Clang 6.0.0 Release Notes¶
- Introduction
- What’s New in Clang 6.0.0?
- Major New Features
- Non-comprehensive list of changes in this release
- New Compiler Flags
- Deprecated Compiler Flags
- New Pragmas in Clang
- Attribute Changes in Clang
- Windows Support
- C Language Changes in Clang
- C++ Language Changes in Clang
- Objective-C Language Changes in Clang
- OpenCL C Language Changes in Clang
- OpenMP Support in Clang
- Internal API Changes
- AST Matchers
- clang-format
- libclang
- Static Analyzer
- Undefined Behavior Sanitizer (UBSan)
- Core Analysis Improvements
- New Issues Found
- Significant Known Problems
- Additional Information
Written by the LLVM Team
Introduction¶
This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 6.0.0. Here we describe the status of Clang in some detail, including major improvements from the previous release and new feature work. For the general LLVM release notes, see the LLVM documentation. All LLVM releases may be downloaded from the LLVM releases web site.
For more information about Clang or LLVM, including information about the latest release, please see the Clang Web Site or the LLVM Web Site.
What’s New in Clang 6.0.0?¶
Some of the major new features and improvements to Clang are listed here. Generic improvements to Clang as a whole or to its underlying infrastructure are described first, followed by language-specific sections with improvements to Clang’s support for those languages.
Major New Features¶
- ...
Improvements to Clang’s diagnostics¶
-Wpragma-pack
is a new warning that warns in the following cases:- When a translation unit is missing terminating
#pragma pack (pop)
directives. - When leaving an included file that changes the current alignment value,
i.e. when the alignment before
#include
is different to the alignment after#include
. -Wpragma-pack-suspicious-include
(disabled by default) warns on an#include
when the included file contains structures or unions affected by a non-default alignment that has been specified using a#pragma pack
directive prior to the#include
.
- When a translation unit is missing terminating
-Wobjc-messaging-id
is a new, non-default warning that warns about message sends to unqualifiedid
in Objective-C. This warning is useful for projects that would like to avoid any potential future compiler errors/warnings, as the system frameworks might add a method with the same selector which could make the message send toid
ambiguous.-Wtautological-compare
now warns when comparing an unsigned integer and 0 regardless of whether the constant is signed or unsigned.”-Wtautological-compare
now warns about comparing a signed integer and 0 when the signed integer is coerced to an unsigned type for the comparison.-Wsign-compare
was adjusted not to warn in this case.-Wtautological-constant-compare
is a new warning that warns on tautological comparisons between integer variable of the typeT
and the largest/smallest possible integer constant of that same type.- For C code,
-Wsign-compare
,-Wsign-conversion
,-Wtautological-constant-compare
and-Wtautological-constant-out-of-range-compare
were adjusted to use the underlying datatype ofenum
. -Wnull-pointer-arithmetic
now warns about performing pointer arithmetic on a null pointer. Such pointer arithmetic has an undefined behavior if the offset is nonzero. It also now warns about arithmetic on a null pointer treated as a cast from integer to pointer (GNU extension).-Wzero-as-null-pointer-constant
was adjusted not to warn on null pointer constants that originate from system macros, exceptNULL
macro.-Wdelete-non-virtual-dtor
can now fire in system headers, so thatstd::unique_ptr<>
deleting through a non-virtual dtor is now diagnosed.-Wunreachable-code
can now reason about__try
,__except
and__leave
.
Non-comprehensive list of changes in this release¶
- Bitrig OS was merged back into OpenBSD, so Bitrig support has been removed from Clang/LLVM.
- The default value of _MSC_VER was raised from 1800 to 1911, making it compatible with the Visual Studio 2015 and 2017 C++ standard library headers. Users should generally expect this to be regularly raised to match the most recently released version of the Visual C++ compiler.
- clang now defaults to
.init_array
if no gcc installation can be found. If a gcc installation is found, it still prefers.ctors
if the found gcc is older than 4.7.0. - The new builtin preprocessor macros
__is_target_arch
,__is_target_vendor
,__is_target_os
, and__is_target_environment
can be used to to examine the individual components of the target triple.
