SummaryThe declare variant directive declares a specialized variant of a base function and specifies the context in which that specialized variant is used. The declare variant directive is a declarative directive.
Syntax
The syntax of the declare variant directive is as follows:
#pragma omp declare variant(variant-func-id) clause new-line
[#pragma omp declare variant(variant-func-id) clause new-line]
[ ... ]
function definition or declaration
where clause is one of the following:
match(context-selector-specification)
and where variant-func-id is the name of a function variant that is either a base language identifier or, for C++, a template-id.
DescriptionThe declare variant directive declares the base function to have the specified function variant. The context selector in the match clause is associated with the variant. The OpenMP context for a call to a given base function is defined according to Section 2.3.1. If the context selector that is associated with a declared function variant is compatible with the OpenMP context of a call to a base function according to the matching rules defined in Section 2.3.3 then a call to the variant is a candidate to replace the base function call. For any call to the base function for which candidate variants exist, the variant with the highest score is selected from all compatible variants. If multiple variants have the highest score, the selected variant is implementation defined. If a compatible variant exists, the call to the base function is replaced with a call to the selected variant. If no compatible variants exist then the call to the base function is not changed.
Different declare variant directives may be specified for different declarations of the same base function.
Any differences that the specific OpenMP context requires in the prototype of the variant from the base function prototype are implementation defined.
The function variant is determined by base language standard name lookup rules ([basic.lookup]) of variant-func-id with arguments that correspond to the argument types in the base function declaration.
The variant-func-id and any expressions inside of the match clause are interpreted as if they appeared at the scope of the trailing return type of the base function.
RestrictionsRestrictions to the declare variant directive are as follows:
∙ If the function has any declarations, then the declare variant directives for any declarations that have one must be equivalent. If the function definition has a declare variant, it must also be equivalent. Otherwise, the result is unspecified.
∙ The declare variant directive cannot be specified for a virtual function. ∙ The type of the function variant must be compatible with the type of the base function after the implementation-defined transformation for its OpenMP context.
∙ base-proc-name must not be a generic name, procedure pointer, or entry name. ∙ If base-proc-name is omitted then the declare variant directive must appear in the specification part of a subroutine subprogram or a function subprogram. ∙ Any declare variant directive must appear in the specification part of a subroutine, subprogram, function subprogram, or interface body to which it applies. ∙ If a declare variant directive is specified in an interface block for a procedure then it must match a declare variant directive in the definition of the procedure. ∙ If a procedure is declared via a procedure declaration statement then the procedure base-proc-name should appear in the same specification. ∙ If a declare variant directive is specified for a procedure name with an explicit interface and a declare variant directive is also specified for the definition of the procedure, the two declare variant directives must match. Otherwise the result is unspecified.