Dynamic linking of device code

This document describes purpose and design of dynamic linking of device code feature.

NOTE: This is not a final version. The document is still in progress.

Background

Sometimes users want to link device code dynamically at run time. One possible use case for such linkage - providing device functions via shared libraries. The example below shows how device function LibDeviceFunc can be dynamically linked to a SYCL app:

// app.cpp
SYCL_EXTERNAL int LibDeviceFunc(int i);

/* ... */
Q.submit([&](sycl::handler &CGH) {
CGH.parallel_for(/* ... */ [=](sycl::item i) {
  out[i] = LibDeviceFunc(i.get_id(0));
}); /* ... */
std::cout << out[i] << " ";

// lib.cpp
SYCL_EXTERNAL int LibDeviceFunc(int i) {
  return i * 2;
}
; Commands
clang++ -fsycl lib.cpp -shared -o helpers.so
clang++ -fsycl app.cpp -lhelpers -o a.out
./a.out
Output: 0 2 4 6 ...

The first invocation of clang++ driver will create a “fat” shared library which contains both host code and device code. The second invocation of clang++ driver will create a “fat” application binary that also contains embedded device code. Host part of the application and library will be linked automatically by standard C++ toolchain and system linker, while linking of device part of the application and library requires new functionality which is described in this document.

It is possible to manually create sycl::program in both app and shared library, then use link SYCL API to get a single program and launch kernels using it. But this approach is a noticeable deviations from regular C/C++ workflow and requires some amount of source code boilerplate.

Another possible scenario - use functions defined in a pre-compiled device image provided by the user. Example:

// a.cpp
SYCL_EXTERNAL void LibDeviceFunc();
...
Q.submit([&](sycl::handler &CGH) {
  CGH.parallel_for([]() { LibDeviceFunc(); });
});

// b.cpp
/*no SYCL_EXTERNAL*/ void LibDeviceFunc() { ... }

We have a SYCL_EXTERNAL function LibDeviceFunc called from a kernel, but the application defined only host version of this function. Then user adds device image with definition of LibDeviceFunc to the fat object via special compiler option (like -fsycl-add-targets).

The main purpose of this feature is to provide a user-friendly mechanism which allows to link device code dynamically at runtime, such as in the scenarios above.

Requirements

User’s device code can be compiled into some form and not linked statically with device code of application. It can be embedded as a device image into a shared library or supplied as a separate device image with attached properties. This code is linked dynamically at run time with device image of a user’s application in order to resolve dependencies. The requirements listed below must be satisfied to enable such use case.

The presented dynamic device code linkage mechanism must:

  • Allow to link device code represented as device binary image dynamically at runtime with other device binary images. In order to use this functionality the user can create and supply device binary image to DPC++ Runtime library via following ways:

    • Create a “fat” shared library by standard SYCL compiler driver invocation

    • Supply host binary or shared object with device binary image using manual invocations of SYCL tools such as clang-offload-wrapper and linker

    • Load device binary image into memory via dlopen-like API

      • This is a TODO item, since SYCL standard doesn’t define such API yet. Example how such API may look like:

      // suppose, mylib.spv defines SYCL_EXTERNAL function foo, then this call:
      device_image img = device_dlopen("mylib.spv");
      // will make foo available for dynamic symbol resolution. If any subsequent
      // JIT compilations try to compile device code with external reference to
      // foo, it can now be resolved following the resolution mechanism described
      // in this doc, and JIT compilation will succeed.
      
  • Allow different format for device code - e.g. it can be SPIR-V or native device binary

  • Provide automatic runtime resolution of SYCL_EXTERNAL function references within the SYCL app to their definitions (if found) within any suitable dynamically linked device binary image

  • Support pointers to SYCL_EXTERNAL functions across the dynamic linkage boundaries within the device code - taking a pointer, call through a pointer.

  • Specific code changes are not required, i.e. the mechanism of linking works as close as possible to host shared libraries.

Design

The overall idea:

  • Each device image is supplied with a list of imported and exported symbol names through device image properties mechanism

  • In order to create a program executable from device image DPC++ RT will check if this device image has a list of imported symbols and if it has, then RT will search for device images which define required symbols using lists of exported symbols.

