What registers must be preserved by an x86 function?

Question

I'm writing a function in x86 assembly that should be callable from c code, and I'm wondering which registers i have to restore before i return to the caller.

Currently I'm only restoring esp and ebp, while the return value is in eax.

Are there any other registers I should be concerned about, or could I leave whatever pleases me in them?

Answer 1

Using Microsoft's 32 bit ABI (cdecl or stdcall or other calling conventions), EAX, EDX and ECX are scratch registers (call clobbered). The other general-purpose integer registers are call-preserved.

The condition codes in EFLAGS are call-clobbered. DF=0 is required on call/return so you can use rep movsb without a cld first. The x87 stack must be empty on call, or on return from a function that doesn't return an FP value. (FP return values go in st0, with the x87 stack empty other than that.) XMM6 and 7 are call-preserved, the rest are call-clobbered scratch registers.

Outside of Windows, most 32-bit calling conventions (including i386 System V on Linux) agree with this choice of EAX, EDX and ECX as call-clobbered, but all the xmm registers are call-clobbered.

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For x64 under Windows, you only need to restore RBX, RBP, RDI, RSI, R12, R13, R14, and R15. XMM6..15 are call-preserved. (And you have to reserve 32 bytes of shadow space for use by the callee, whether or not there are any args that don't fit in registers.) xmm6..15 are call-preserved.
See https://en.wikipedia.org/wiki/X86_calling_conventions#Microsoft_x64_calling_convention for more details.

Other OSes use the x86-64 System V ABI (see figure 3.4), where the call-preserved integer registers are RBP, RBX, RSP, R12, R13, R14, and R15. All the XMM/YMM/ZMM registers are call-clobbered.

EFLAGS and the x87 stack are the same as in 32-bit conventions: DF=0, condition flags are clobbered, and x87 stack is empty. (x86-64 conventions return FP values in XMM0, so the x87 stack registers always need to be empty on call/return.)

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For links to official calling convention docs, see https://stackoverflow.com/tags/x86/info

Answer 2

32-bit: EBX, ESI, EDI, EBP
64-bit Windows: RBX, RSI, RDI, RBP, R12-R15, XMM6-XMM15
64-bit Linux,BSD,Mac: RBX, RBP, R12-R15

For details see "Software optimization resources" by Agner Fog. Calling conventions are described in this pdf.