proposal: cmd/compile, cmd/cgo: support targeting arm64_32

[sandler31]

I've tried to compile a go module using the "c-archive" mode in order to include in an Xcode project, targeted for the WatchOS (running on the Apple Watch).

After performing all the necessary steps to do so, I've discovered that it's not currently possible, due to the fact that the WatchOS is expecting binaries to target the special "ARM64_32" CPU type, which according to them means "64-bit hardware with 32-bit types".

I used the following script in my go module compilation attempt:

#!/bin/sh -eu

export CGO_ENABLED=1
export GOOS=ios
export GOARCH=arm64
export SDK=watchos
export CGO_CFLAGS="-fembed-bitcode"

go build -buildmode=c-archive -trimpath -o out.a ./

With the following code:

package main

import "C"

//export foo
func foo() *C.char {
	return C.CString("bar")
}

func main() {}

Of course, as I already said, this produces the expected binary as per the GOOS/GOARCH values, but not the one needed to run on WatchOS:

otool -h out.a                  
Archive : out.a
out.a(go.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4       1880 0x00000000
out.a(000000.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        600 0x00002000
out.a(000001.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        440 0x00002000
out.a(000002.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        440 0x00002000
out.a(000003.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        440 0x00002000
out.a(000004.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        520 0x00002000
out.a(000005.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        680 0x00002000
out.a(000006.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        760 0x00002000
out.a(000007.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        520 0x00002000
out.a(000008.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        520 0x00002000
out.a(000009.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        520 0x00002000
out.a(000010.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        680 0x00002000
out.a(000011.o):
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedfacf 16777228          0  0x00           1     4        520 0x00000000

16777228 being (CPU_TYPE_ARM | CPU_ARCH_ABI64)

The correct binary format can be achieved by compiling a simple c file, targeting the correct CPU type:

#!/bin/sh -eu

SDK=watchos
SDK_PATH=`xcrun --sdk $SDK --show-sdk-path`
CLANG=`xcrun --sdk $SDK --find clang`
CARCH=arm64_32
$CLANG -arch $CARCH -isysroot $SDK_PATH -mwatchos-version-min=9.0 -c main.c -o c_out.a

int foo (int a) {
    return a;
}

Which results with the following:

otool -h c_out.a 
c_out.a:
Mach header
      magic  cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
 0xfeedface 33554444          1  0x00           1     4        320 0x00002000

33554444 being (CPU_TYPE_ARM | CPU_ARCH_ABI64_32), and CPU sutype CPU_SUBTYPE_ARM64_V8

I realize this might require some serious work to be supported, and would be happy to try and work on this myself, unfortunately this is not really my area of expertise, and I have very minimal knowledge of the golang/cgo compilation internals.
If there is no possibility this would be supported in the foreseeable future, I would be happy to receive some pointers on the areas where said work needs to be done.

On a side note, when researching this topic, I found out that support for targeting this architecture has been implemented in llvm (https://reviews.llvm.org/D99822), but seeing as gollvm does not even seem to support compiling to darwin, this seems like a dead end.

[ianlancetaylor]

CC @golang/runtime