OpenMP Extensions for OpenACC

OpenACC provides some functionality that OpenMP does not. In some cases, Clang supports OpenMP extensions to provide similar functionality, taking advantage of the runtime implementation already required for OpenACC. This section documents those extensions.

By default, Clang recognizes these extensions. The command-line option -fno-openmp-extensions can be specified to disable all OpenMP extensions, including those described in this section.

Motivation

There are multiple benefits to exposing OpenACC functionality as LLVM OpenMP extensions:

  • OpenMP applications can take advantage of the additional functionality.
  • As LLVM’s implementation of these extensions matures, it can serve as a basis for including these extensions in the OpenMP standard.
  • Source-to-source translation from certain OpenACC features to OpenMP is otherwise impossible.
  • Runtime tests can be written in terms of OpenMP instead of OpenACC or low-level runtime calls.
  • More generally, there is a clean separation of concerns between OpenACC and OpenMP development in LLVM. That is, LLVM’s OpenMP developers can discuss, modify, and debug LLVM’s extended OpenMP implementation and test suite without directly considering OpenACC’s language and execution model, which are handled by LLVM’s OpenACC developers.

ompx_hold Map Type Modifier

Example

#pragma omp target data map(ompx_hold, tofrom: x) // holds onto mapping of x throughout region
{
  foo(); // might have map(delete: x)
  #pragma omp target map(present, alloc: x) // x is guaranteed to be present
  printf("%d\n", x);
}

The ompx_hold map type modifier above specifies that the target data directive holds onto the mapping for x throughout the associated region regardless of any target exit data directives executed during the call to foo. Thus, the presence assertion for x at the enclosed target construct cannot fail.

Behavior

  • Stated more generally, the ompx_hold map type modifier specifies that the associated data is not unmapped until the end of the construct. As usual, the standard OpenMP reference count for the data must also reach zero before the data is unmapped.
  • If ompx_hold is specified for the same data on lexically or dynamically enclosed constructs, there is no additional effect as the data mapping is already held throughout their regions.
  • The ompx_hold map type modifier is permitted to appear only on target constructs (and associated combined constructs) and target data constructs. It is not permitted to appear on target enter data or target exit data directives because there is no associated statement, so it is not meaningful to hold onto a mapping until the end of the directive.
  • The runtime reports an error if omp_target_disassociate_ptr is called for a mapping for which the ompx_hold map type modifier is in effect.
  • Like the present map type modifier, the ompx_hold map type modifier applies to an entire struct if it’s specified for any member of that struct even if other map clauses on the same directive specify other members without the ompx_hold map type modifier.
  • ompx_hold support is not yet provided for defaultmap.

Implementation

  • LLVM uses the term dynamic reference count for the standard OpenMP reference count for host/device data mappings.
  • The ompx_hold map type modifier selects an alternate reference count, called the hold reference count.
  • A mapping is removed only once both its reference counts reach zero.
  • Because ompx_hold can appear only constructs, increments and decrements of the hold reference count are guaranteed to be balanced, so it is impossible to decrement it below zero.
  • The dynamic reference count is used wherever ompx_hold is not specified (and possibly cannot be specified). Decrementing the dynamic reference count has no effect if it is already zero.
  • The runtime determines that the ompx_hold map type modifier is in effect (see Behavior above) when the hold reference count is greater than zero.

Relationship with OpenACC

OpenACC specifies two reference counts for tracking host/device data mappings. Which reference count is used to implement an OpenACC directive is determined by the nature of that directive, either dynamic or structured:

  • The dynamic reference count is always used for enter data and exit data directives and corresponding OpenACC routines.
  • The structured reference count is always used for data and compute constructs, which are similar to OpenMP’s target data and target constructs.

Contrast with OpenMP, where the dynamic reference count is always used unless the application developer specifies an alternate behavior via our map type modifier extension. We chose the name hold for that map type modifier because, as demonstrated in the above example, hold concisely identifies the desired behavior from the application developer’s perspective without referencing the implementation of that behavior.

The hold reference count is otherwise modeled after OpenACC’s structured reference count. For example, calling acc_unmap_data, which is similar to omp_target_disassociate_ptr, is an error when the structured reference count is not zero.

While Flang and Clang obviously must implement the syntax and semantics for selecting OpenACC reference counts differently than for selecting OpenMP reference counts, the implementation is the same at the runtime level. That is, OpenACC’s dynamic reference count is OpenMP’s dynamic reference count, and OpenACC’s structured reference count is our OpenMP hold reference count extension.