    • Besides symbol names, additional attributes are taken into account (like device image format: SPIR-V or native device binary)

    • No logical binding between host module and export/import lists, i.e. resolution is performed w/o regard to containing host modules

  • All found device images are used to create program objects and then these programs are linked together.

  • Actual linking is performed by underlying backend (OpenCL/L0/etc.)

    • Underlying backend is the backend used by DPC++ RT to create program from device binary image, perform JIT compilation (if required for chosen device image format) and linking with other programs in order to resolve symbols.

Next sections describe details of changes in each component.

DPC++ front-end changes

Now during device code split process SYCL_EXTERNAL functions are considered as entry points (as well as kernels). For this purpose DPC++ front-end generates module-id attribute on each SYCL_EXTERNAL function.

sycl-post-link changes

In order to support dynamic linking of device code, sycl-post-link performs 2 main tasks:

  • Supplies device images containing exports with an information about exported symbols

  • Supplies device images with an information about imported symbols

In addition, SYCL_EXTERNAL functions as well as kernels are considered as entry points during device code split. If device code split is enabled SYCL_EXTERNAL functions defined in shared libraries and used within it can be duplicated. Example:

// Shared library

// A.cpp
SYCL_EXTERNAL int LibDeviceFunc(int i) {
  return i * 2;
}

// B.cpp
/* ... */
Q.submit([&](sycl::handler &CGH) {
CGH.parallel_for(/* ... */ [=](sycl::item i) {
  out[i] = LibDeviceFunc(i.get_id(0));
} /* ... */

If user requested per-source device code split, then for this shared library sycl-post-link will create two device images and both of them will define LibDeviceFunc function. However LibDeviceFunc won’t be exported from device image that corresponds to source file B.cpp and it will be exported only from device image that corresponds to source file where LibDeviceFunc was defined, i.e. A.cpp.

Such duplication is needed for two reasons:

  • We aim to make device images with kernels self-contained so no JIT linker invocations would be needed if we have definitions of all called functions.

  • We could export SYCL_EXTERNAL functions from device images with kernels, but it would mean that when user’s app calls SYCL_EXTERNAL function, it has to link a whole kernel and all its dependencies - so we leave a possibility for user to arrange code on per-source basis.

Non-SYCL_EXTERNAL functions used by SYCL_EXTERNAL functions are copied to device images corresponding to those SYCL_EXTERNAL functions to make them self-contained - in the same way as it is done when splitting kernels across device images. In case one SYCL_EXTERNAL function uses another SYCL_EXTERNAL function with different value in sycl-module-id attribute, the second one is not copied to the device image with the first function, but dependency between those device images is recorded instead.

After device code split, all non-SYCL_EXTERNAL functions and copied SYCL_EXTERNAL functions left in device images with kernels marked with internal linkage to avoid multiple definition errors during runtime linking. After that sycl-post-link records list of names of exported functions, i.e. functions with sycl-module-id attribute and external linkage.

In order to collect information about imported symbols sycl-post-link looks through LLVM IR and for each declared but not defined symbol records its name, except the following cases:

  • Declarations with __ prefix in demangled name are not recorded as imported functions

    • Declarations with __spirv_* prefix should not be recorded as dependencies since they represent SPIR-V operations and will be transformed to SPIR-V instructions during LLVM->SPIR-V translation.

  • Based on some attributes (which could be defined later) we may want to avoid listing some functions as imported ones

    • This is needed to have possibility to call device-specific builtins not starting with __ by forward-declaring them in DPC++ code

NOTE: If device code split is enabled, imports collection is performed after split and it is performed on separated images.

All collected information is attached to a device image via properties mechanism.

