Clang Offload Wrapper

Introduction

This tool is used in OpenMP offloading toolchain to embed device code objects (usually ELF) into a wrapper host llvm IR (bitcode) file. The wrapper host IR is then assembled and linked with host code objects to generate the executable binary. See Multi-image Binary Embedding and Execution for OpenMP for more details.

Usage

This tool can be used as follows:

$ clang-offload-wrapper -help
OVERVIEW: A tool to create a wrapper bitcode for offload target binaries.
Takes offload target binaries as input and produces bitcode file containing
target binaries packaged as data and initialization code which registers
target binaries in offload runtime.

USAGE: clang-offload-wrapper [options] <input files>

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-wrapper options:
  -o <filename>                      - Output filename
  --target=<triple>                  - Target triple for the output module

Example

clang-offload-wrapper -target host-triple -o host-wrapper.bc --offload-arch=gfx906 gfx906-binary.out --offload-arch=gfx90a gfx90a-binary.out

OpenMP Device Binary Embedding

Various structures and functions used in the wrapper host IR form the interface between the executable binary and the OpenMP runtime.

Enum Types

Offloading Declare Target Flags Enum lists different flag for offloading entries.

Offloading Declare Target Flags Enum

Name	Value	Description
OMP_DECLARE_TARGET_LINK	0x01	Mark the entry as having a ‘link’ attribute (w.r.t. link clause)
OMP_DECLARE_TARGET_CTOR	0x02	Mark the entry as being a global constructor
OMP_DECLARE_TARGET_DTOR	0x04	Mark the entry as being a global destructor

Structure Types

__tgt_offload_entry structure, __tgt_device_image structure, __tgt_bin_desc structure, and __tgt_image_info structure are the structures used in the wrapper host IR.

__tgt_offload_entry structure

Type	Identifier	Description
void*	addr	Address of global symbol within device image (function or global)
char*	name	Name of the symbol
size_t	size	Size of the entry info (0 if it is a function)
int32_t	flags	Flags associated with the entry (see Offloading Declare Target Flags Enum)
int32_t	reserved	Reserved, to be used by the runtime library.

__tgt_device_image structure

Type	Identifier	Description
void*	ImageStart	Pointer to the target code start
void*	ImageEnd	Pointer to the target code end
__tgt_offload_entry*	EntriesBegin	Begin of table with all target entries
__tgt_offload_entry*	EntriesEnd	End of table (non inclusive)

__tgt_bin_desc structure

Type	Identifier	Description
int32_t	NumDeviceImages	Number of device types supported
__tgt_device_image*	DeviceImages	Array of device images (1 per dev. type)
__tgt_offload_entry*	HostEntriesBegin	Begin of table with all host entries
__tgt_offload_entry*	HostEntriesEnd	End of table (non inclusive)

__tgt_image_info structure

Type	Identifier	Description
int32_t	version	The version of this struct
int32_t	image_number	Image number in image library starting from 0
int32_t	number_images	Number of images, used for initial allocation
char*	offload_arch	Target ID for which this image was compiled
char*	compile_opts	reserved for future use

Global Variables

Global Variables lists various global variables, along with their type and their explicit ELF sections, which are used to store device images and related symbols.

Global Variables

Variable	Type	ELF Section	Description
__start_omp_offloading_entries	__tgt_offload_entry	.omp_offloading_entries	Begin symbol for the offload entries table.
__stop_omp_offloading_entries	__tgt_offload_entry	.omp_offloading_entries	End symbol for the offload entries table.
__dummy.omp_offloading.entry	__tgt_offload_entry	.omp_offloading_entries	Dummy zero-sized object in the offload entries section to force linker to define begin/end symbols defined above.
.omp_offloading.device_image	__tgt_device_image	.omp_offloading_entries	ELF device code object of the first image.
.omp_offloading.device_image.N	__tgt_device_image	.omp_offloading_entries	ELF device code object of the (N+1)th image.
.omp_offloading.device_images	__tgt_device_image	.omp_offloading_entries	Array of images.
.omp_offloading.descriptor	__tgt_bin_desc	.omp_offloading_entries	Binary descriptor object (see details below).
__offload_arch	string	.offload_arch_list	Target ID string of the first image.
.offload_image_info	__tgt_image_info	.omp_offloading_entries	Object containing target ID of the first image.
__offload_arch.N	string	.offload_arch_list	Target ID string of the (N+1)th image.
.offload_image_info.N	__tgt_image_info	.omp_offloading_entries	Object containing target ID of the (N+1)th image.

Binary Descriptor for Device Images

This object is passed to the offloading runtime at program startup and it describes all device images available in the executable or shared library. It is defined as follows:

__attribute__((visibility("hidden")))
extern __tgt_offload_entry *__start_omp_offloading_entries;
__attribute__((visibility("hidden")))
extern __tgt_offload_entry *__stop_omp_offloading_entries;

static const char Image0[] = { <Bufs.front() contents> };
...
static const char ImageN[] = { <Bufs.back() contents> };

static const __tgt_device_image Images[] = {
  {
    Image0,                            /*ImageStart*/
    Image0 + sizeof(Image0),           /*ImageEnd*/
    __start_omp_offloading_entries,    /*EntriesBegin*/
    __stop_omp_offloading_entries      /*EntriesEnd*/
  },
  ...
  {
    ImageN,                            /*ImageStart*/
    ImageN + sizeof(ImageN),           /*ImageEnd*/
    __start_omp_offloading_entries,    /*EntriesBegin*/
    __stop_omp_offloading_entries      /*EntriesEnd*/
  }
};

static const __tgt_bin_desc BinDesc = {
  sizeof(Images) / sizeof(Images[0]),  /*NumDeviceImages*/
  Images,                              /*DeviceImages*/
  __start_omp_offloading_entries,      /*HostEntriesBegin*/
  __stop_omp_offloading_entries        /*HostEntriesEnd*/
};

Global Constructor and Destructor

Global constructor (.omp_offloading.descriptor_reg()) registers the library of images with the runtime by calling __tgt_register_lib() function. The cunstructor is explicitly defined in .text.startup section. It calls __tgt_register_image_info() function for each .offload_image_info.N before calling registration function. Similarly, global destructor (.omp_offloading.descriptor_unreg()) calls __tgt_unregister_lib() for the unregistration and is also defined in .text.startup section.

Multi-image Binary Embedding and Execution for OpenMP

For each offloading target, device ELF code objects are generated by clang, opt, llc, and lld pipeline. These code objects along with the target id of the offloading target devices are passed to the clang-offload-wrapper.

• At compile time, the clang-offload-wrapper tool takes the following actions:

  • It embeds the ELF code objects for the device into the host code (see OpenMP Device Binary Embedding).

  • It creates internal labels to these embedded device code objects (.offload_image_info.N).

  • It creates a global constructor to get the address of the embedded device code through .offload_image_info.N structure and to register the device code.

  • It also creates a new ELF section .offload_arch_list with an array of null-terminated strings where each string (__offload_arch.N) provides the target ID of an image.

• At execution time:

  • The global constructor gets run and it registers the device image.

  • The runtime looks for an image that is compatible with the offload environment. It uses the offload-arch library to obtain underlying system’s environment. It’s the target ID for AMDGPU and the processor name for other offloading targets.