New Compiler Flags¶
- –autocomplete was implemented to obtain a list of flags and its arguments. This is used for shell autocompletion.
- The
-fdouble-square-bracket-attributes
and corresponding-fno-double-square-bracket-attributes
flags were added to enable or disable [[]] attributes in any language mode. Currently, only a limited number of attributes are supported outside of C++ mode. See the Clang attribute documentation for more information about which attributes are supported for each syntax. - Added the
-std=c17
,-std=gnu17
, and-std=iso9899:2017
language mode flags for compatibility with GCC. This enables support for the next version of the C standard, expected to be published by ISO in 2018. The only difference between the-std=c17
and-std=c11
language modes is the value of the__STDC_VERSION__
macro, as C17 is a bug fix release. - Added the
-fexperimental-isel
and-fno-experimental-isel
flags to enable/disable the new GlobalISel instruction selection framework. This feature is enabled by default for AArch64 at the-O0
optimization level. Support for other targets or optimization levels is currently incomplete. - New
-nostdlib++
flag to disable linking the C++ standard library. Similar to usingclang
instead ofclang++
but doesn’t disable-lm
.
Deprecated Compiler Flags¶
The following options are deprecated and ignored. They will be removed in future versions of Clang.
- ...
New Pragmas in Clang¶
Clang now supports the ...
Attribute Changes in Clang¶
- Clang now supports the majority of its attributes under both the GNU-style
spelling (
__attribute((name))
) and the double square-bracket spelling in theclang
vendor namespace ([[clang::name]]
). Attributes whose syntax is specified by some other standard (such as CUDA and OpenCL attributes) continue to follow their respective specification. - Added the
__has_c_attribute()
builtin preprocessor macro which allows users to dynamically detect whether a double square-bracket attribute is supported in C mode. This attribute syntax can be enabled with the-fdouble-square-bracket-attributes
flag. - The presence of __attribute__((availability(...))) on a declaration no longer implies default visibility for that declaration on macOS.
- Clang now supports configuration files. These are collections of driver options, which can be applied by specifying the configuration file, either using command line option –config foo.cfg or encoding it into executable name foo-clang. Clang behaves as if the options from this file were inserted before the options specified in command line. This feature is primary intended to facilitate cross compilation. Details can be found in Clang Compiler User’s Manual <http://clang.llvm.org/docs/UsersManual.html#configuration-files>.
- ...
Windows Support¶
- Clang now has initial, preliminary support for targeting Windows on ARM64.
- clang-cl now exposes the
--version
flag.
C++ Language Changes in Clang¶
- Clang’s default C++ dialect is now
gnu++14
instead ofgnu++98
. This means Clang will by default accept code using features from C++14 and conforming GNU extensions. Projects incompatible with C++14 can add-std=gnu++98
to their build settings to restore the previous behaviour.
C++1z Feature Support¶
...
OpenCL C Language Changes in Clang¶
- Added subgroup builtins to enqueue kernel support.
- Added CL2.0 atomics as Clang builtins that now accept an additional memory scope parameter propagated to atomic IR instructions (this is to align with the corresponding change in LLVM IR) (see spec s6.13.11.4).
- Miscellaneous fixes in the CL header.
- Allow per target selection of address space during CodeGen of certain OpenCL types. Default target implementation is provided mimicking old behavior.
- Macro
__IMAGE_SUPPORT__
is now automatically added (as per spec s6.10). - Added
cl_intel_subgroups
andcl_intel_subgroups_short
extensions. - All function calls are marked by the convergent attribute to prevent optimizations that break SPMD program semantics. This will be removed by LLVM passes if it can be proved that the function does not use convergent operations.
- Create a kernel wrapper for enqueued blocks, which simplifies enqueue support by providing common functionality.
- Added private address space explicitly in AST and refactored address space support with several simplifications and bug fixes (PR33419 and PR33420).
- OpenCL now allows functions with empty parameters to be treated as if they had a void parameter list (inspired from C++ support). OpenCL C spec update to follow.
