RealView Compilation Tools - ARM Information Center

Compiler-specific Features
• one global register variable in Thumb-1
• half the number of available floating-point registers as global floating-point
register variables.
If you declare too many global variables, code size increases significantly. In some
cases, your program might not compile.
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 on page 4-9 produces an error because global registers must be specified
in all declarations of the same variable.
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