RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
4.1.6 __global_reg
The __global_reg storage class specifier allocates the declared variable to a global
variable register.
Syntax
__global_reg(n) type varName
Where:
n
type
varName
Is an integer between one and eight.
Is one of the following types:
• any integer type, except long long
• any char type
• any pointer type.
Is the name of a variable.
Restrictions
If you use this storage class, you cannot use any additional storage class such as extern,
static, or typedef.
In C, global register variables cannot be qualified or initialized at declaration. In C++,
any initialization is treated as a dynamic initialization.
The number of available registers varies depending on the variant of the AAPCS being
used, there are between five and seven registers available for use as global variable
registers.
In practice, it is recommended that you do not use more than:
• three global register variables in ARM or Thumb-2
• 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.
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