Each device image is supplied with an array of property sets:

struct pi_device_binary_struct {
...
  // Array of property sets
  pi_device_binary_property_set PropertySetsBegin;
  pi_device_binary_property_set PropertySetsEnd;
};

Each property set is represented by the following struct:

// Named array of properties.
struct _pi_device_binary_property_set_struct {
  char *Name;                                // the name
  pi_device_binary_property PropertiesBegin; // array start
  pi_device_binary_property PropertiesEnd;   // array end
};

It contains name of property set and array of properties. Each property is represented by the following struct:

struct _pi_device_binary_property_struct {
  char *Name;       // null-terminated property name
  void *ValAddr;    // address of property value
  uint32_t Type;    // _pi_property_type
  uint64_t ValSize; // size of property value in bytes
};

List of imported symbols is represented as a single property set with name SYCL/imported symbols recorded in the Name field of property set. Each property in this set holds name of the particular imported symbol recorded in the Name field of the property. List of exported symbols is represented in the same way, except the corresponding set has the name SYCL/exported symbols.

DPC++ runtime changes

DPC++ RT performs device images collection task by grouping all device images required to execute a kernel based on the list of exports/imports, creates programs using collected images and links them together using PI API. Resulting program is then added to the cache to avoid repetition of symbol resolution, compilation, and linking processes for any future attempts to invoke kernels defined by this program.

DPC++ runtime plugin interface (PI) changes

During device images collection process RT considers modules as available for linking using information about ability of chosen device backend to compile and link programs created from particular device image format. The information about ability to compile and link particular format of device code is provided by PI plugin implementation for concrete backend. For this purpose piDeviceGetInfo API is used. For each device image format supported by DPC++ RT PI device extension is defined. Each extension is a string that can be returned by piDeviceGetInfo call with query PI_DEVICE_INFO_EXTENSIONS. Mapping of extension strings and formats that can be linked: | Device image format | Extension string | Meaning | |———————|——————|———| | __SYCL_PI_DEVICE_BINARY_TARGET_SPIRV64 | “pi_ext_spirv64_linking” | Linking of SPIR-V 64-bit programs is supported| | __SYCL_PI_DEVICE_BINARY_TARGET_SPIRV64_X86_64 | “pi_ext_spirv64_x86_64_linking” | Linking of 64-bit programs that were AOT compiled for CPU device is supported| | __SYCL_PI_DEVICE_BINARY_TARGET_SPIRV64_GEN | “pi_ext_spirv64_gen_linking” | Linking of 64-bit programs that were AOT compiled for GPU device is supported| | __SYCL_PI_DEVICE_BINARY_TARGET_SPIRV64_FPGA | “pi_ext_spirv64_fpga_linking” | Linking of 64-bit programs that were AOT compiled for FPGA device is supported|

To link several device images together piProgramLink API will be used. Depending on concrete plugin implementation and set of device image formats that can be linked at run-time, piProgramLink API may receive programs made from device images in different formats as inputs (including SPIR-V and native code).

Support of runtime linking in backends
  • The initial implementation will support dynamic linking of device code in SPIR-V format on OpenCL backend:

    • OpenCL plugin will use the existing OpenCL clLinkProgram() API to online link the SPIR-V modules together.

    • A new Level Zero API to online link programs on SPIR-V level is required for better performance.

    • While there is no Level Zero API to link programs on SPIR-V level, existing zeModuleDynamicLink() can be used as fallback.

  • In order to support dynamic linking of AOT compiled device code the following should be implemented on backends site:

    • AOT compilers must allow to compile SPIR-V modules with unresolved symbols and produce device code in format that can be linked in run time and allows to reduce JIT overhead

    • OpenCL program binary type CL_PROGRAM_BINARY_TYPE_[COMPILED_OBJECT/LIBRARY] should have native code format or any other format that can be emitted by AOT compiler and allows to reduce JIT overhead

Device images collection and linking

Device images collection and linking of programs is performed by DPC++ Runtime class named ProgramManager.

When the program manager gets a request to create a program object using device image, it examines its list of imported symbols and finds device images which export those symbols, then program manager creates programs for each required device image and links them all together.

All needed device images are found by iterating through all available OS modules without predefined order and searching for first unresolved symbol in list of imports of target device image. During search device image format is taken into account, i.e. only device images that have the same format as target device image will be considered as suitable ones. Once suitable device image that contains first symbol is met, remaining exported symbols are checked in found image. If they match some imported symbols then these matched symbols will be marked as resolved. The procedure repeats until all imported symbols are marked as resolved. In case all available device images are viewed, but some imported symbols remain unresolved, exception will be thrown.

