Issue 0444: Issues with alignment in C11, part 1

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: WG 14, Joseph Myers
Date: 2013-07-23
Reference document: N1731
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

There are various deficiencies in the C11 text about alignment requirements.

Issue 1: Existence of over-aligned types

6.2.8#3 defines the concept of an over-aligned type, with a footnote saying "Every over-aligned type is, or contains, a structure or union type with a member to which an extended alignment has been applied.". But there is no way in the syntax to apply such an alignment to a member. _Alignas appears in the syntax for alignment-specifier, which in turn appears in that for declaration-specifiers (6.7#1). But structure and union members instead use struct-declaration which uses specifier-qualifier-list which doesn't include a case for alignment-specifier at all. So for the reference to over-aligned types, and the reference in 6.7.5#6 to the "declared object or member", to be meaningful, something needs adding to the syntax for struct-declaration. (Note that specifier-qualifier-list is also used in the syntax for type-name, and it seems less likely that a type-name was intended to be able to include alignment-specifiers.)

Comment from WG14 on 2017-11-03:

Oct 2013 meeting

Committee Discussion

• The desired and intended syntax is indeed missing.
• Whereas one could achieve the desired effect by placing the directive on the aggregate itself and controlling it by the maximum alignment of each of its members, this is far from the intended goal, and we do consider this a defect.
• We solicit a suggested technical corrigendum from the author.
• Modifying the definition of type name will require considerable thought.
• Applying the directive beyond aggregate members, such as a simple scalar declaration, is not well defined, and brings in the various storage durations and how or whether all or any are supported.

Apr 2014 meeting

Committee Discussion

• The author provided the following technical corrigendum as input to N1804:
  6.7.2.1#1: Move the definition of specifier-qualifier-list to 6.7.7#1. Insert a new definition in 6.7.2.1#1:

  specifier-qualifier-alignment-list:

  type-specifier specifier-qualifier-alignment-list_opt
  type-qualifier specifier-qualifier-alignment-list_opt
  alignment-specifier specifier-qualifier-alignment-list_opt

  Change the use of specifier-qualifier-list in the syntax for struct-declaration to specifier-qualifier-alignment-list.

  6.7.2#2:

  Change "and in the specifier-qualifier list in each struct declaration and type name" to ", in the specifier-qualifier-alignment list in each struct declaration and in the specifier-qualifier list in each type name".

  6.7.3#5:

  Change "specifier-qualifier-list" to "specifier-qualifier-list, specifier-qualifier-alignment-list or declaration-specifiers" (twice).

  (That the wording about duplicate qualifiers and qualifiers used with _Atomic doesn't deal with declaration-specifiers, the syntax production relevant to normal declarations, is a pre-existing problem noticed in the course of preparing this wording.)

  (I believe all the semantics and constraints required for alignment specifiers on members are in place, including 6.2.7#1 dealing with cross-translation-unit type compatibility and references to bit-fields and members in 6.7.5.)

• The committee agrees with this as the proper direction and will solicit further review from implementors.

Oct 2014 meeting

Committee Discussion

There still has not been adequate review of these changes. The Project Editor and others have been asked to examine these changes closely prior to our next meeting.

Apr 2015 meeting

Committee Discussion

The suggested syntax provided by the author has been adopted on a trial basis in at least one implementation. It does not, however, provide for compound literals.

A simpler syntax change was discussed, to wit

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

alignment-specifier specifier-qualifier-list_opt

where specifier-qualifier-list is used in the grammar in only two productions: struct-declaration (which relates to the primary purpose of this DR), and type-name, which is used only in the definitions of these constructs:

• generic association (generic selection)
• compound literal
• sizeof expression
• _Alignof expression
• cast expression
• atomic type specifier
• alignment specifier

A constraint could be added to 6.7.7 type-name after paragraph 1 disallowing the use of alignment-specifier in a type-name except in the case of compound literal which was deemed useful by the committee. The following principles were elucidated:

• _Alignas needs to be applied wherever objects are laid out in memory. On modern architectures page and cache line alignment of data structures is increasingly critical for performance.
• Alignment is incorporated into the type system for structure (and union) members, but otherwise is not considered part of the type.

In 6.7.3p5, there are two references to specifier-qualifier-list, which should also reference declaration specifiers.

In 6.7.5 paragraphs 2 and 4, there are occurrences of the phrase “alignment attribute” which should instead read “alignment specifier”

Oct 2015 meeting

Committee Discussion

The committee did not discuss the direction from the last meeting in any substantial manner. It has solicited a paper from the author of the direction expressing these ideas as a Suggested Technical Corrigendum.

Apr 2016 meeting

Committee Discussion

A new paper N2028 was submitted that embodied the direction above and the committee accepted it.

Oct 2016 meeting

Committee Discussion

It was noted that C++ allows one additional production for alignment-specifier between struct and tag.

The paper N2028 was presented which had an alternate suggestion for a resolution, but the committee preferred the following.

Proposed Technical Correigendum

Change 6.7.2.1p1 from

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

to

specifier-qualifier-list:

type-specifier specifier-qualifier-list_opt

type-qualifier specifier-qualifier-list_opt

alignment-specifier specifier-qualifier-list_opt

Change 6.7.5p2 from

An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or a function, or a parameter, or an object declared with the register storage-class specifier.

to

An alignment specifier shall appear only in the declaration specifiers of a declaration, or in the specifier-qualifier list of a member declaration, or in the type name of a compound literal. An alignment specifier shall not be used in conjunction with either of the storage-class specifiers typedef or register, nor in a declaration of a function or bit-field.

Change 6.7.3p5 from

If the same qualifier appears more than once in the same specifier-qualifier-list, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier in a specifier-qualifier-list, the resulting type is the so-qualified atomic type.

to

If the same qualifier appears more than once in the same specifier-qualifier-list or as declaration-specifiers, either directly or via one or more typedefs, the behavior is the same as if it appeared only once. If other qualifiers appear along with the _Atomic qualifier the resulting type is the so-qualified atomic type.

Change 6.7.5p4 from

The combined effect of all alignment attributes in a declaration shall not...

to

The combined effect of all alignment specifiers in a declaration shall not...