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 Variables in specified registers  

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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 which have a couple of global variables that are accessed very often.
• Local register variables in specific registers do not reserve the registers. 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 data flow 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 Assembler instructions with C expression operands), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the asm.)