The following assumption is made: each device image represents some combination of defined symbols (kernels or SYCL_EXTERNAL functions) and different device images either contain exactly the same symbols or not overlapping list of defined symbols. If this assumption is not correct, there can be two cases:

  • Same symbols have the same definitions. In this case it doesn’t matter which device image is taken to use duplicated symbol

  • Same symbols have different definitions. In this case ODR violation takes place, such situation leads to undefined behaviour. For more details refer to ODR violations section.

    • The situation when two device images of different formats define the same symbols with two different definitions is not considered as ODR violation. In this case the suitable device image will be picked.

So, it is valid to pick the met first device image which defines required symbol during search.

Program caching

Existing support for device code caching can be re-used to cache dynamically linked programs with slight changes.

In-memory cache

The existing mechanism of caching can be re-used in presence of dynamic linking. Example of code when caching mechanism is successfully re-used for dynamically linked code:

// Application
SYCL_EXTERNAL void LibFunc();

Q.submit([&](sycl::handler &CGH) {
  CGH.parallel_for<InternalKernel>( ... )
}); // 1. Program is compiled, linked and saved in cache
    // 2. Prepared program is used to enqueue kernel

Q.submit([&](sycl::handler &CGH) {
  handler.parallel_for([] { LibFunc(); }); // Prepared program is used to enqueue kernel
});

// Library
SYCL_EXTERNAL void LibFunc() {
// ...
}

In current cache structure the programs map’s key consists of four components: kernel set id, specialization constants values, the device this program is built for, build options id. In this example Id of kernel set where application’s kernels can be used to access program cache. However when shared library defines kernels and these kernels are run by the application unchanged cache structure may lead to double compilation of the same code. Example of code that leads to double compilation of library code:

// Application
SYCL_EXTERNAL void LibFunc();

Q.submit([&](sycl::handler &CGH) {
  handler.parallel_for([] { LibFunc(); });  // Device code for library is
                                            // compiled and linked together with
                                            // device code for application, i.e.
                                            // LibFunc1 and ExternalKernel exist
                                            // in prepared state
});
// ...
EnqueueLibraryKernel(Q); // If cache mechanism is not changed, this line will
                         // lead to second compilation of ExternalKernel and
                         // LibFunc

// Library
SYCL_EXTERNAL void LibFunc() {
// ...
}

EnqueueLibraryKernel(queue) {
  queue.submit(parallel_for<ExternalKernel>(...));
}

Such case can be optimized by bringing nesting into cache keys structure. Kernel set id can be found for each kernel using its name and OS module it is coming from. In presence of dynamic linking resulting program can be combined out of device images which come from different OS modules. So, it should be possible to find needed program by kernel name and any OS module that was involved in this program. The new mapping structure is:

{kernel name} =>
  {OSModuleHandle, spec const, opts, dev} => program

I.e. each kernel name is mapped to a set of tuples that consists of OS module, spec constant values, JIT compiler options and device. Then concrete tuple is mapped to a program object. Several tuples can be mapped to the same program object. These tuples are created during process of compilation and symbols resolution for concrete device image. When some program is a result of linking several programs from device images with different OS modules, a tuple is created for each OS module ID. These tuples are used as nested cache entries after kernel name. Example of modified cache structure when dynamic linking is involved:

// Application
// OSModule = 1

SYCL_EXTERNAL void LibFunc();
queue.submit(parallel_for<InternalKernel>( ... ));

Q.submit([&](sycl::handler &CGH) {
  CGH.parallel_for([] { LibFunc(); });
});

EnqueueLibraryKernel(q);

// Library
// OSModule = 2

SYCL_EXTERNAL LibFunc();

EnqueueLibraryKernel(queue) {
  queue.submit(parallel_for<ExternalKernel>(...));
}
Program cache will have the following structure:
 "InternalKernel" =>
   {1, ...} => program 1
 "ExternalKernel" =>
   {1, ...} => program 1
   {2, ...} => program 1

However the library code will be compiled twice if kernel from the library was enqueued before kernels from the application, i.e. in such case:

// Application
SYCL_EXTERNAL void LibFunc();

EnqueueLibraryKernel(Q); // First, library code is compiled alone since it
                         // doesn't have any dependencies
// ...
Q.submit([&](sycl::handler &CGH) {
  handler.parallel_for([] { LibFunc(); });  // Second, library code is compiled
                                            // and linked together with code of
                                            // the application
});

The kernel caching mechanism is re-used without changes.

