What registers to save in the ARM C calling convention?

Question

It's been a while since I last coded arm assembler and I'm a little rusty on the details. If I call a C function from arm, I only have to worry about saving r0-r3 and lr, right?

If the C function uses any other registers, is it responsible for saving those on the stack and restoring them? In other words, the compiler would generate code to do this for C functions.

For example if I use r10 in an assembler function, I don't have to push its value on the stack, or to memory, and pop/restore it after a C call, do I?

This is for arm-eabi-gcc 4.3.0.

Answer 1

It depends on the ABI for the platform you are compiling for. On Linux, there are two ARM ABIs; the old one and the new one. AFAIK, the new one (EABI) is in fact ARM's AAPCS. The complete EABI definitions currently live here on ARM's infocenter.

From the AAPCS, §5.1.1:

• r0-r3 are the argument and scratch registers; r0-r1 are also the result registers
• r4-r8 are callee-save registers
• r9 might be a callee-save register or not (on some variants of AAPCS it is a special register)
• r10-r11 are callee-save registers
• r12-r15 are special registers

A callee-save register must be saved by the callee (in opposition to a caller-save register, where the caller saves the register); so, if this is the ABI you are using, you do not have to save r10 before calling another function (the other function is responsible for saving it).

Edit: Which compiler you are using makes no difference; gcc in particular can be configured for several different ABIs, and it can even be changed on the command line. Looking at the prologue/epilogue code it generates is not that useful, since it is tailored for each function and the compiler can use other ways of saving a register (for instance, saving it in the middle of a function).

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Terminology: "callee-save" is a synonym for "non-volatile" or "call-preserved": What are callee and caller saved registers?
When making a function call, you can assume that the values in r4-r11 (except maybe r9) are still there after (call-preserved), but not for r0-r3 (call-clobbered / volatile).