Reproduced with 46c5856
go version devel +46c5856 Mon Dec 25 00:07:22 2017 +0000 linux/amd64.

More generally, any of pseudo registers will cause runtime panic
if it is used as scaled index in memory operand.
This includes FP, SB and PC, but not SP (at least for x86).

Should (Base)(PseudoReg*Scale) result in error like
"illegal addressing mode for FP/SB/PC"?

If we are going for error, should this check be x86-specific,
or should ARM code fail to compile too?
Currently, it accepts things like MOVW (R20)(PC*2), R0.

Another note: empty scaling will result in asmidx: bad address for x86.
Here is more complete error reproducer:

// Included p.String() output (note that pseudo registers are not recognized).
TEXT asmfunc(SB), $0
        CALL (AX)(PC*1) // CALL	(AX)(R???-4*1) | runtime error: index out of range
        CALL (AX)(SB*1) // CALL	(AX)(R???-2*1) | runtime error: index out of range
        CALL (AX)(FP*1) // CALL	(AX)(R???-1*1) | runtime error: index out of range
        RET

If patch like this is applied:

diff --git a/src/cmd/asm/internal/asm/parse.go b/src/cmd/asm/internal/asm/parse.go
index 1d5d073..a278de0 100644
--- a/src/cmd/asm/internal/asm/parse.go
+++ b/src/cmd/asm/internal/asm/parse.go
@@ -790,6 +790,10 @@ func (p *Parser) registerIndirect(a *obj.Addr, prefix rune) {
                if r2 != 0 {
                        p.errorf("unimplemented two-register form")
                }
+               if p.arch.InFamily(sys.I386, sys.AMD64) && r1 < 0 {
+                       // Reject (R)(Pseudo*scale) forms for x86.
+                       p.errorf("illegal addressing mode for pseudo-register")
+               }
                a.Index = r1
                if scale == 0 && p.arch.Family == sys.ARM64 {
                        // scale is 1 by default for ARM64

Attempt to compile example above will result in:

GOARCH=386 ./bin/go tool asm foo.s
foo.s:2: illegal addressing mode for pseudo-register
foo.s:3: illegal addressing mode for pseudo-register
foo.s:4: illegal addressing mode for pseudo-register
asm: assembly of foo.s failed

Change https://golang.org/cl/85419 mentions this issue: cmd/asm: disallow (Reg)(Pseudo*scale) indirect for x86

Change https://golang.org/cl/107835 mentions this issue: cmd/internal/obj/x86: disallow PC/FP/SB scaled index