There are two problems here, first, the operand is encoded incorrectly, which makes cmd/internal/obj/... bail out correctly, but with wrong error message.

The second problem is that the printing of the Addr is wrong as well.

And cmd/asm seems to abort after the first call to
ctxt.Diag, making further output after "illegal combination"
unavailable from cmd/internal/obj.

I'm also hit by this problem (I added another call to ctxt.Diag
and it never comes out.)

This one is tricky. I was thrown by the R520, but that's being parsed as a symbol, so in amd64 terms, the error is caused by
ADDQ xxx, R1

The issue is that this is a syntax error, but is not caught as one because the operand parser has no context, and it is illegal to have a bare symbol like that except as a jump label. I made a tiny change to catch that case, and it failed on arm64 because of this obscure syntax:

CSINV CS, R1, R2, R3

here the CS is a magic word, not a register, that is parsing as TYPE_MEM. It really shouldn't be. We should fix that somehow or special-case it more precisely in the assembler - it works only by accident. If that's fixed, the fix can be made so that xxx is allowed only as a label and you get this nice error:

x.s:2: illegal addressing mode for symbol xxx

This has been fixed by other changes.

x.s:2: unrecognized instruction "ADD"
x.s:5: unrecognized instruction "ADD"
asm: asm: assembly of x.s failed

I'm sorry, but this is not fixed, in fact, for many instruction the form is silently accepted creating hard to debug defects.

: iridium:tmp; cat a0.s
TEXT    foo0(SB),7,$-8
    MOVD    R520, R5
    RET
: iridium:tmp; GOOS=linux GOARCH=arm64 go tool asm -o /tmp/a.o -S /Users/aram/tmp/a0.s
foo0 t=1 dupok nosplit size=16 value=0 args=0xffffffff80000000 locals=0xfffffffffffffff8 leaf
    0x0000 00000 (/Users/aram/tmp/a0.s:1)   TEXT    foo0(SB), 7, $-8
    0x0000 00000 (/Users/aram/tmp/a0.s:2)   MOVD    0, R5
    0x0004 00004 (/Users/aram/tmp/a0.s:3)   RET (R30)
    0x0000 05 00 40 f9 c0 03 5f d6 00 00 00 00 00 00 00 00  ..@..._.........
    rel 4+0 t=8 +0
: iridium:tmp; 

The problem is about the syntax
R520
I cannot put in a fix for that until someone fixes the arm64 obj representation for condition codes. That is issue #12632. Fix that and the invalid syntax here can be rejected cleanly. I have assigned it to you to encourage progress.

P.S. Read the longer message from me above.

I'll fix the arm64 problem.

CL https://golang.org/cl/14680 mentions this issue.

CL https://golang.org/cl/14691 mentions this issue.