DRAFT: Fix issue #2670, support for extern(Objective-C)

[MrcSnm]

Most here are obviously WIP, there is a bunch of forwarded things and as I'm only getting started with LLVM I would like you guys to review what I'm doing right now.

@kinke @thewilsonator @jacob-carlborg
#2670

I'm trying to replicate those structs defined by the following objc code:

#import <objc/runtime.h>
#import <Foundation/Foundation.h>

int main()
{
  [NSObject alloc];
  return 0;
}

; ModuleID = 'test.m'
source_filename = "test.m"
target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx12.0.0"

; Trying to create those strructs
%0 = type opaque
%struct._class_t = type { %struct._class_t*, %struct._class_t*, %struct._objc_cache*, i8* (i8*, i8*)**, %struct._class_ro_t* }
%struct._objc_cache = type opaque
%struct._class_ro_t = type { i32, i32, i32, i8*, i8*, %struct.__method_list_t*, %struct._objc_protocol_list*, %struct._ivar_list_t*, i8*, %struct._prop_list_t* }
%struct.__method_list_t = type { i32, i32, [0 x %struct._objc_method] }
%struct._objc_method = type { i8*, i8*, i8* }
%struct._objc_protocol_list = type { i64, [0 x %struct._protocol_t*] }
%struct._protocol_t = type { i8*, i8*, %struct._objc_protocol_list*, %struct.__method_list_t*, %struct.__method_list_t*, %struct.__method_list_t*, %struct.__method_list_t*, %struct._prop_list_t*, i32, i32, i8**, i8*, %struct._prop_list_t* }
%struct._ivar_list_t = type { i32, i32, [0 x %struct._ivar_t] }
%struct._ivar_t = type { i64*, i8*, i8*, i32, i32 }
%struct._prop_list_t = type { i32, i32, [0 x %struct._prop_t] }
%struct._prop_t = type { i8*, i8* }

@"OBJC_CLASS_$_NSObject" = external global %struct._class_t
@"OBJC_CLASSLIST_REFERENCES_$_" = internal global %struct._class_t* @"OBJC_CLASS_$_NSObject", section "__DATA,__objc_classrefs,regular,no_dead_strip", align 8
@llvm.compiler.used = appending global [1 x i8*] [i8* bitcast (%struct._class_t** @"OBJC_CLASSLIST_REFERENCES_$_" to i8*)], section "llvm.metadata"

; Function Attrs: noinline optnone ssp uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  %2 = load %struct._class_t*, %struct._class_t** @"OBJC_CLASSLIST_REFERENCES_$_", align 8
  %3 = bitcast %struct._class_t* %2 to i8*
  %4 = call i8* @objc_alloc(i8* %3)
  %5 = bitcast i8* %4 to %0*
  ret i32 0
}

declare i8* @objc_alloc(i8*)

attributes #0 = { noinline optnone ssp uwtable "darwin-stkchk-strong-link" "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "probe-stack"="___chkstk_darwin" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+cx8,+fxsr,+mmx,+sahf,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "tune-cpu"="generic" }

!llvm.module.flags = !{!0, !1, !2, !3, !4, !5, !6, !7, !8, !9, !10}
!llvm.ident = !{!11}

!0 = !{i32 2, !"SDK Version", [2 x i32] [i32 13, i32 1]}
!1 = !{i32 1, !"Objective-C Version", i32 2}
!2 = !{i32 1, !"Objective-C Image Info Version", i32 0}
!3 = !{i32 1, !"Objective-C Image Info Section", !"__DATA,__objc_imageinfo,regular,no_dead_strip"}
!4 = !{i32 1, !"Objective-C Garbage Collection", i8 0}
!5 = !{i32 1, !"Objective-C Class Properties", i32 64}
!6 = !{i32 1, !"Objective-C Enforce ClassRO Pointer Signing", i8 0}
!7 = !{i32 1, !"wchar_size", i32 4}
!8 = !{i32 7, !"PIC Level", i32 2}
!9 = !{i32 7, !"uwtable", i32 1}
!10 = !{i32 7, !"frame-pointer", i32 2}
!11 = !{!"Apple clang version 14.0.0 (clang-1400.0.29.202)"}

[MrcSnm]

Right now I would like to know what is the correct way to self reference types, function pointers, and create structs. Most of the handiwork is there but I'm still quite lost in the details

[jacob-carlborg]

Unfortunately I'm not familiar with the LLVM or the LDC internals, but I can help you with the Objective-C specific parts.

As a suggestion, I would start with the disabled Objective-C tests and implement what's necessary to get them to pass. The most basic ones are the ones related to calling instance methods, which are https://github.com/ldc-developers/ldc/blob/master/tests/dmd/runnable/objc_call.d and https://github.com/ldc-developers/ldc/blob/master/tests/dmd/runnable/objc_objc_msgSend.d. As you can see, you can use the Objective-C runtime (objc_lookUpClass in this case) to help bootstrapping.

BTW, GitHub has support for draft pull requests.