Implementation design for “device_global”

This document describes the implementation design for the DPC++ extension sycl_ext_oneapi_device_global, which allows applications to declare global variables in device code.

Requirements

The extension specification document referenced above contains the full set of requirements for this feature, but some requirements that are particularly relevant to the design are called out here.

The first issue relates to the mechanism for integrating host and device code. Like specialization constants, device global variables are referenced in both host and device code, so they require some mechanism to correlate the variable instance in device code with the variable instance in host code. The API for referencing a device global variable, however, is different from the API for specialization constants. Whereas specialization constants are referenced through a templated member function:

sycl::specialization_id<int> spec_var;

void func(sycl::queue q) {
  q.submit([&](sycl::handler &cgh) {
    cgh.set_specialization_constant<spec_var>(42);
    cgh.single_task([=](sycl::kernel_handler kh) {
      int i = kh.get_specialization_constant<spec_var>();
    });
  });
}

Device global variables, by contrast, are referenced by their address:

sycl::ext::oneapi::experimental::device_global<int> dev_var;

void func(sycl::queue q) {
  int val = 42;
  q.copy(&val, dev_var).wait();      // The 'dev_var' parameter is by reference
  q.submit([&](sycl::handler &cgh) {
    cgh.single_task([=] {
      int i = dev_var;
    });
  });
}

This is a key difference because the compiler does not statically know which device global variable is being referenced; we only know the address at runtime. As we will see later, this has a ramification on the integration headers and on the mechanism that connects instances of device global variables in host code with their corresponding instances in device code.

Another issue relates to the device_image_scope property which can be applied to a device global variable declaration. The intent of this property is to allow a device global variable to be implemented directly on top of a SPIR-V module scope global variable. When this property is not present, an instance of a device global variable is shared across all device images that are loaded onto a particular device. By contrast, when this property is present, each device image has its own instance of the device global variable. However, since multiple variable instances have confusing semantics, the API requires the user to ensure that each such variable exists in exactly one device image. The extension specification has more details on this property.

The important impact on the design, though, is that device global variables declared with the device_image_scope property have an implementation that is quite different from device global variables that are not declared with this property. The sections below describe both implementations.

Design

Changes to DPC++ headers

Class specializations based on device_image_scope

The headers, of course, include the declaration of the new device_global class, which is described in the extension specification. The declaration of this class uses partial specialization to define the class differently depending on whether it has the device_image_scope property. When the property is not present, the class has a member variable which is a pointer to the underlying type. Member functions which return a reference to the value (e.g. get) return the value of this pointer:

template<typename T, typename PropertyListT>
class device_global {
  T *usmptr;
 public:
  T& get() noexcept { return *usmptr; }
  /* other member functions */
};

However, when the property is present, it has a member variable which is the type itself, and member functions return a reference to this value.

template<typename T, typename PropertyListT>
class device_global {
  T val{};
 public:
  T& get() noexcept { return val; }
  /* other member functions */
};

Note that the val member has a default initializer that causes it to be “value initialized”. Since the type T is limited to types that are trivially constructible, this means that val will be zero initialized.

In both cases the member variable (either usmptr or val) must be the first member variable in the class. As we will see later, the runtime assumes that the address of the device_global variable itself is the same as the address of this member variable.

Attributes attached to the class

The device_global class declaration contains three C++ attributes which convey information to the front-end. These attributes are only needed for the device compiler, and the #ifdef __SYCL_DEVICE_ONLY__ allows the customer to use another host compiler, even if it does not recognize these attributes. Also note that these attributes are all in the __sycl_detail__ namespace, so they are considered implementation details of DPC++. We do not intend to support them as general attributes that customer code can use.

template <typename T, typename PropertyListT = property_list<>>
class device_global {/*...*/};

// Partial specialization to make PropertyListT visible as a parameter pack
// of properties.
template <typename T, typename ...Props>
class
#ifdef __SYCL_DEVICE_ONLY__
  [[__sycl_detail__::add_ir_global_variable_attributes(
    "sycl-device-global-size",
    Props::meta_name...,
    sizeof(T),
    Props::meta_value...
    )]]
  [[__sycl_detail__::global_variable_allowed]]
  [[__sycl_detail__::device_global]]
#endif
  device_global<T, property_list<Props...>> {/*...*/};

The [[__sycl_detail__::add_ir_global_variable_attributes()]] attribute is used to convey information about the compile-time properties to the front-end, and it is described more fully by the compile-time properties design document. This attribute is also used for other classes that have properties, so it is not specific to the device_global class.

