Question: How does your message sending work?

[madsmtm]

I see that you're emitting a call to a objc_msgSend function, but you somehow move the selector into the link name. How does that work? I assume it's dyld magic, but do you have some documentation on it, and some information on since when it has been supported?

cidre/cidre-macros/src/lib.rs

Lines 575 to 585 in 0985976

	extern \"C\" {{
	#[link_name = \"objc_msgSend${extern_name}\"]
	fn msg_send();
	}}
	unsafe {{
	let fn_ptr = msg_send as *const std::ffi::c_void;
	let sig: extern \"C\" fn{fn_args} {ret} = std::mem::transmute(fn_ptr);
	{call_args}
	}}

[yury]

Starting with Xcode 14 Apple changed how they generate code for selectors. More info at WWDC video 3:10.

I noticed that change, tested and rewrote my old msg_send to new one.

// clang -ObjC -S test.h
#import <Foundation/Foundation.h>

void test(void) {
  NSNumber * num = [NSNumber numberWithBool:YES];
}

        .section        __TEXT,__text,regular,pure_instructions
        .build_version macos, 13, 0     sdk_version 13, 3
        .globl  _test                           ; -- Begin function test
        .p2align        2
_test:                                  ; @test
        .cfi_startproc
; %bb.0:
        sub     sp, sp, #32
        .cfi_def_cfa_offset 32
        stp     x29, x30, [sp, #16]             ; 16-byte Folded Spill
        add     x29, sp, #16
        .cfi_def_cfa w29, 16
        .cfi_offset w30, -8
        .cfi_offset w29, -16
                                        ; implicit-def: $x1
        adrp    x8, _OBJC_CLASSLIST_REFERENCES_$_@PAGE
        ldr     x0, [x8, _OBJC_CLASSLIST_REFERENCES_$_@PAGEOFF]
        mov     w8, #1
        and     w2, w8, #0x1
        bl      "_objc_msgSend$numberWithBool:"
        str     x0, [sp, #8]
        ldp     x29, x30, [sp, #16]             ; 16-byte Folded Reload
        add     sp, sp, #32
        ret
        .cfi_endproc
                                        ; -- End function
        .section        __DATA,__objc_classrefs,regular,no_dead_strip
        .p2align        3                               ; @"OBJC_CLASSLIST_REFERENCES_$_"
_OBJC_CLASSLIST_REFERENCES_$_:
        .quad   _OBJC_CLASS_$_NSNumber

        .section        __DATA,__objc_imageinfo,regular,no_dead_strip
L_OBJC_IMAGE_INFO:
        .long   0
        .long   64

.subsections_via_symbols

One downside - they turn it as optimization. So I have to add opt-level = 1

Another - this is very platform specific, but I'm ok with that if we get zero cost.

[yury]

Also implement objc_release optimization.

[madsmtm]

Oh that's very, very cool! Thanks for the video link, I'll probably incorporate something similar into objc2 soon, behind an experimental feature-flag.

Do you know if it only works on Aarch64, or are these also available for x86_64?

[yury]

Not sure about 86_64.

[madsmtm]

I've implemented parts of this in madsmtm/objc2#462, the message sending changes will take a bit more to do.

[yury]

Closing this issue. Feel free to reopen if needed.