How to prevent useless save/restore of a callee save?

[flint ocean]

See https://godbolt.org/z/1MxnqKsYs

export fn sampleTailCaller() noreturn {
    asm volatile("" : : : "rbx");
    const next: *const fn() callconv(.C) noreturn = @ptrFromInt(0xabcdef);
    @call(.always_tail, next, .{});
}

sampleTailCaller:
        push    rbx
        mov     eax, 11259375
        pop     rbx
        jmp     rax

Is it possible to avoid push rbx/pop rbx pair here without removing the asm statement?

This comes up when writing a VM in zig. As zig doesn't have computed gotos, you have to rely on tail calls. There are a few problems with callee-saves it seems though

[nova galleon]

iirc zig's "auto" calling convention is a derivative of sysv, which says that rbx must be callee-saved

if you don't want that, choose a calling convention that suits your purpose

[flint ocean]

well, what I am trying to do here is to make sure that function can use any registers without preserving their value first

this should be fine, as all these functions are noreturn, so they really shouldn't bother about preserving values from callee

[nova galleon]

do the functions have any arguments?

[flint ocean]

they would, yeah

[nova galleon]

on x86_64?

[flint ocean]

preferrably any arch

I want this code to be generic

[nova galleon]

unfortunately that certainly isn't possible, I looked around a bit and it seems like you're probably going to have to use naked functions and make your own "calling convention"

most calling conventions have callee saved registers

[flint ocean]

that's understandable, what I do not seem to understand is why does zig bother with saving those in the first place - it is not like its going to return back to the function

the type is noreturn

it's probably llvm's fault I imagine

I will just hope that I don't use many registers and llvm's regalloc will never pick callee saves

[nova galleon]

thats a fair point, it shouldn't need to save them in a noreturn; as for the last point: there are very few registers that are caller saved, so good luck

[vapid tundra]

The push/pop rbx is precisely because the asm indicates it as a clobber

rbx is caller saved, so if you clobber that then the compiler has to save/restore it. Something like rax OTOH is callee saved so if you clobber that no save/restore is needed as the compiler doesnt use rax before that asm

[flint ocean]

right, but there is no need to save/restore it - we will never make it to the callee

How to prevent useless save/restore of a caller save?

How to prevent useless save/restore of a callee save?

[stoic ridge]

but your asm is 100% opaque to zig

[nova galleon]

Their argument is that callee saved registers shouldn't be saved in a noreturn function, because they're only saved because the caller may be using them: but in a noreturn function you never return to the caller