Note that the parameter list to [[__sycl_detail__::add_ir_global_variable_attributes()]] contains one additional property named "sycl-device-global-size". The sycl-post-link tool uses this property to distinguish device global variables from other module scope variables, and the property tells the size of the underlying data type of the device global variable.

The [[__sycl_detail__::global_variable_allowed]] attribute informs the front-end that global variables of this type are allowed to be referenced in device code. By default, the front-end diagnoses an error if device code references a global variable unless the variable is constexpr or const and constant initialized. However, the presence of this attribute informs the front-end that variables of this type are an exception to this rule, so the front-end does not diagnose an error when device code references a device_global variable. This attribute could also be used by other types, so it is also not specific to the device_global class.

NOTE: The implementation of device-side asserts recently introduced a new C++ attribute sycl_global_var for a similar purpose. The design for device global variables cannot use that attribute because sycl_global_var is intended to be specified on the variable definition (not the type declaration), and we do not want to force users to add an attribute to each definition of a device_global variable. However, the implementation of device-side asserts could be changed to use [[__sycl_detail__::global_variable_allowed]]. We could then remove the support for sycl_global_var.

The last attribute [[__sycl_detail__::device_global]] controls error reporting for variables declared of this type. The device global extension specification places restrictions on where a device_global variable can be declared. Rather than have the front-end recognize the name of the device_global type, the front-end uses this attribute to know which restrictions to enforce for this type.

Declaration of member functions to copy device global variables

The headers are also updated to add the new copy() and memcpy() member functions to handler and queue which copy data to or from a device global variable. These declarations use SFINAE such that they are conditionally available depending on the host_access property.

Changes to the DPC++ front-end

There are several changes to the device compiler front-end:

  • The front-end adds a new LLVM IR attribute sycl-unique-id to the definition of each device_global variable, which provides a unique string identifier for each device global variable. The rules for creating this string are the same as __builtin_sycl_unique_stable_id, so the front-end can use the same algorithm when generating the string.

  • The front-end checks for restrictions on variable declarations using the device_global type. As described above, the front-end uses the [[__sycl_detail__::device_global]] attribute (rather than the class name) to know which set of restrictions to check. The restrictions specific to device global variables are documented in the extension specification.

  • The front-end avoids diagnosing an error when variables of type device_global are referenced in device code because the type is decorated with the [[__sycl_detail__::global_variable_allowed]] attribute.

  • The front-end generates new content in both the integration header and the integration footer, which is described in more detail below.

Handling shadowed device global variables

The example above shows a simple case where the user’s device global variables can all be uniquely referenced via fully qualified lookup (e.g. ::inner::Bar). However, it is possible for users to construct applications where this is not the case, for example:

sycl::device_global<int> FuBar;
namespace {
  sycl::device_global<int> FuBar;
}

In this example, the FuBar variable in the global namespace shadows a variable with the same name in the unnamed namespace. The integration footer can reference the variable in the global namespace as ::FuBar, but there is no way to reference the variable in the unnamed namespace using fully qualified lookup.

