0043 - `groupshared` Arguments

• Planned Version: 202x

Introduction

This proposal introduces a new use of the groupshared keyword for function arguments to allow passing groupshared arguments by address rather than by value or copy-in/copy-out.

Motivation

DXC’s implementation of HLSL includes a set of Interlocked functions implementing atomic operations on groupshared memory. These functions are not expressible in HLSL and rely on special case implementation in DXC.

Proposed solution

HLSL 202x will allow the groupshared type annotation keyword on function parameter declarations. The keyword when applied to a parameter declaration of type T, alters the qualified type of the parameter to a groupshared T & (a reference to groupshared memory of type T).

No implicit or explicit conversion can change the memory space of an object. To perform such a conversion, a user must declare a new object in the destination memory space and initialize it appropriately. For overload resolution, the parameter type must be an exact match in order for overload resolution to succeed since no conversions will be valid.

Alternatives considered

Reference types is an obvious alternative. This proposal introduces a slightly conflicting syntax from what we would prefer with reference types available.

This more minimal feature has material benefit today for both DXC and Clang, and can avoid Clang requiring special case handling for library functions. As such, this proposal is preferred to waiting until references can be finalized.

Outstanding Questions

• Can we enable this feature in earlier language modes? I think yes, but we should explore.
• DXC does some odd handling for user-defined data types, does this work in DXC or do we need to limit the types it can apply to?