OpenHMPP
OpenHMPP (HMPP for Hybrid Multicore Parallel Programming) - programming standard for heterogeneous computing. Based on a set of compiler directives, standard is a programming model designed to handle hardware accelerators without the complexity associated with GPU programming. This approach based on directives has been implemented because they enable a loose relationship between an application code and the use of a hardware accelerator (HWA). Introduction. The OpenHMPP directive-based programming model offers a syntax to offload computations on hardware accelerators and to optimize data movement to/from the hardware memory. The model is based on works initialized by CAPS (Compiler and  Architecture for Embedded and Superscalar Processors), a common project from INRIA, CNRS, the University of Rennes 1 and the INSA of Rennes. OpenHMPP concept. OpenHMPP is based on the concept of codelets, functions that can be remotely executed on HWAs. The OpenHMPP codelet concept. A codelet has the following properties: These properties ensure that a codelet RPC can be remotely executed by a HWA. This RPC and its associated data transfers can be asynchronous. Codelet RPCs. HMPP provides synchronous and asynchronous RPC. Implementation of asynchronous operation is hardware dependent. HMPP Memory Model. HMPP considers two address spaces: the host processor  one and the HWA memory. Directives concept. The OpenHMPP directives may be seen as “meta-information” added in the application source code. They are safe meta-information i.e. they do not change the original code behavior. They address the remote execution (RPC) of a function as well as the transfers of data to/from the HWA memory. The table below introduces the OpenHMPP directives. OpenHMPP directives address different needs: some of them are dedicated to declarations and others are dedicated to the management of the execution. Concept of set of directives. One of the fundamental points of the HMPP approach  is the concept of directives and their associated labels which makes it possible to expose a coherent structure on a whole set of directives disseminated in an application. There are two kinds of labels: OpenHMPP Directives Syntax. In order to simplify the notations, regular expressions will be used to describe the syntax of the HMPP directives. The color convention below is used for the description of syntax directives: General syntax. The general syntax of OpenHMPP directives is: #pragma hmpp <grp_label> [codelet_label]? directive_type [,directive_parameters]* [&] !$hmpp <grp_label> [codelet_label]? directive_type [,directive_parameters]* [&] Where: Directive parameters. The parameters associated to  a directive may be of different types. Below are the directive parameters defined in OpenHMPP: OpenHMPP directives. Directives for declaring and executing a codelet. A codice_17 directive declares a computation to be remotely executed on a hardware accelerator. For the codice_17 directive: The syntax of the directive is: More than one codelet directive can be added to a function in order to specify different uses or different execution contexts. However, there can be only one codelet directive for a given call site label. The codice_19 directive specifies how the use a codelet at a given point in the program. The  syntax of the directive is: An example is shown here : In some cases, a specific management of the data throughout the application is required (CPU/GPU data movements optimization, shared variables...). The codice_20 directive allows the declaration of a group of codelets. The parameters defined in this directive are applied to all codelets belonging to the group. The syntax of the directive is: Data transfers directives to optimize communication overhead. When using a HWA, the main bottleneck is often the data transfers between the HWA and the main processor. To limit the communication overhead, data transfers can be overlapped with  successive executions of the same codelet by using the asynchronous property of the HWA. The codice_21 directive locks the HWA and allocates the needed amount of memory. The codice_22 directive specifies when to release the HWA for a group or a stand-alone codelet. The codice_23 directive prefetches data before the remote execution of the codelet. The codice_24 directive is a synchronization barrier to wait for an asynchronous codelet execution to complete and to then download the results. The codice_25 directive specifies to wait until the completion of an asynchronous callsite execution. For the synchronize directive, the codelet label is  always mandatory and the group label is required if the codelet belongs to a group. In the following example, the device initialization, memory allocation and upload of the input data are done only once outside the loop and not in each iteration of the loop. The codice_25 directive allows to wait for the asynchronous execution of the codelet to complete before launching another iteration. Finally the codice_24 directive outside the loop uploads the sgemm result. Sharing data between codelets. Those directives map together all the arguments sharing the given name for all the group. The types and dimensions of all  mapped arguments must be identical. The codice_28 directive maps several arguments on the device. This directive is quite similar as the codice_28 directive except that the arguments to be mapped are directly specified by their name. The codice_30 directive is equivalent to multiple codice_28 directives. Global variable. The codice_32 directive declares some variables as global within a group. Those variables can then be directly accessed from any codelet belonging to the group. This directive applies to the declaration statement just following it in the source code. The syntax of this directive is: The notation codice_33 with the prefix codice_34, indicates  an application's variable declared as resident. Acceleration of regions. A region is a merge of the codelet/callsite directives. The goal is to avoid code restructuration to build the codelet. Therefore, all the attributes available for codice_17 or codice_19 directives can be used on codice_37 directives. In C language: Implementations. The OpenHMPP Open Standard is based on HMPP Version 2.3 (May 2009, CAPS entreprise). The OpenHMPP directive-based programming model is implemented in: OpenHMPP is used by HPC actors in Oil & Gas, Energy, Manufacturing, Finance, Education & Research.