Such programs are still legal, though. The integration footer can support cases like this by defining a shim function that returns a reference to the shadowed device global:

namespace {
namespace __sycl_detail {

static constexpr decltype(FuBar) &__shim_1() {
  return FuBar;   // References 'FuBar' in the unnamed namespace
}

} // namespace __sycl_detail
} // namespace (unnamed)

namespace sycl::detail {

__sycl_device_global_registration::__sycl_device_global_registration() noexcept {
  device_global_map::add(&::FuBar,
    /* same string returned from __builtin_sycl_unique_stable_id(::FuBar) */);
  device_global_map::add(&::__sycl_detail::__shim_1(),
    /* same string returned from __builtin_sycl_unique_stable_id(::(unnamed)::FuBar) */);
}

} // namespace sycl::detail

The __shim_1() function is defined in the same namespace as the second FuBar device global, so it can reference the variable through unqualified name lookup. Furthermore, the name of the shim function is globally unique, so it is guaranteed not to be shadowed by any other name in the translation unit. This problem with variable shadowing is also a problem for the integration footer we use for specialization constants. See the specialization constant design document for more details on this topic.

Changes to the DPC++ driver

A new command line argument, --device-globals must be passed to the sycl-post-link tool to enable processing device global variables.

Changes to the sycl-post-link tool

The sycl-post-link tool performs its normal algorithm to identify the set of kernels and device functions that are bundled together into each module. Once it identifies the functions in each module, it scans those functions looking for references to global variables that are decorated with the LLVM IR attribute "sycl-device-global-size" (these are the variables of type device_global). If any device global variable decorated with the LLVM IR attribute corresponding to the device_image_scope property appears in more than one module, the sycl-post-link tool issues an error diagnostic:

error: device_global variable <name> with property "device_image_scope"
       is used in more than one device image.

Assuming that no error diagnostic is issued, the sycl-post-link tool includes the IR definition of each device_global variable in the modules that reference that variable.

As described in the design for compile-time properties, the sycl-post-link tool is responsible for generating idiomatic LLVM IR for any compile-time properties that need to be generated in SPIR-V.

The HostAccessINTEL decoration is required for all device global variables because it provides the name that the DPC++ runtime uses to access the variable. Therefore, the sycl-post-link tool always generates idiomatic LLVM IR for this decoration. The first SPIR-V operand is set according to the host_access property (or set to Read/Write if the device global doesn’t have that property). The second SPIR-V operation is set to the value of the device global’s sycl-unique-id.

The sycl-post-link tool also generates idiomatic LLVM IR for the InitModeINTEL decoration (if the device global has the init_mode property) and for the ImplementInCSRINTEL decoration (if the device global has the implement_in_csr property). See the SPV_INTEL_global_variable_decorations SPIR-V extension for details about all of these decorations.

The sycl-post-link tool also create a “SYCL/device globals” property set for each device code module that contains at least one device global variable.

New “SYCL/device globals” property set

Each device code module that references one or more device global variables has an associated “SYCL/device globals” property set. The name of each property in this set is the sycl-unique-id string of a device_global variable that is contained by the module. The value of each property has property type PI_PROPERTY_TYPE_BYTE_ARRAY and contains a structure with the following fields:

struct {
  uint32_t size;
  uint8_t device_image_scope;
};

The size field contains the size (in bytes) of the underlying type T of the device global variable. The sycl-post-link tool gets this value from the LLVM IR attribute "sycl-device-global-size".

The device_image_scope field is either 1 (true) or 0 (false), telling whether the device global variable was declared with the device_image_scope property.

Changes to the DPC++ runtime

Several changes are needed to the DPC++ runtime

  • As noted in the requirements section, an instance of a device global variable that does not have the device_image_scope property is shared by all device images on a device. To satisfy this requirement, the device global variable contains a pointer to a buffer allocated from USM device memory, and the content of the variable is stored in this buffer. All device images on a particular device point to the same buffer, so the variable’s state is shared. The runtime, therefore, must allocate this USM buffer for each such device global variable.

  • As we noted above, the front-end generates new content in the integration footer which calls the function sycl::detail::device_global_map::add(). The runtime defines this function and maintains information about all the device global variables in the application. This information includes:

    • The host address of the variable.

    • The string which uniquely identifies the variable.

    • The size (in bytes) of the underlying T type for the variable.

    • A boolean telling whether the variable is decorated with the device_image_scope property.