Persistent cache

The documented approach to persistent cache needs to be expanded in presence of dynamic linking support. One of the identifiers for built image hash is hash made out of device image used as input for the JIT compilation. In case when “main” image have imports information, device image hash should be created from all device images that are necessary to build it, i.e. hash out of “main” device image and set of images that define all symbols imported by “main” device image. The hash string is a result of appending device images. To make order of device images defined and persistent across runs of the same application, device images are sorted before they are used to create hash string. A string made out of names of defined symbols defined by a device image is used to compare device images during sorting process.

Corner cases and limitations

It is not guaranteed that behaviour of host shared libraries and device shared libraries will always match. There are several cases when behaviours don’t match, the next sections will cover details of such cases.

ODR violations

C++ standard defines One Definition Rule as:

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement; no diagnostic required. The definition can appear explicitly in the program, it can be found in the standard or a user-defined library, or (when appropriate) it is implicitly defined.

Here is an example:

ODR violation

Both libraries libB and libC provide two different definitions of function b(), so this example illustrates ODR violation. Technically this case has undefined behaviour, however it is possible to run and compile this example on Linux and Windows. Whereas on Linux only function b() from library libB is called, on Windows both versions of function b() are used. Most of backends online linkers act like static linkers, i.e. just merge device code from different programs, so it is not possible to correctly imitate Windows behaviour in device code linking because attempts to do it will result in multiple definition errors.

Given that, it is not guaranteed that behaviour of shared host libraries and shared device libraries will always match in case of such ODR violations.

LD_PRELOAD

Another way to violate ODR is LD_PRELOAD environment variable on Linux. It allows to load specified shared library before any other shared libraries so it will be searched for symbols before other shared libraries. It allows to substitute functions from regular shared libraries by functions from preloaded library. Device code registration is implemented using global constructors. Order of global constructors calling is not defined across different translation units, so with current design of device shared libraries and device code registration mechanism it is not possible to understand which device code comes from preloaded library and which comes from regular shared libraries.

Here is an example:

LD_PRELOAD

“libPreload” library is preloaded using LD_PRELOAD environment variable. In this example, device code from “libPreload” might be registered after device code from “libA”.

To implement basic support, for each device image we can record name of the library where this device image comes from and parse content of LD_PRELOAD environment variable to choose the proper images. However such implementation will only allow to substitute a whole device image and not an arbitrary function (unless it is the only function in a device image), because partial substitution will cause multiple definition errors during runtime linking.

Run-time libraries loading

It is possible to load shared library during run-time. Both Linux and Windows provide a way to do so (for example dlopen() on Linux or LoadLibrary on Windows). In case run-time loading is used to load some shared library, the symbols from this shared library do not appear in the namespace of the main program. It means that even though shared library is loaded successfully in run-time, it is not possible to use symbols from it directly. The symbols from run-time loaded library can be accessed by address which can be obtained using corresponding OS-dependent API (for example dlsym() on Linux).

The problem here is that even though symbols from run-time loaded shared library are not part of application’s namespace, the library is loaded through standard mechanism, i.e. its global constructors are invoked which means that device code from it is registered, so it is not possible to understand whether device code comes from run-time loaded library or not. If such run-time loaded library exports device symbols and they somehow match with symbols that actually directly used in device code somewhere, it is possible that symbols from run-time loaded library will be unexpectedly used.

To resolve this problem we need to ensure that device code registered from run-time loaded library appears at the end of symbols search list, however having that device code registration is triggered by global constructors, it doesn’t seem possible.

One more possible mitigation would be to record name of the library from which each symbol should be imported, but it still won’t resolve all potential issues with run-time library loading, because user can load the library with the same name as one of the explicitly linked libraries.