0006 - Restricted Unbounded Arrays
| Status | Accepted |
|---|---|
| Author | |
| Sponsor |
- Planned Version: 202x
- PRs: #634
Implementation Status
| DXC | Clang | |
|---|---|---|
| Restricting unbound array declarations | Not Started | Not Started |
Introduction
New restrictions imposed on where declarators of arrays of unknown size can be used legally in HLSL.
Motivation
Arrays of unknown size cause a variety of problems within HLSL. When used as parameters they cannot be copied in and out which violates the base calling convention. For resource arrays, passing arrays of unknown size makes resource initialization fragile across call boundaries.
Since the behavior of out of bounds accesses is undefined, losing the array bound information at the source level and in the calling convention is rife with potential pitfalls.
Proposed solution
This proposal suggests limiting declarators of arrays of unknown size to:
- Global declarations of resources.
- Variables where an initializer is provided and the array bound is implied by the initializer.
Detailed design
Subscript [Expr.Post.Subscript]
A postfix-expression followed by an expression in square brackets
([ ]) is a subscript expression. In an array subscript expression of
the form E1[E2], E1 must have array, vector, or matrix of T[]
type, or type T where T provides an overloaded implementation of
operator[] (\ref{Overload}).
If the postfix expression E1 is of array, vector or matrix type, the
expression E2 must be a value of integer type, or of a type that is
implicitly convertible to integer type. If the value is known at compile
time to be outside the range [0, N-1] where N is the number of
elements in the vector, array, or matrix, the program is ill-formed. If the value is outside the
range at runtime, the behavior is undefined.
Declarators [Decl.Decl]
\Sec{Declarators}{Decl.Decl}
\begin{grammar}
\define{init-declarator-list}\br
init-declarator\br
init-declarator-list \terminal{,} init-declarator\br
\define{init-declarator}\br
declarator \opt{initializer}\br
\define{declarator}\br
declarator-id \opt{attribute-specifier-seq}\br
declarator parameters-and-qualifiers\br
declarator \terminal{\lbrack} \opt{constant-expression} \terminal{\rbrack} \opt{attribute-specifier-seq}\br
\define{parameters-and-qualifiers}\br
\terminal{(} parameter-declaration-clause \terminal{)} \opt{cv-qualifier-seq} \opt{attribute-specifier-seq}\br
\define{cv-qualifier-seq}\br
cv-qualifier \opt{cv-qualifier-seq}\br
\define{cv-qualifier}\br
\terminal{const}\br
\define{declarator-id}\br
id-expression
\end{grammar}
Arrays [Decl.Array]
In a declaration $T D$ where $D$ has the form:
\begin{innergrammar}
\terminal{D1} \terminal{\lbrack} \opt{constant-expression} \terminal{\rbrack} \opt{attribute-specifier-seq}
\end{innergrammar}
The type of the entity declared in a declaration $T D1$ is $T\prime$, the derived type $T$. The type of the entity declared in the declarator $D$ is $T\prime[N]$. The constant-expression shall be a converted constant expression of unsigned integer type. Its value $N$ specifies the number of elements in the array, called the array bound.
If present, $N$ must be greater than zero. If not present, the declaration must be a global declaration of an object of a resource type (\ref{Resources}); otherwise the program is ill-formed.