Clang Offload Packager — Clang 15.0.7 documentation

# Clang 15.0.7 documentation

## Clang Offload Packager

## Introduction

This tool bundles device files into a single image containing necessary metadata. We use a custom binary format for bundling all the device images together. The image format is a small header wrapping around a string map. This tool creates bundled binaries so that they can be embedded into the host to create a fat-binary.

## Binary Format

The binary format is marked by the `0x10FF10AD` magic bytes, followed by a version. Each created binary contains its own magic bytes. This allows us to locate all the embedded offloading sections even after they may have been merged by the linker, such as when using relocatable linking. Conceptually, this binary format is a serialization of a string map and an image buffer. The binary header is described in the following table.

Offloading Binary Header
Type | Identifier | Description
--- | --- | ---
uint8_t | magic | The magic bytes for the binary format (0x10FF10AD)
uint32_t | version | Version of this format (currently version 1)
uint64_t | size | Size of this binary in bytes
uint64_t | entry offset | Absolute offset of the offload entries in bytes
uint64_t | entry size | Size of the offload entries in bytes

Once identified through the magic bytes, we use the size field to take a slice of the binary blob containing the information for a single offloading image. We can then use the offset field to find the actual offloading entries containing the image and metadata. The offload entry contains information about the device image. It contains the fields shown in the following table.

Offloading Entry Table
Type | Identifier | Description
--- | --- | ---
uint16_t | image kind | The kind of the device image (e.g. bc, cubin)
uint16_t | offload kind | The producer of the image (e.g. openmp, cuda)
uint32_t | flags | Generic flags for the image
uint64_t | string offset | Absolute offset of the string metadata table
uint64_t | num strings | Number of string entries in the table
uint64_t | image offset | Absolute offset of the device image in bytes
uint64_t | image size | Size of the device image in bytes

This table contains the offsets of the string table and the device image itself along with some other integer information. The image kind lets us easily identify the type of image stored here without needing to inspect the binary. The offloading kind is used to determine which registration code or linking semantics are necessary for this image. These are stored as enumerations with the following values for the offload kind and the image kind.

Image Kind
Name | Value | Description
--- | --- | ---
IMG_None | 0x00 | No image information provided
IMG_Object | 0x01 | The image is a generic object file
IMG_Bitcode | 0x02 | The image is an LLVM-IR bitcode file
IMG_Cubin | 0x03 | The image is a CUDA object file
IMG_Fatbinary | 0x04 | The image is a CUDA fatbinary file
IMG_PTX | 0x05 | The image is a CUDA PTX file

Offload Kind
Name | Value | Description
--- | --- | ---
OFK_None | 0x00 | No offloading information provided
OFK_OpenMP | 0x01 | The producer was OpenMP offloading
OFK_CUDA | 0x02 | The producer was CUDA
OFK_HIP | 0x03 | The producer was HIP

The flags are used to signify certain conditions, such as the presence of debugging information or whether or not LTO was used. The string entry table is used to generically contain any arbitrary key-value pair. This is stored as an array of the string entry format.

Offloading String Entry
Type | Identifier | Description
--- | --- | ---
uint64_t | key offset | Absolute byte offset of the key in th string table
uint64_t | value offset | Absolute byte offset of the value in the string table

The string entries simply provide offsets to a key and value pair in the binary images string table. The string table is simply a collection of null terminated strings with defined offsets in the image. The string entry allows us to create a key-value pair from this string table. This is used for passing arbitrary arguments to the image, such as the triple and architecture.

All of these structures are combined to form a single binary blob, the order does not matter because of the use of absolute offsets. This makes it easier to extend in the future. As mentioned previously, multiple offloading images are bundled together by simply concatenating them in this format. Because we have the magic bytes and size of each image, we can extract them as-needed.

## Usage

This tool can be used with the following arguments. Generally information is passed as a key-value pair to the `image=` argument. The `file`, `triple`, and `arch` arguments are considered mandatory to make a valid image.

    OVERVIEW: A utility for bundling several object files into a single binary.
    The output binary can then be embedded into the host section table
    to create a fatbinary containing offloading code.

    USAGE: clang-offload-packager [options]

    OPTIONS:

    Generic Options:

      --help                      - Display available options (--help-hidden for more)
      --help-list                 - Display list of available options (--help-list-hidden for more)
      --version                   - Display the version of this program

    clang-offload-packager options:

      --image=<<key>=<value>,...> - List of key and value arguments. Required
                                    keywords are 'file' and 'triple'.
      -o <file>                   - Write output to <file>.

## Example

This tool simply takes many input files from the `image` option and creates a single output file with all the images combined.

    clang-offload-packager -o out.bin --image=file=input.o,triple=nvptx64,arch=sm_70