cxx_generic_lambdas, LangOpts.CPlusPlus14)
▲ Show 20 Lines • Show All 87 Lines • Show Last 20 Lines

clang/include/clang/Basic/LangOptions.def

Show First 20 Lines • Show All 144 Lines • ▼ Show 20 Lines
LANGOPT(MSBitfields , 1, 0, "Microsoft-compatible structure layout") LANGOPT(MSBitfields , 1, 0, "Microsoft-compatible structure layout")
LANGOPT(Freestanding, 1, 0, "freestanding implementation") LANGOPT(Freestanding, 1, 0, "freestanding implementation")
LANGOPT(NoBuiltin , 1, 0, "disable builtin functions") LANGOPT(NoBuiltin , 1, 0, "disable builtin functions")
LANGOPT(NoMathBuiltin , 1, 0, "disable math builtin functions") LANGOPT(NoMathBuiltin , 1, 0, "disable math builtin functions")
LANGOPT(GNUAsm , 1, 1, "GNU-style inline assembly") LANGOPT(GNUAsm , 1, 1, "GNU-style inline assembly")
LANGOPT(Coroutines , 1, 0, "C++20 coroutines") LANGOPT(Coroutines , 1, 0, "C++20 coroutines")
LANGOPT(DllExportInlines , 1, 1, "dllexported classes dllexport inline methods") LANGOPT(DllExportInlines , 1, 1, "dllexported classes dllexport inline methods")
LANGOPT(RelaxedTemplateTemplateArgs, 1, 0, "C++17 relaxed matching of template template arguments") LANGOPT(RelaxedTemplateTemplateArgs, 1, 0, "C++17 relaxed matching of template template arguments")
LANGOPT(Unstable , 1, 0, "Enable unstable and experimental features")
ldionneUnsubmitted
Not Done
ReplyInline Actions

The documentation of this option should be way more beefy. Something like:

Enable unstable and experimental language and library features.

This option enables various language and library features that are either experimental (also known as TSes), or have been standardized but are not stable yet. This option should not be used in production code, since neither ABI nor API stability are guaranteed.

We should also provide documentation inside clang/docs/ClangCommandLineReference.rst.

ldionne: The documentation of this option should be way more beefy. Something like: ``` Enable unstable…
MordanteUnsubmitted
Not Done
ReplyInline Actions

+1

Mordante: +1
LANGOPT(DoubleSquareBracketAttributes, 1, 0, "'[[]]' attributes extension for all language standard modes") LANGOPT(DoubleSquareBracketAttributes, 1, 0, "'[[]]' attributes extension for all language standard modes")
COMPATIBLE_LANGOPT(RecoveryAST, 1, 1, "Preserve expressions in AST when encountering errors") COMPATIBLE_LANGOPT(RecoveryAST, 1, 1, "Preserve expressions in AST when encountering errors")
COMPATIBLE_LANGOPT(RecoveryASTType, 1, 1, "Preserve the type in recovery expressions") COMPATIBLE_LANGOPT(RecoveryASTType, 1, 1, "Preserve the type in recovery expressions")
BENIGN_LANGOPT(ThreadsafeStatics , 1, 1, "thread-safe static initializers") BENIGN_LANGOPT(ThreadsafeStatics , 1, 1, "thread-safe static initializers")
LANGOPT(POSIXThreads , 1, 0, "POSIX thread support") LANGOPT(POSIXThreads , 1, 0, "POSIX thread support")
▲ Show 20 Lines • Show All 288 Lines • Show Last 20 Lines

