Clang 3.5 documentation

Clang 3.5 (In-Progress) Release Notes

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Clang 3.5 (In-Progress) Release Notes

Written by the LLVM Team


These are in-progress notes for the upcoming Clang 3.5 release. You may prefer the Clang 3.4 Release Notes.


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.

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.
  • ...

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.

The option -u is 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, interleaving, and unrolling to be enabled or disabled. Vector width as well as interleave and unrolling count can be manually specified. See language extensions for details.

Clang now supports the #pragma unroll and #pragma nounroll directives to specify loop unrolling optimization hints. Placed just prior to the desired loop, #pragma unroll directs the loop unroller to attempt to fully unroll the loop. The pragma may also be specified with a positive integer parameter indicating the desired unroll count: #pragma unroll _value_. The unroll count parameter can be optionally enclosed in parentheses. The directive #pragma nounroll indicates that the loop should not be unrolled.

C Language Changes in Clang


C11 Feature Support


C++ Language Changes in Clang

  • ...

C++11 Feature Support


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.


Static Analyzer

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.


New Issues Found

  • ...

Python Binding Changes

The following methods have been added:

  • ...

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.

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