    • The associated per-device USM buffer pointer, if this variable does not have the device_image_scope property.

    We refer to this information as the “device global database” below.

  • The runtime also implements the new copy and memcpy functions in the queue and handler classes which copy to or from device_global variables.

Initializing the device global variables in device code

When a DPC++ application submits a kernel, the runtime constructs a pi_program containing this kernel that is compiled for the target device, if such a pi_program does not yet exist. If the kernel resides in a device code module that calls into a shared library, the runtime identifies a set of device code modules that need to be online-linked together in order to construct the pi_program.

After creating a pi_program and before invoking any kernel it contains, the runtime does the following:

  • Scan the entries of the “SYCL/device globals” property sets of each device code module that contributes to the the pi_program to get information about each device global variable that is used by the pi_program. This information is added to device global database.

  • For each device global variable that is not decorated with the device_image_scope property:

    • Check if a USM buffer has already been created for the variable on this target device. If not, the runtime allocates the buffer from USM device memory using the size from the database and zero-initializes the content of the buffer. The pointer to this buffer is saved in the database for future reuse.

    • Regardless of whether the USM buffer has already been created for the variable, the runtime initializes the usmptr member in the device instance of the variable by using a new PI interface which copies data from the host to a global variable in a pi_program. It is a simple matter to use this interface to overwrite the usmptr member with the address of the USM buffer.

Note that the runtime does not need to initialize the val member variable of device global variables that are decorated with device_image_scope because the val default initializer already guarantees that this member variable is zero initialized.

Implementing the copy and memcpy functions in queue and handler

Each of these functions accepts a (host) pointer to a device global variable as one of its parameters, and the runtime uses this pointer to find the associated information for this variable in the device global database. The code in the integration footer ensures that the database will at least contain the address of the variable and its unique string, even if no kernel referencing this variable has been submitted yet.

Each of these functions is templated on the variable’s underlying type T, so it knows the size of this type. Each function is also templated on the variable’s property list, so it knows whether the variable has the device_image_scope property.

The remaining behavior depends on whether the variable is decorated with the device_image_scope property.

If the variable is not decorated with this property, the runtime uses the database to determine if a USM buffer has been allocated yet for this variable on this device. If not, the runtime allocates the buffer using sizeof(T) and zero-initializes the buffer. Regardless, the runtime implements the copy / memcpy function by copying to or from this USM buffer, using the normal mechanism for copying to / from a USM pointer.

The runtime avoids the future cost of looking up the variable in the database by caching the USM pointer in the host instance of the variable’s usmptr member.

If the variable is decorated with the device_image_scope property, the variable’s value exists directly in the device code module, not in a USM buffer. The runtime first uses the variable’s unique string identifier to see if there is a pi_program that contains the variable. If there is more than one such pi_program, the runtime diagnoses an error by throwing errc::invalid. If there is no such pi_program, the runtime scans all “SYCL/device globals” property sets to find the device code module that contains this variable and uses its normal mechanism for creating a pi_program from this device code module. (The algorithm for creating device code modules in the sycl-post-link tool ensures that there will be no more than one module that contains the variable.) Finally, the runtime uses the new PI interface to copy to or from the contents of the variable in this pi_program.

It is possible that a device global variable with device_image_scope is not referenced by any kernel, in which case the variable’s unique string will not exist in any property set. In this case, the runtime simply uses the host instance of the device_global variable to hold the value and copies to or from the val member.

In all cases, the runtime uses sizeof(T) to determine if the copy operation will read or write beyond the end of the device global variable’s storage. If so, the runtime diagnoses an error by throwing errc::invalid.

