Clang 8.0.0 (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 8 release. Release notes for previous releases can be found on the Download Page.

Introduction

This document contains the release notes for the Clang C/C++/Objective-C frontend, part of the LLVM Compiler Infrastructure, release 8.0.0. 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.

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 8.0.0?

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

  • Clang supports use of a profile remapping file, which permits profile data captured for one version of a program to be applied when building another version where symbols have changed (for example, due to renaming a class or namespace). See the Clang Compiler User’s Manual for details.

Improvements to Clang’s diagnostics

  • -Wextra-semi-stmt is a new diagnostic that diagnoses extra semicolons, much like -Wextra-semi. This new diagnostic diagnoses all unnecessary null statements (expression statements without an expression), unless: the semicolon directly follows a macro that was expanded to nothing or if the semicolon is within the macro itself. This applies to macros defined in system headers as well as user-defined macros.

    #define MACRO(x) int x;
    #define NULLMACRO(varname)
    
    void test() {
      ; // <- warning: ';' with no preceding expression is a null statement
    
      while (true)
        ; // OK, it is needed.
    
      switch (my_enum) {
      case E1:
        // stuff
        break;
      case E2:
        ; // OK, it is needed.
      }
    
      MACRO(v0;) // Extra semicolon, but within macro, so ignored.
    
      MACRO(v1); // <- warning: ';' with no preceding expression is a null statement
    
      NULLMACRO(v2); // ignored, NULLMACRO expanded to nothing.
    }
    
  • -Wempty-init-stmt is a new diagnostic that diagnoses empty init-statements of if, switch, range-based for, unless: the semicolon directly follows a macro that was expanded to nothing or if the semicolon is within the macro itself (both macros from system headers, and normal macros). This diagnostic is in the -Wextra-semi-stmt group and is enabled in -Wextra.

    void test() {
      if(; // <- warning: init-statement of 'if' is a null statement
         true)
        ;
    
      switch (; // <- warning: init-statement of 'switch' is a null statement
              x) {
        ...
      }
    
      for (; // <- warning: init-statement of 'range-based for' is a null statement
           int y : S())
        ;
    }
    

Non-comprehensive list of changes in this release

  • The experimental feature Pretokenized Headers (PTH) was removed in its entirely from Clang. The feature did not properly work with about 1/3 of the possible tokens available and was unmaintained.
  • The internals of libc++ include directory detection on MacOS have changed. Instead of running a search based on the -resource-dir flag, the search is now based on the path of the compiler in the filesystem. The default behaviour should not change. However, if you override -resource-dir manually and rely on the old behaviour you will need to add appropriate compiler flags for finding the corresponding libc++ include directory.

New Compiler Flags

  • -fprofile-filter-files=[regexes] and -fprofile-exclude-files=[regexes].

    Clang has now options to filter or exclude some files when instrumenting for gcov-based profiling. See the Clang Compiler User’s Manual for details.

Deprecated Compiler Flags

The following options are deprecated and ignored. They will be removed in future versions of Clang.

Modified Compiler Flags

  • As of clang 8, alignof and _Alignof return the ABI alignment of a type, as opposed to the preferred alignment. __alignof still returns the preferred alignment. -fclang-abi-compat=7 (and previous) will make alignof and _Alignof return preferred alignment again.

New Pragmas in Clang

  • Clang now supports adding multiple #pragma clang attribute attributes into a scope of pushed attributes.

Windows Support

  • clang-cl now supports the use of the precompiled header options /Yc and /Yu without the filename argument. When these options are used without the filename, a #pragma hdrstop inside the source marks the end of the precompiled code.
  • clang-cl has a new command-line option, /Zc:dllexportInlines-, similar to -fvisibility-inlines-hidden on non-Windows, that makes class-level dllexport and dllimport attributes not apply to inline member functions. This can significantly reduce compile and link times. See the User’s Manual for more info.

C Language Changes in Clang

C11 Feature Support

C++ Language Changes in Clang

C++1z Feature Support

ABI Changes in Clang

  • _Alignof and alignof now return the ABI alignment of a type, as opposed to the preferred alignment.
    • This is more in keeping with the language of the standards, as well as being compatible with gcc
    • __alignof and __alignof__ still return the preferred alignment of a type
    • This shouldn’t break any ABI except for things that explicitly ask for alignas(alignof(T)).
    • If you have interfaces that break with this change, you may wish to switch to alignas(__alignof(T)), instead of using the -fclang-abi-compat switch.

OpenMP Support in Clang

  • Support relational-op != (not-equal) as one of the canonical forms of random access iterator.
  • Added support for mapping of the lambdas in target regions.
  • Added parsing/sema analysis for OpenMP 5.0 requires directive.
  • Various bugfixes and improvements.

New features supported for Cuda devices:

  • Added support for the reductions across the teams.
  • Extended number of constructs that can be executed in SPMD mode.
  • Fixed support for lastprivate/reduction variables in SPMD constructs.
  • General performance improvement.

Internal API Changes

These are major API changes that have happened since the 7.0.0 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.

Undefined Behavior Sanitizer (UBSan)

  • The Implicit Conversion Sanitizer (-fsanitize=implicit-conversion) group was extended. One more type of issues is caught - implicit integer sign change. (-fsanitize=implicit-integer-sign-change). This makes the Implicit Conversion Sanitizer feature-complete, with only missing piece being bitfield handling. While there is a -Wsign-conversion diagnostic group that catches this kind of issues, it is both noisy, and does not catch all the cases.

    bool consume(unsigned int val);
    
    void test(int val) {
      (void)consume(val); // If the value was negative, it is now large positive.
      (void)consume((unsigned int)val); // OK, the conversion is explicit.
    }
    

    Like some other -fsanitize=integer checks, these issues are not undefined behaviour. But they are not always intentional, and are somewhat hard to track down. This group is not enabled by -fsanitize=undefined, but the -fsanitize=implicit-integer-sign-change check is enabled by -fsanitize=integer. (as is -fsanitize=implicit-integer-truncation check)

  • The Implicit Conversion Sanitizer (-fsanitize=implicit-conversion) has learned to sanitize compound assignment operators.

  • alignment check has learned to sanitize the assume_aligned-like attributes:

    typedef char **__attribute__((align_value(1024))) aligned_char;
    struct ac_struct {
      aligned_char a;
    };
    char **load_from_ac_struct(struct ac_struct *x) {
      return x->a; // <- check that loaded 'a' is aligned
    }
    
    char **passthrough(__attribute__((align_value(1024))) char **x) {
      return x; // <- check the pointer passed as function argument
    }
    
    char **__attribute__((alloc_align(2)))
    alloc_align(int size, unsigned long alignment);
    
    char **caller(int size) {
      return alloc_align(size, 1024); // <- check returned pointer
    }
    
    char **__attribute__((assume_aligned(1024))) get_ptr();
    
    char **caller2() {
      return get_ptr(); // <- check returned pointer
    }
    
    void *caller3(char **x) {
      return __builtin_assume_aligned(x, 1024);  // <- check returned pointer
    }
    
    void *caller4(char **x, unsigned long offset) {
      return __builtin_assume_aligned(x, 1024, offset);  // <- check returned pointer accounting for the offest
    }
    
    void process(char *data, int width) {
        #pragma omp for simd aligned(data : 1024) // <- aligned clause will be checked.
        for (int x = 0; x < width; x++)
        data[x] *= data[x];
    }
    

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.