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ClangOffloadBundler.rst

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Clang Offload Bundler

Introduction

For heterogeneous single source programming languages, use one or more --offload-arch=<target-id> Clang options to specify the target IDs of the code to generate for the offload code regions.

The tool chain may perform multiple compilations of a translation unit to produce separate code objects for the host and potentially multiple offloaded devices. The clang-offload-bundler tool may be used as part of the tool chain to combine these multiple code objects into a single bundled code object.

The tool chain may use a bundled code object as an intermediate step so that each tool chain step consumes and produces a single file as in traditional non-heterogeneous tool chains. The bundled code object contains the code objects for the host and all the offload devices.

A bundled code object may also be used to bundle just the offloaded code objects, and embedded as data into the host code object. The host compilation includes an init function that will use the runtime corresponding to the offload kind (see :ref:`clang-offload-kind-table`) to load the offload code objects appropriate to the devices present when the host program is executed.

Supported File Formats

Several text and binary file formats are supported for bundling/unbundling. See :ref:`supported-file-formats-table` for a list of currently supported formats.

Supported File Formats
File Format File Extension Text/Binary
CPP output i Text
C++ CPP output ii Text
CUDA/HIP output cui Text
Dependency d Text
LLVM ll Text
LLVM Bitcode bc Binary
Assembler s Text
Object o Binary
Archive of objects a Binary
Precompiled header gch Binary
Clang AST file ast Binary

Bundled Text File Layout

The format of the bundled files is currently very simple: text formats are concatenated with comments that have a magic string and bundle entry ID in between.

"Comment OFFLOAD_BUNDLER_MAGIC_STR__START__ 1st Bundle Entry ID"
Bundle 1
"Comment OFFLOAD_BUNDLER_MAGIC_STR__END__ 1st Bundle Entry ID"
...
"Comment OFFLOAD_BUNDLER_MAGIC_STR__START__ Nth Bundle Entry ID"
Bundle N
"Comment OFFLOAD_BUNDLER_MAGIC_STR__END__ 1st Bundle Entry ID"

Bundled Binary File Layout

The layout of a bundled code object is defined by the following table:

Bundled Code Object Layout
Field Type Size in Bytes Description
Magic String string 24 __CLANG_OFFLOAD_BUNDLE__
Number Of Bundle Entries integer 8 Number of bundle entries.
1st Bundle Entry Code Object Offset integer 8 Byte offset from beginning of bundled code object to 1st code object.
1st Bundle Entry Code Object Size integer 8 Byte size of 1st code object.
1st Bundle Entry ID Length integer 8 Character length of bundle entry ID of 1st code object.
1st Bundle Entry ID string 1st Bundle Entry ID Length Bundle entry ID of 1st code object. This is not NUL terminated. See :ref:`clang-bundle-entry-id`.
...      
Nth Bundle Entry Code Object Offset integer 8  
Nth Bundle Entry Code Object Size integer 8  
Nth Bundle Entry ID Length integer 8  
Nth Bundle Entry ID string 1st Bundle Entry ID Length  
1st Bundle Entry Code Object bytes 1st Bundle Entry Code Object Size  
...      
Nth Bundle Entry Code Object bytes Nth Bundle Entry Code Object Size  

Bundle Entry ID

Each entry in a bundled code object (see :ref:`clang-bundled-code-object-layout`) has a bundle entry ID that indicates the kind of the entry's code object and the runtime that manages it.

Bundle entry ID syntax is defined by the following BNF syntax:

<bundle-entry-id> ::== <offload-kind> "-" <target-triple> [ "-" <target-id> ]

Where:

offload-kind

The runtime responsible for managing the bundled entry code object. See :ref:`clang-offload-kind-table`.

