Clang 3.7 Release Notes¶
- What’s New in Clang 3.7?
- Additional Information
Written by the LLVM Team
This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 3.7. 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.
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.
- Use of the __declspec language extension for declaration attributes now requires passing the -fms-extensions or -fborland compiler flag. This language extension is also enabled when compiling CUDA code, but its use should be viewed as an implementation detail that is subject to change.
- On Windows targets, some uses of the __try, __except, and __finally language constructs are supported in Clang 3.7. MSVC-compatible C++ exceptions are not yet supported, however.
- Clang 3.7 fully supports OpenMP 3.1 and reported to work on many platforms, including x86, x86-64 and Power. Also, pragma omp simd from OpenMP 4.0 is supported as well. See below for details.
- Clang 3.7 includes an implementation of control flow integrity, a security hardening mechanism.
Clang’s diagnostics are constantly being improved to catch more issues, explain them more clearly, and provide more accurate source information about them. The improvements since the 3.6 release include:
- -Wrange-loop-analysis analyzes the loop variable type and the container type to determine whether copies are made of the container elements. If possible, suggest a const reference type to prevent copies, or a non-reference type to indicate a copy is made.
- -Wredundant-move warns when a parameter variable is moved on return and the return type is the same as the variable. Returning the variable directly will already make a move, so the call is not needed.
- -Wpessimizing-move warns when a local variable is moved on return and the return type is the same as the variable. Copy elision cannot take place with a move, but can take place if the variable is returned directly.
- -Wmove is a new warning group which has the previous two warnings, -Wredundant-move and -Wpessimizing-move, as well as previous warning -Wself-move. In addition, this group is part of -Wmost and -Wall now.
- -Winfinite-recursion, a warning for functions that only call themselves, is now part of -Wmost and -Wall.
- -Wobjc-circular-container prevents creation of circular containers, it covers NSMutableArray, NSMutableSet, NSMutableDictionary, NSMutableOrderedSet and all their subclasses.
The sized deallocation feature of C++14 is now controlled by the -fsized-deallocation flag. This feature relies on library support that isn’t yet widely deployed, so the user must supply an extra flag to get the extra functionality.
- objc_boxable attribute was added. Structs and unions marked with this attribute can be used with boxed expressions (@(...)) to create NSValue.
Clang now accepts GCC-compatible flags for profile guided optimization (PGO). You can now use -fprofile-generate=<dir>, -fprofile-use=<dir>, -fno-profile-generate and -fno-profile-use. These flags have the same semantics as their GCC counterparts. However, the generated profile is still LLVM-specific. PGO profiles generated with Clang cannot be used by GCC and vice-versa.
Clang now emits function entry counts in profile-instrumented binaries. This has improved the computation of weights and frequencies in profile analysis.
OpenMP 3.1 is fully supported, but disabled by default. To enable it, please use the -fopenmp=libomp command line option. Your feedback (positive or negative) on using OpenMP-enabled clang would be much appreciated; please share it either on cfe-dev or openmp-dev mailing lists.
In addition to OpenMP 3.1, several important elements of the 4.0 version of the standard are supported as well:
- omp simd, omp for simd and omp parallel for simd pragmas
- atomic constructs
- proc_bind clause of omp parallel pragma
- depend clause of omp task pragma (except for array sections)
- omp cancel and omp cancellation point pragmas
- omp taskgroup pragma
These are major API changes that have happened since the 3.6 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.
- Some of the PPCallbacks interface now deals in MacroDefinition objects instead of MacroDirective objects. This allows preserving full information on macros imported from modules.
- clang-c/Index.h no longer #includes clang-c/Documentation.h. You now need to explicitly #include "clang-c/Documentation.h" if you use the libclang documentation API.
- The generated plists now contain the name of the check that generated it.
- Configuration options can now be passed to the checkers (not just the static analyzer core).
- New check for dereferencing object that the result of a zero-length allocation.
- Also check functions in precompiled headers.
- Properly handle alloca() in some checkers.
- Various improvements to the retain count checker.
- Clang will now always default to the z10 processor when compiling without any -march= option. Previous releases used to automatically detect the current host CPU when compiling natively. If you wish to still have clang detect the current host CPU, you now need to use the -march=native option.
- Clang now provides the <s390intrin.h> header file.
- Clang now supports the transactional-execution facility and provides associated builtins and the <htmintrin.h> and <htmxlintrin.h> header files. Support is enabled by default on zEC12 and above, and can additionally be enabled or disabled via the -mhtm / -mno-htm command line options.
- Clang now supports the vector facility. This includes a change in the ABI to pass arguments and return values of vector types in vector registers, as well as a change in the default alignment of vector types. Support is enabled by default on z13 and above, and can additionally be enabled or disabled via the -mvx / -mno-vx command line options.
- Clang now supports the System z vector language extension, providing a “vector” keyword to define vector types, and a set of builtins defined in the <vecintrin.h> header file. This can be enabled via the -fzvector command line option. For compatibility with GCC, Clang also supports the -mzvector option as an alias.
- Several cases of ABI incompatibility with GCC have been fixed.
Added new checks:
- google-global-names-in-headers: flag global namespace pollution in header files.
- misc-assert-side-effect: detects assert() conditions with side effects which can cause different behavior in debug / release builds.
- misc-assign-operator-signature: finds declarations of assign operators with the wrong return and/or argument types.
- misc-inaccurate-erase: warns when some elements of a container are not removed due to using the erase() algorithm incorrectly.
- misc-inefficient-algorithm: warns on inefficient use of STL algorithms on associative containers.
- misc-macro-parentheses: finds macros that can have unexpected behavior due to missing parentheses.
- misc-macro-repeated-side-effects: checks for repeated argument with side effects in macros.
- misc-noexcept-move-constructor: flags user-defined move constructors and assignment operators not marked with noexcept or marked with noexcept(expr) where expr evaluates to false (but is not a false literal itself).
- misc-static-assert: replaces assert() with static_assert() if the condition is evaluable at compile time.
- readability-container-size-empty: checks whether a call to the size() method can be replaced with a call to empty().
- readability-else-after-return: flags conditional statements having the else branch, when the true branch has a return as the last statement.
- readability-redundant-string-cstr: finds unnecessary calls to std::string::c_str().
- readability-shrink-to-fit: replaces copy and swap tricks on shrinkable containers with the shrink_to_fit() method call.
- readability-simplify-boolean-expr: looks for boolean expressions involving boolean constants and simplifies them to use the appropriate boolean expression directly (if (x == true) ... -> if (x), etc.)
This is expected to the be the last major release of Clang that will support running on Windows XP and Windows Vista. For the next major release the minimum Windows version requirement will be Windows 7.
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.