clang/include/clang/Driver/Options.td

  • This file is larger than 256 KB, so syntax highlighting is disabled by default.
Show First 20 Lines • Show All 1,154 Lines • ▼ Show 20 Linesdef offload_EQ : CommaJoined<["--"], "offload=">, Flags<[NoXarchOption]>,
HelpText<"Specify comma-separated list of offloading target triples (CUDA and HIP only)">; HelpText<"Specify comma-separated list of offloading target triples (CUDA and HIP only)">;
// C++ Coroutines TS // C++ Coroutines TS
defm coroutines_ts : BoolFOption<"coroutines-ts", defm coroutines_ts : BoolFOption<"coroutines-ts",
LangOpts<"Coroutines">, Default<cpp20.KeyPath>, LangOpts<"Coroutines">, Default<cpp20.KeyPath>,
PosFlag<SetTrue, [CC1Option], "Enable support for the C++ Coroutines TS">, PosFlag<SetTrue, [CC1Option], "Enable support for the C++ Coroutines TS">,
NegFlag<SetFalse>>; NegFlag<SetFalse>>;
defm unstable : BoolFOption<"unstable",
LangOpts<"Unstable">, DefaultFalse,
PosFlag<SetTrue, [CC1Option, CoreOption], "Enable unstable and experimental features">,
NegFlag<SetFalse>>;
def fembed_offload_object_EQ : Joined<["-"], "fembed-offload-object=">, def fembed_offload_object_EQ : Joined<["-"], "fembed-offload-object=">,
Group<f_Group>, Flags<[NoXarchOption, CC1Option]>, Group<f_Group>, Flags<[NoXarchOption, CC1Option]>,
HelpText<"Embed Offloading device-side binary into host object file as a section.">, HelpText<"Embed Offloading device-side binary into host object file as a section.">,
MarshallingInfoStringVector<CodeGenOpts<"OffloadObjects">>; MarshallingInfoStringVector<CodeGenOpts<"OffloadObjects">>;
def fembed_bitcode_EQ : Joined<["-"], "fembed-bitcode=">, def fembed_bitcode_EQ : Joined<["-"], "fembed-bitcode=">,
Group<f_Group>, Flags<[NoXarchOption, CC1Option, CC1AsOption]>, MetaVarName<"<option>">, Group<f_Group>, Flags<[NoXarchOption, CC1Option, CC1AsOption]>, MetaVarName<"<option>">,
HelpText<"Embed LLVM bitcode (option: off, all, bitcode, marker)">, HelpText<"Embed LLVM bitcode (option: off, all, bitcode, marker)">,
Values<"off,all,bitcode,marker">, NormalizedValuesScope<"CodeGenOptions">, Values<"off,all,bitcode,marker">, NormalizedValuesScope<"CodeGenOptions">,
▲ Show 20 Lines • Show All 5,435 Lines • Show Last 20 Lines

clang/lib/Driver/ToolChains/Clang.cpp

  • This file is larger than 256 KB, so syntax highlighting is disabled by default.
Show First 20 Lines • Show All 5,758 Lines • ▼ Show 20 Lines if (Arg *A = Args.getLastArg(options::OPT_fconstexpr_depth_EQ)) {
CmdArgs.push_back(A->getValue()); CmdArgs.push_back(A->getValue());
} }
if (Arg *A = Args.getLastArg(options::OPT_fconstexpr_steps_EQ)) { if (Arg *A = Args.getLastArg(options::OPT_fconstexpr_steps_EQ)) {
CmdArgs.push_back("-fconstexpr-steps"); CmdArgs.push_back("-fconstexpr-steps");
CmdArgs.push_back(A->getValue()); CmdArgs.push_back(A->getValue());
} }
if (Args.hasArg(options::OPT_funstable)) {
CmdArgs.push_back("-funstable");
if (!Args.hasArg(options::OPT_fno_coroutines_ts))
CmdArgs.push_back("-fcoroutines-ts");
MordanteUnsubmitted
Not Done
ReplyInline Actions

Is this option still useful? In LLVM 14 the <coroutine> header is no longer experimental. The experimental header will soon be removed.

Mordante: Is this option still useful? In LLVM 14 the `<coroutine>` header is no longer experimental. The…
CmdArgs.push_back("-fmodules-ts");
ldionneUnsubmitted
Not Done
ReplyInline Actions

We should also be adding -lc++experimental if the standard library is libc++.

ldionne: We should also be adding `-lc++experimental` if the standard library is libc++.
}
if (Args.hasArg(options::OPT_fexperimental_new_constant_interpreter)) if (Args.hasArg(options::OPT_fexperimental_new_constant_interpreter))
CmdArgs.push_back("-fexperimental-new-constant-interpreter"); CmdArgs.push_back("-fexperimental-new-constant-interpreter");
if (Arg *A = Args.getLastArg(options::OPT_fbracket_depth_EQ)) { if (Arg *A = Args.getLastArg(options::OPT_fbracket_depth_EQ)) {
CmdArgs.push_back("-fbracket-depth"); CmdArgs.push_back("-fbracket-depth");
CmdArgs.push_back(A->getValue()); CmdArgs.push_back(A->getValue());
} }
▲ Show 20 Lines • Show All 2,579 Lines • Show Last 20 Lines

clang/test/Driver/unstable-flag.cpp

  • This file was added.
// RUN: %clang -funstable -### %s 2>&1 | FileCheck %s
// CHECK: -funstable
// CHECK: -fcoroutines-ts
// CHECK: -fmodules-ts

clang/test/Lexer/has_feature_cxx_unstable.cpp

  • This file was added.
// RUN: %clang_cc1 -funstable -E %s -o - | FileCheck --check-prefix=CHECK-UNSTABLE %s
// RUN: %clang_cc1 -E %s -o - | FileCheck --check-prefix=CHECK-NO-UNSTABLE %s
#if __has_feature(cxx_unstable)
int has_cxx_unstable();
#else
int has_no_cxx_unstable();
#endif
// CHECK-UNSTABLE: int has_cxx_unstable();
// CHECK-NO-UNSTABLE: int has_no_cxx_unstable();