Skip to content

Latest commit

 

History

History
78 lines (62 loc) · 3.07 KB

5_Metadata.md

File metadata and controls

78 lines (62 loc) · 3.07 KB
Raw

Metadata

Here we use "metadata" to signify both LLVM metadata and attributes. While metadata is more flexible, in some cases attributes may be preferred either because passes are required to keep them or because there are existing LLVM attributes with the required semantics.

Representing Source-Language Attributes

Many languages allow attributes to be placed on callable and type definitions. For instance, in Q# attributes are compile-time constant values of specific user-defined types that themselves have the Microsoft.Quantum.Core.Attribute attribute.

The language compiler should represent these attributes as LLVM metadata associated with the callable or type. For callables, the metadata representing the attribute should be attached to the LLVM global symbol that defines the implementation table for the callable. The identifier of the metadata node should be "!quantum.", where "!" is the LLVM standard prefix for a metadata value, followed by the namespace-qualified name of the attribute. For example, a callable Your.Opwith two attributes, My.Attribute(6, "hello") and Their.Attribute(2.1), applied to it would be represented in LLVM as follows:

@Your.Op = constant
  [void (%Tuple*, %Tuple*, %Tuple*)*]
  [
    void (%Tuple*, %Tuple*, %Tuple*)*
        @Your.Op-body,
    void (%Tuple*, %Tuple*, %Tuple*)*
        @Your.Op-adj,
    void (%Tuple*, %Tuple*, %Tuple*)*
        @Your.Op-ctl,
    void (%Tuple*, %Tuple*, %Tuple*)*
        @Your.Op-ctladj
  ], !quantum.My.Attribute {i64 6, !"hello\00"},
     !quantum.Their.Attribute {double 2.1}

LLVM does not allow metadata to be associated with structure definitions, so there is no direct way to represent attributes attached to user-defined types. Thus, attributes on types are represented as named (module-level) metadata, where the metadata node's name is "quantum." followed by the namespace-qualified name of the type. The metadata itself is the same as for callables, but wrapped in one more level of LLVM structure in order to handle multiple attributes on the same structure. For example, a type Your.Typewith two attributes, My.Attribute(6, "hello") and Their.Attribute(2.1), applied to it would be represented in LLVM as follows:

!quantum.Your.Type = !{ !{!"quantum.My.Attribute\00", i64 6, !"hello\00"},
                        !{ !"quantum.Their.Attribute\00", double 2.1} }

Standard LLVM Metadata

Debugging Information

Compilers are strongly urged to follow the recommendations in Source Level Debugging with LLVM.

Branch Prediction

Compilers are strongly urged to follow the recommendations in LLVM Branch Weight Metadata.

Other Compiler-Generated Metadata

If the QIR includes a function that is the quantum entry point, it should be marked with an LLVM "EntryPoint" attribute.

Discussion

It is likely that there is other useful information that could be represented as LLVM metadata in QIR. We anticipate that this will become clearer through use.

Back to index