Assembly with clang:

So far I've found it very buggy - even for simple things:

Code:

.intel_syntax noprefix
.globl WinMainCRTStartup
WinMainCRTStartup:
        pop rax

        xor ecx,ecx
        lea rdx,[rip + _message]
        lea r8,[rip + _caption]
        mov r9,rcx
        call MessageBoxW

        xchg ecx,eax
        call ExitProcess
        int3

.macro wchars name:req foo:vararg
        .align 2,0
        \name :
        .irpc bar,"\foo"
        .ifnc \bar\bar,""
                .ascii "\bar"
                .byte 0
        .else # without this else cc1as generates incorrect output
                .print "\\bar"
        .endif
        .endr
        .byte 0,0
.endmacro

.data
wchars _caption Win64 assembly program
wchars _message Hello World!

# commands for the linker
.section .drectve,"yn"
.ascii  " /SUBSYSTEM:WINDOWS"
.ascii  " /DEFAULTLIB:kernel32.lib"
.ascii  " /DEFAULTLIB:user32.lib"

Build with...

Code:

clang -cc1as -triple x86_64-pc-windows-msvc19.28.29910 -filetype obj -o thisfile.obj thisfile.s
lld-link -out:thisfile.exe thisfile.obj

I'm kind of leery about trying anything more challenging. There doesn't appear to be a directive to output wide chars, or any UTF-8 support of any kind. All strings must be escapable ASCII - otherwise they are interpreted as symbols.