Datasheet

ARM Compiler Reference

number (for example, r4). In C, global register variables cannot be

qualified or initialized at declaration. In C++, any initialization is treated

as a dynamic initialization. Valid types are:

• any integer type, except

long long

• any pointer type.

For example, to declare a global integer register variable allocated to r5

(the ATPCS register v2), use the following:

__global_reg(2) int x;

The global register must be specified in all declarations of the same

variable. For example, the following is an error:

int x;

__global_reg(1) int x; // error

Also,

__global_reg

variables in C cannot be initialized at definition. For

example, the following is an error in C, though not in C++:

__global_reg(1) int x=1; // error in C

Depending on the ATPCS variant used, between five and seven integer

registers, and four floating-point registers are available for use as global

Thumb code, or more than two global floating-point register variables is

not recommended.

Note

In Thumb,

__global_reg(4)

is not allowed.

Unlike register variables declared with the standard

keyword,

the compiler does not move global register variables to memory as

required. 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

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.