Provides the compiler with information about the access of objects by a function such that certain properties of function calls can be deduced.
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[[unsequenced]][[__unsequenced__]] | (1) | ||||||||
[[reproducible]][[__reproducible__]] | (2) | ||||||||
These attributes apply to a function declarator or to a type specifier that has a function type. The corresponding attribute is a property of the function type.
An evaluation of a function call is effectless if any store operationthat is sequenced during the call is the modification of an object that synchronizes with the call; if additionally the operation is observable, all access to the object must be based on a unique pointer parameter of the function.
An evaluationE is idempotent if a second evaluation ofE can be sequenced immediately after the original one without changing the resulting value, if any, or the observable state of the execution.
A functionF is stateless if any definition of an object of static or threadstorage duration inF or in a function that is called byF isconst but notvolatile qualified.
A functionF is independent if for any objectX that is observed by a call toFthrough an lvalue that is not based on a parameter of the call, all accesses toX in all calls toF during the same program execution observe the same value;otherwise if the access is based on a pointer parameter, there shall be a unique such pointer parameterP such that any access toX shall be to an lvalue that is based onP.
An objectX is observed by a function call if both synchronize, ifX is not local to the call, ifX has a lifetime that starts before the function call, and if an access ofX is sequenced during the call; the last value ofX, if any, that is stored before the call is said to be the value ofX that is observed by the call.
These attributes exist for the purpose of compiler optimization.
If a function is reproducible, multiple subsequent calls can be treated as a single call.
If a function is unsequenced, multiple subsequent calls can be treated as a single call, and the calls can be parallelized and reordered arbitrarily.
