RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
Caution
You must take care when using global register variables because:
• There is no check at link time to ensure that direct calls between different
compilation units are sensible. If possible, define global register variables used in
a program in each compilation unit of the program. In general, it is best to place
the definition in a global header file. You must set up the value in the global
register early in your code, before the register is used.
• A global register variable maps to a callee-saved register, so its value is saved and
restored across a call to a function in a compilation unit that does not use it as a
global register variable, such as a library function.
• Calls back into a compilation unit that uses a global register variable are
dangerous. For example, if a function using a global register is called from a
compilation unit that does not declare the global register variable, the function
reads the wrong values from its supposed global register variables.
• This storage class can only be used at file scope.
Example
Example 4-1 declares a global variable register allocated to r5.
Example 4-1 Declaring a global integer register variable
__global_reg(2) int x
; v2 is the synonym for r5
Example 4-2 produces an error because global register must be specified in all
declarations of the same variable.
Example 4-2 Global register - declaration error
int x;
__global_reg(1) int x; // error
In C, __global_reg variables cannot be initialized at definition. Example 4-3 on
page 4-9 produces an error in C, but not in C++.
4-8 Copyright © 2007, 2010 ARM Limited. All rights reserved. ARM DUI 0348A
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