-qisolated_call
Category
Purpose
Specifies functions in the source file that have no side effects other than those implied by their parameters.
Essentially, any change in the state of the runtime environment is considered a side effect, including:
- Accessing a volatile object
- Modifying an external object
- Modifying a static object
- Modifying a file
- Accessing a file that is modified by another process or thread
- Allocating a dynamic object, unless it is released before returning
- Releasing a dynamic object, unless it was allocated during the same invocation
- Changing system state, such as rounding mode or exception handling
- Calling a function that does any of the above
Marking a function as isolated indicates to the optimizer that external and static variables cannot be changed by the called function and that pessimistic references to storage can be deleted from the calling function where appropriate. Instructions can be reordered with more freedom, resulting in fewer pipeline delays and faster execution in the processor. Multiple calls to the same function with identical parameters can be combined, calls can be deleted if their results are not needed, and the order of calls can be changed.
Syntax
Option syntax .-:--------. V | >>- -q--isolated_call--=----function-+-------------------------><
Defaults
Not applicable.
Parameters
- function
- The name of a function that does not have side effects or does
not rely on functions or processes that have side effects. function is a primary expression that can be an identifier, operator function, conversion function, or qualified name.
An identifier must be of type function or a typedef of function.
If the name refers
to an overloaded function, all variants of that function are marked
as isolated calls. 
Usage
The only side effect that is allowed for a function named in the option or pragma is modifying the storage pointed to by any pointer arguments passed to the function, that is, calls by reference. The function is also permitted to examine nonvolatile external objects and return a result that depends on the nonvolatile state of the runtime environment. Do not specify a function that causes any other side effects; that calls itself; or that relies on local static storage. If a function is incorrectly identified as having no side effects, the program behavior might be unexpected or produce incorrect results.
Predefined macros
None.
Examples
xlc myprogram.c -qisolated_call=myfunction:classfunctionRelated information