- General miscellaneous refactoring and cleanup of blocks support for OpenCL to remove unused parts inherited from Objective C implementation.
- Miscellaneous improvements in vector diagnostics.
- Added half float load and store builtins without enabling half as a legal type
(
__builtin_store_half for double
,__builtin_store_halff
for double,__builtin_load_half for double
,__builtin_load_halff
for float).
OpenMP Support in Clang¶
- Added options -f[no]-openmp-simd that support code emission only for OpenMP SIMD-based directives, like #pragma omp simd, #pragma omp parallel for simd etc. The code is emitted only for simd-based part of the combined directives and clauses.
- Added support for almost all target-based directives except for #pragma omp target teams distribute parallel for [simd]. Although, please note that depend clauses on target-based directives are not supported yet. Clang supports offloading to X86_64, AArch64 and PPC64[LE] devices.
- Added support for reduction-based clauses on task-based directives from upcoming OpenMP 5.0.
- The LLVM OpenMP runtime libomp now supports the OpenMP Tools Interface (OMPT) on x86, x86_64, AArch64, and PPC64 on Linux, Windows, and macOS. If you observe a measurable performance impact on one of your applications without a tool attached, please rebuild the runtime library with -DLIBOMP_OMPT_SUPPORT=OFF and file a bug at LLVM’s Bugzilla or send a message to the OpenMP development list.
Internal API Changes¶
These are major API changes that have happened since the 4.0.0 release of Clang. If upgrading an external codebase that uses Clang as a library, this section should help get you past the largest hurdles of upgrading.
- ...
AST Matchers¶
The hasDeclaration matcher now works the same for Type and QualType and only ever looks through one level of sugaring in a limited number of cases.
There are two main patterns affected by this:
- qualType(hasDeclaration(recordDecl(...))): previously, we would look through sugar like TypedefType to get at the underlying recordDecl; now, we need to explicitly remove the sugaring: qualType(hasUnqualifiedDesugaredType(hasDeclaration(recordDecl(...))))
- hasType(recordDecl(...)): hasType internally uses hasDeclaration; previously, this matcher used to match for example TypedefTypes of the RecordType, but after the change they don’t; to fix, use:
- ::
- hasType(hasUnqualifiedDesugaredType(
- recordType(hasDeclaration(recordDecl(...)))))
- templateSpecializationType(hasDeclaration(classTemplateDecl(...))): previously, we would directly match the underlying ClassTemplateDecl; now, we can explicitly match the ClassTemplateSpecializationDecl, but that requires to explicitly get the ClassTemplateDecl:
- ::
- templateSpecializationType(hasDeclaration(
- classTemplateSpecializationDecl(
- hasSpecializedTemplate(classTemplateDecl(...)))))
clang-format¶
Option IndentPPDirectives added to indent preprocessor directives on conditionals.
Before After #if FOO #if BAR #include <foo> #endif #endif
#if FOO # if BAR # include <foo> # endif #endif
Option -verbose added to the command line. Shows the list of processed files.
Option IncludeBlocks added to merge and regroup multiple
#include
blocks during sorting.Before (Preserve) Merge Regroup #include "b.h" #include "a.b" #include <lib/main.h>
#include "a.h" #include "b.h" #include <lib/main.h>
#include "a.h" #include "b.h" #include <lib/main.h>
Static Analyzer¶
- Static Analyzer can now properly detect and diagnose unary pre-/post- increment/decrement on an uninitialized value.
...
Undefined Behavior Sanitizer (UBSan)¶
- A minimal runtime is now available. It is suitable for use in production
environments, and has a small attack surface. It only provides very basic
issue logging and deduplication, and does not support
-fsanitize=vptr
checking.
Additional Information¶
A wide variety of additional information is available on the Clang web
page. The web page contains versions of the
API documentation which are up-to-date with the Subversion version of
the source code. You can access versions of these documents specific to
this release by going into the “clang/docs/
” directory in the Clang
tree.
If you have any questions or comments about Clang, please feel free to contact us via the mailing list.