5.38.2 Specifying Registers for Local Variables

You can define a local register variable with a specified register like this:

register int *foo asm ("a5");

Here a5 is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see section 5.36 Assembler Instructions with C Expression Operands). Both of these things generally require that you conditionalize your program according to cpu type.

In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register %a5.

Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.

This option does not guarantee that GNU CC will generate code that has this variable in the register you specify at all times. You may not code an explicit reference to this register in an asm statement and assume it will always refer to this variable.

Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.