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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!
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  • Arm Employee Badge
    Note: This was originally posted on 19th July 2011 at http://forums.arm.com

    The called function is allowed to corrupt r0-r3, and r12. The caller must store to stack before the call, and load from stack after the call. Nothing automatic about it - these are real instructions emitted by the compiler.

    In many cases these registers won't contain anything the caller cares about though, so it doesn't matter if they get corrupted by the called function.
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  • Arm Employee Badge
    Note: This was originally posted on 19th July 2011 at http://forums.arm.com

    The called function is allowed to corrupt r0-r3, and r12. The caller must store to stack before the call, and load from stack after the call. Nothing automatic about it - these are real instructions emitted by the compiler.

    In many cases these registers won't contain anything the caller cares about though, so it doesn't matter if they get corrupted by the called function.
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