Clang 3.5 Release Notes¶
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 3.5. 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 check out the main please see the Clang Web Site or the LLVM Web Site.
Note that if you are reading this file from a Subversion checkout or the main Clang web page, this document applies to the next release, not the current one. To see the release notes for a specific release, please see the releases page.
What’s New in Clang 3.5?¶
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¶
- Clang uses the new MingW ABI GCC 4.7 changed the mingw ABI. Clang 3.4 and older use the GCC 4.6 ABI. Clang 3.5 and newer use the GCC 4.7 abi.
- The __has_attribute feature test is now target-aware. Older versions of Clang would return true when the attribute spelling was known, regardless of whether the attribute was available to the specific target. Clang now returns true only when the attribute pertains to the current compilation target.
- Clang 3.5 now has parsing and semantic-analysis support for all OpenMP 3.1 pragmas (except atomics and ordered). LLVM’s OpenMP runtime library, originally developed by Intel, has been modified to work on ARM, PowerPC, as well as X86. Code generation support is minimal at this point and will continue to be developed for 3.6, along with the rest of OpenMP 3.1. Support for OpenMP 4.0 features, such as SIMD and target accelerator directives, is also in progress. Contributors to this work include AMD, Argonne National Lab., IBM, Intel, Texas Instruments, University of Houston and many others.
Improvements to Clang’s diagnostics¶
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 3.4 release include:
GCC compatibility: Clang displays a warning on unsupported gcc optimization flags instead of an error.
Remarks system: Clang supports -R flags for enabling remarks. These are diagnostic messages that provide information about the compilation process, but don’t suggest that a problem has been detected. As such, they cannot be upgraded to errors with -Werror or -Rerror. A -Reverything flag is provided (paralleling -Weverything) to turn on all remarks.
New remark -Rpass: Clang provides information about decisions made by optimization passes during compilation. See Options to Emit Optimization Reports.
New warning -Wabsolute-value: Clang warns about incorrect or useless usage of the absolute functions (abs, fabsf, etc).
#include <stdlib.h> void foo() { unsigned int i=0; abs(i); }
returns warning: taking the absolute value of unsigned type ‘unsigned int’ has no effect [-Wabsolute-value]
or
#include <stdlib.h> void plop() { long long i=0; abs(i); }
returns warning: absolute value function ‘abs’ given an argument of type ‘long long’ but has parameter of type ‘int’ which may cause truncation of value [-Wabsolute-value] use function ‘llabs’ instead
New warning -Wtautological-pointer-compare:
#include <stddef.h> void foo() { int arr[5]; int x; // warn on these conditionals if (foo); if (arr); if (&x); if (foo == NULL); if (arr == NULL); if (&x == NULL); }
returns warning: comparison of address of ‘x’ equal to a null pointer is always false [-Wtautological-pointer-compare]
New warning -Wtautological-undefined-compare:
#include <stddef.h> void f(int &x) { if (&x == nullptr) { } }
returns warning: reference cannot be bound to dereferenced null pointer in well-defined C++ code; comparison may be assumed to always evaluate to false [-Wtautological-undefined-compare]
...
New Compiler Flags¶
The integrated assembler is now turned on by default on ARM (and Thumb), so the use of the option -fintegrated-as is now redundant on those architectures. This is an important move to both eat our own dog food and to ease cross-compilation tremendously.
We are aware of the problems that this may cause for code bases that rely on specific GNU syntax or extensions, and we’re working towards getting them all fixed. Please, report bugs or feature requests if you find anything. In the meantime, use -fno-integrated-as to revert back the call to GNU assembler.
In order to provide better diagnostics, the integrated assembler validates inline assembly when the integrated assembler is enabled. Because this is considered a feature of the compiler, it is controlled via the fintegrated-as and fno-integrated-as flags which enable and disable the integrated assembler respectively. -integrated-as and -no-integrated-as are now considered legacy flags (but are available as an alias to prevent breaking existing users), and users are encouraged to switch to the equivalent new feature flag.
Deprecated flags -faddress-sanitizer, -fthread-sanitizer, -fcatch-undefined-behavior and -fbounds-checking were removed in favor of -fsanitize= family of flags.
It is now possible to get optimization reports from the major transformation passes via three new flags: -Rpass, -Rpass-missed and -Rpass-analysis. These flags take a POSIX regular expression which indicates the name of the pass (or passes) that should emit optimization remarks.
Options -u and -z are forwarded to the linker on gnutools toolchains.
New Pragmas in Clang¶
Loop optimization hints can be specified using the new #pragma clang loop directive just prior to the desired loop. The directive allows vectorization and interleaving to be enabled or disabled. Vector width as well as interleave count can be manually specified. See Extensions for loop hint optimizations for details.
C++ Language Changes in Clang¶
- Reference parameters and return values from functions are more aggressively assumed to refer to valid objects when optimizing. Clang will attempt to issue a warning by default if it sees null checks being performed on references, and -fsanitize=null can be used to detect null references being formed at runtime.
C++17 Feature Support¶
Clang has experimental support for some proposed C++1z (tentatively, C++17) features. This support can be enabled using the -std=c++1z flag. The supported features are:
- static_assert(expr) with no message
- for (identifier : range) as a synonym for for (auto &&identifier : range)
- template<template<...> typename> as a synonym for template<template<...> class>
Additionally, trigraphs are not recognized by default in this mode. -ftrigraphs can be used if you need to parse legacy code that uses trigraphs. Note that these features may be changed or removed in future Clang releases without notice.
Internal API Changes¶
These are major API changes that have happened since the 3.4 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.
- Clang uses std::unique_ptr<T> in many places where it used to use raw T * pointers.
Static Analyzer¶
Check for code testing a variable for 0 after using it as a denominator. This new checker, alpha.core.TestAfterDivZero, catches issues like this:
int sum = ...
int avg = sum / count; // potential division by zero...
if (count == 0) { ... } // ...caught here
The -analyzer-config options are now passed from scan-build through to ccc-analyzer and then to Clang.
With the option -analyzer-config stable-report-filename=true, instead of report-XXXXXX.html, scan-build/clang analyzer generate report-<filename>-<function, method name>-<function position>-<id>.html. (id = i++ for several issues found in the same function/method).
List the function/method name in the index page of scan-build.
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.