Bundled Code Object Offload Kind
Offload Kind Description
host Host code object. clang-offload-bundler always includes this entry as the first bundled code object entry. For an embedded bundled code object this entry is not used by the runtime and so is generally an empty code object.
hip Offload code object for the HIP language. Used for all HIP language offload code objects when the clang-offload-bundler is used to bundle code objects as intermediate steps of the tool chain. Also used for AMD GPU code objects before ABI version V4 when the clang-offload-bundler is used to create a fat binary to be loaded by the HIP runtime. The fat binary can be loaded directly from a file, or be embedded in the host code object as a data section with the name .hip_fatbin.
hipv4 Offload code object for the HIP language. Used for AMD GPU code objects with at least ABI version V4 when the clang-offload-bundler is used to create a fat binary to be loaded by the HIP runtime. The fat binary can be loaded directly from a file, or be embedded in the host code object as a data section with the name .hip_fatbin.
openmp Offload code object for the OpenMP language extension.
target-triple
The target triple of the code object.
target-id
The canonical target ID of the code object. Present only if the target supports a target ID. See :ref:`clang-target-id`.

Each entry of a bundled code object must have a different bundle entry ID. There can be multiple entries for the same processor provided they differ in target feature settings. If there is an entry with a target feature specified as Any, then all entries must specify that target feature as Any for the same processor. There may be additional target specific restrictions.

Target ID

A target ID is used to indicate the processor and optionally its configuration, expressed by a set of target features, that affect ISA generation. It is target specific if a target ID is supported, or if the target triple alone is sufficient to specify the ISA generation.

It is used with the -mcpu=<target-id> and --offload-arch=<target-id> Clang compilation options to specify the kind of code to generate.

It is also used as part of the bundle entry ID to identify the code object. See :ref:`clang-bundle-entry-id`.

Target ID syntax is defined by the following BNF syntax:

<target-id> ::== <processor> ( ":" <target-feature> ( "+" | "-" ) )*

Where:

processor
Is a the target specific processor or any alternative processor name.
target-feature

Is a target feature name that is supported by the processor. Each target feature must appear at most once in a target ID and can have one of three values:

Any
Specified by omitting the target feature from the target ID. A code object compiled with a target ID specifying the default value of a target feature can be loaded and executed on a processor configured with the target feature on or off.
On
Specified by +, indicating the target feature is enabled. A code object compiled with a target ID specifying a target feature on can only be loaded on a processor configured with the target feature on.
Off
specified by -, indicating the target feature is disabled. A code object compiled with a target ID specifying a target feature off can only be loaded on a processor configured with the target feature off.

There are two forms of target ID:

Non-Canonical Form
The non-canonical form is used as the input to user commands to allow the user greater convenience. It allows both the primary and alternative processor name to be used and the target features may be specified in any order.
Canonical Form
The canonical form is used for all generated output to allow greater convenience for tools that consume the information. It is also used for internal passing of information between tools. Only the primary and not alternative processor name is used and the target features are specified in alphabetic order. Command line tools convert non-canonical form to canonical form.

Target Specific information

Target specific information is available for the following:

AMD GPU
AMD GPU supports target ID and target features. See User Guide for AMDGPU Backend which defines the processors and target features supported.

Most other targets do not support target IDs.

Archive Unbundling

Unbundling of heterogeneous device archive is done to create device specific archives. Heterogeneous Device Archive is in a format compatible with GNU ar utility and contains a collection of bundled device binaries where each bundle file will contain device binaries for a host and one or more targets. The output device specific archive is in a format compatible with GNU ar utility and contains a collection of device binaries for a specific target.

Heterogeneous Device Archive, HDA = {F1.X, F2.X, ..., FN.Y}
where, Fi = Bundle{Host-DeviceBinary, T1-DeviceBinary, T2-DeviceBinary, ...,
                   Tm-DeviceBinary},
       Ti = {Target i, qualified using Bundle Entry ID},
       X/Y = \*.bc for AMDGPU and \*.cubin for NVPTX

Device Specific Archive, DSA(Tk) = {F1-Tk-DeviceBinary.X, F2-Tk-DeviceBinary.X, ...
                                    FN-Tk-DeviceBinary.Y}
where, Fi-Tj-DeviceBinary.X represents device binary of i-th bundled device
binary file for target Tj.

clang-offload-bundler extracts compatible device binaries for a given target from the bundled device binaries in a heterogeneous device archive and creates a target specific device archive without bundling.

clang-offload-bundler determines whether a device binary is compatible with a target by comparing bundle ID's. Two bundle ID's are considered compatible if:

  • Their offload kind are the same
  • Their target triple are the same
  • Their GPUArch are the same