Clang 11.0.0 (In-Progress) Release Notes

Written by the LLVM Team

Warning

These are in-progress notes for the upcoming Clang 11 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 11.0.0. 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 Git 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 11.0.0?

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

  • -Wpointer-to-int-cast is a new warning group. This group warns about C-style casts of pointers to a integer type too small to hold all possible values.

Non-comprehensive list of changes in this release

  • For the ARM target, C-language intrinsics are now provided for the full Arm v8.1-M MVE instruction set. <arm_mve.h> supports the complete API defined in the Arm C Language Extensions.

New Compiler Flags

  • -fstack-clash-protection will provide a protection against the stack clash attack for x86 architecture through automatic probing of each page of allocated stack.
  • -ffp-exception-behavior={ignore,maytrap,strict} allows the user to specify the floating-point exception behavior. The default setting is ignore.
  • -ffp-model={precise,strict,fast} provides the user an umbrella option to simplify access to the many single purpose floating point options. The default setting is precise.

Deprecated Compiler Flags

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

Modified Compiler Flags

  • -fno-common has been enabled as the default for all targets. Therefore, C code that uses tentative definitions as definitions of a variable in multiple translation units will trigger multiple-definition linker errors. Generally, this occurs when the use of the extern keyword is neglected in the declaration of a variable in a header file. In some cases, no specific translation unit provides a definition of the variable. The previous behavior can be restored by specifying -fcommon.
  • -Wasm-ignored-qualifier (ex. asm const (“”)) has been removed and replaced with an error (this matches a recent change in GCC-9).
  • -Wasm-file-asm-volatile (ex. asm volatile (“”) at global scope) has been removed and replaced with an error (this matches GCC’s behavior).
  • Duplicate qualifiers on asm statements (ex. asm volatile volatile (“”)) no longer produces a warning via -Wduplicate-decl-specifier, but now an error (this matches GCC’s behavior).

Attribute Changes in Clang

  • Attributes can now be specified by clang plugins. See the Clang Plugins documentation for details.

C Language Changes in Clang

  • The default C language standard used when -std= is not specified has been upgraded from gnu11 to gnu17.
  • Clang now supports the GNU C extension asm inline; it won’t do anything yet, but it will be parsed.

C++ Language Changes in Clang

  • Clang now implements a restriction on giving non-C-compatible anonymous structs a typedef name for linkage purposes, as described in C++ committee paper P1766R1 <http://wg21.link/p1766r1>. This paper was adopted by the C++ committee as a Defect Report resolution, so it is applied retroactively to all C++ standard versions. This affects code such as:

    typedef struct {
      int f() { return 0; }
    } S;
    

    Previous versions of Clang rejected some constructs of this form (specifically, where the linkage of the type happened to be computed before the parser reached the typedef name); those cases are still rejected in Clang 11. In addition, cases that previous versions of Clang did not reject now produce an extension warning. This warning can be disabled with the warning flag -Wno-non-c-typedef-for-linkage.

    Affected code should be updated to provide a tag name for the anonymous struct:

    struct S {
      int f() { return 0; }
    };
    

    If the code is shared with a C compilation (for example, if the parts that are not C-compatible are guarded with #ifdef __cplusplus), the typedef declaration should be retained, but a tag name should still be provided:

    typedef struct S {
      int f() { return 0; }
    } S;
    

C++1z Feature Support

Internal API Changes

These are major API changes that have happened since the 10.0.0 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.

Build System Changes

These are major changes to the build system that have happened since the 10.0.0 release of Clang. Users of the build system should adjust accordingly.

clang-format

  • Option IndentCaseBlocks has been added to support treating the block following a switch case label as a scope block which gets indented itself. It helps avoid having the closing bracket align with the switch statement’s closing bracket (when IndentCaseLabels is false).

  • Option ObjCBreakBeforeNestedBlockParam has been added to optionally apply linebreaks for function arguments declarations before nested blocks.

    switch (fool) {                vs.     switch (fool) {
    case 1:                                case 1: {
      {                                      bar();
         bar();                            } break;
      }                                    default: {
      break;                                 plop();
    default:                               }
      {                                    }
        plop();
      }
    }
    
  • Option InsertTrailingCommas can be set to TCS_Wrapped to insert trailing commas in container literals (arrays and objects) that wrap across multiple lines. It is currently only available for JavaScript and disabled by default (TCS_None).

  • Option BraceWrapping.BeforeLambdaBody has been added to manage lambda line break inside function parameter call in Allman style.

    true:
    connect(
      []()
      {
        foo();
        bar();
      });
    
    false:
    connect([]() {
        foo();
        bar();
      });
    

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 Git 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.