New PI interface to copy to or from a module scope variable

As noted above, we need new PI interfaces that can copy data to or from an instance of a device global variable in a pi_program. This functionality is exposed as two new PI interfaces:

pi_result piextEnqueueDeviceVariableRead(pi_queue Queue, pi_program Program,
                                         const char *Name, pi_bool BlockingRead,
                                         size_t Count, size_t Offset, void *Dst,
                                         pi_uint32 NumEventsInWaitList,
                                         const pi_event *EventsWaitList,
                                         pi_event *Event);

pi_result piextEnqueueDeviceVariableWrite(pi_queue Queue, pi_program Program,
                                          const char *Name,
                                          pi_bool BlockingWrite, size_t Count,
                                          size_t Offset, const void *Src,
                                          pi_uint32 NumEventsInWaitList,
                                          const pi_event *EventsWaitList,
                                          pi_event *Event);

The piextEnqueueDeviceVariableRead function reads Count bytes at byte-offset Offset from a device global variable in Program identified by the name Name. The read data is stored in Dst. Likewise, the piextEnqueueDeviceVariableWrite function reads Count bytes from Dst and stores them at byte-offset Offset in the device global variable in Program identified by the name Name.

Both functions will enqueue the associated memory command on Queue where it will first wait for NumEventsInWaitList events in EventsWaitList to finish. Event will be populated with the event associated with resulting enqueued command. If either BlockingRead or BlockingWrite is true the call will block on the host until the enqueued command finishes execution.

For device_global variables the Name parameter in calls to these functions is the same as the associated sycl-unique-id string.

The Level Zero backend has existing APIs that can implement these PI interfaces. The plugin first calls zeModuleGetGlobalPointer() to get a device pointer for the variable and then calls zeCommandListAppendMemoryCopy() to copy to or from that pointer. However, the documentation (and implementation) of zeModuleGetGlobalPointer() needs to be extended slightly. The description currently says:

  • The application may query global pointer from any module that either exports or imports it.

  • The application must dynamically link a module that imports a global before the global pointer can be queried from it.

This must be changed to say something along these lines:

  • The interpretation of pGlobalName depends on how the module was created. If the module was created from SPIR-V that declares the GlobalVariableDecorationsINTEL capability, the implementation looks first for an OpVariable that is decorated with HostAccessINTEL where the Name operand is the same as pGlobalName. If no such variable is found, the implementation then looks for an OpVariable that is decorated with LinkageAttributes where the Name operand is the same as pGlobalName. (The implementation considers both exported and imported variables as candidates.)

    If the module was created from native code that came from a previous call to zeModuleGetNativeBinary and that other module was created from SPIR-V, then the interpretation of pGlobalName is the same as the SPIR-V case.

  • If pGlobalName identifies an imported SPIR-V variable, the module must be dynamically linked before the variable’s pointer may be queried.

The OpenCL backend has a proposed extension cl_intel_global_variable_access that defines functions clEnqueueReadGlobalVariableINTEL() and clEnqueueWriteGlobalVariableINTEL() which can be easily used to implement these PI interfaces. This DPC++ design depends upon implementation of that OpenCL extension.

The CUDA backend has existing APIs cuModuleGetGlobal() and cuMemcpyAsync() which can be used to implement these PI interfaces.

Design choices

This section captures some of the discussions about aspects of the design.

Should the value be zero-initialized

There was some debate about whether the value in the device_global should always be zero-initialized. We decided to require this in order to be consistent with C++ rules for global variables. We want device_global to model the normal rules for global variables. Since C++ guarantees that a global variable with a trivial constructor is zero-initialized, we want that behavior too.

The downside is that some applications may allocate a very large storage for the underlying type T of a device global variable, and they may not want to pay the cost of zero initializing it. We agree that this is a theoretical problem, but we aren’t sure if this will be an issue for real applications. If it turns out to be a real problem, we propose adding a new property that prevents initialization of the device global value. For example, we could add a new parameter to the init_mode property called none.

Why not include both val and usmptr member variables

Rather than using partial specialization to define device_global differently based on the device_image_scope property, we could instead define both member variables regardless of the properties. This would make the header file implementation easier, but it would lead to wasted space in the case when the device_image_scope property was not specified since the val member is unused in this case. Wasting space on the host may not be such a big problem, but the space would also be wasted on every device that reference the device global variable, and this seems like a bigger problem. We decided that the extra header file complexity of partial specialization is worth avoiding this wasted memory.