Explicit Reg Vars - Using the GNU Compiler Collection (GCC)

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6.43.5 Variables in Specified Registers

GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.

Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters that have a couple of global variables that are accessed very often.
Local register variables in specific registers do not reserve the registers, except at the point where they are used as input or output operands in an asm statement and the asm statement itself is not deleted. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. 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.
These local variables are sometimes convenient for use with the extended asm feature (see Extended Asm), if you want to write one output of the assembler instruction directly into a particular register. (This works provided the register you specify fits the constraints specified for that operand in the asm.)