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Safe to use r13-r15 as general register?

Yang Fang
Note: This was originally posted on 18th July 2011 at http://forums.arm.com

Hello, I am writting a function, which involves complicated algorithm, I want to have a lot of general purpose registers to keep the routine speed. I have read some article that r13-r15 are special registers, but can be used as general purpose registers.

Can I do it like this
   stmfd       sp!, {r4-r15,lr}
:
:
    ldmfd       sp!, {r4-r15,pc}

Thank you very much!
Parents
  • Note: This was originally posted on 19th July 2011 at http://forums.arm.com


    I read some ARM document, it says

    A called routine need not preserve the values of r0-r3, IP (r12) and LR (r14)

    so when subroutine returns, does it mean that it will automatically restore the value of r0-r3, and r12, in the caller function?

    Thank you.


    Not at all!
    It means that the caller don't care about the values of r0, r1, r2 and r3.
    for r12, the caller must not use it. In fact it can use it, but the caller must know that after a sub function call r12 can have changed...
    for r14, the BL instruction will change r14 !


    In all case, the ARM never save any register for you !
    so the caller must know that those register may change after a subroutine call.

Reply
  • Note: This was originally posted on 19th July 2011 at http://forums.arm.com


    I read some ARM document, it says

    A called routine need not preserve the values of r0-r3, IP (r12) and LR (r14)

    so when subroutine returns, does it mean that it will automatically restore the value of r0-r3, and r12, in the caller function?

    Thank you.


    Not at all!
    It means that the caller don't care about the values of r0, r1, r2 and r3.
    for r12, the caller must not use it. In fact it can use it, but the caller must know that after a sub function call r12 can have changed...
    for r14, the BL instruction will change r14 !


    In all case, the ARM never save any register for you !
    so the caller must know that those register may change after a subroutine call.

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