Placing Global Variables in a Register

GCC allows programmers to place global variables in a specific machine register, where the variables will then reside for the duration of the program's execution. GCC calls such variables global register variables.
The syntax requires that the programmer specify the machine register. The following example uses ebx :

register int *foo asm ("ebx");

The programmer must select a variable that is not function-clobbered: that is, the selected variable must be usable by local functions, saved and restored on function call invocation, and not specified for any special purpose by the architecture or operating system's ABI. The compiler will generate a warning if the selected register is inappropriate. If the register is appropriate— ebx , used in this example, is fine for the x86 architecture—the compiler will in turn stop using the register itself.
Such an optimization can provide huge performance boosts if the variable is frequently used. A good example is with a virtual machine. Placing the variable that holds, say, the virtual stack frame pointer in a register might lead to substantial gains. On the other hand, if the architecture is starved of registers to begin with (as the x86 architecture is), this optimization makes little sense.
Global register variables cannot be used in signal handlers, or by more than one thread of execution. They also cannot have initial values because there is no mechanism for executable files to supply default contents for registers. Global register variable declarations should precede any function definitions.