CL https://golang.org/cl/44335 mentions this issue.

What does MOVW.S even mean? I don't think any form of MOVW should take a .S bit.

There is a mov instruction "MOV R1, R2" can take .S suffix.

Now CL 44335 has nothing to do with this issue.

"MOVW $0xffffffaa, R0" is assembled to "MVN $0x55, R0", and MVN can also take a .S suffix.

But I do not find MOVW.S is really used, shall we disable .S for all "MOVW, MOVH, MOVB"?

"MOVW $0xffffffaa, R0" is assembled to "MVN $0x55, R0", and MVN can also take a .S suffix.

The assembler chooses different hardware instructions to materialize constant, which might change (e.g. for better optimization). Sometimes it doesn't take .S bit. The semantic of .S shouldn't depend on the value of the constant.

MOVW has several variants. It is tricky which forms accept .S bit. Punt to Go 1.10.

We have two ways:

1. Check .SWP by optab index instead of by instruction, and warn invalid .S for MOVW
2. Create a new MOVWS along with MOVW

Any better solutions?

This bug can also be assigned to me.

@benshi001 done ;)

I don't think it is necessary to change the existing checking mechanism. It can be checked at corresponding cases of MOVW in asmout.

I don't think creating a new instruction is a good idea. It is not compatible with existing code.

"It can be checked at corresponding cases of MOVW in asmout."

If so, we need not wait to go1.10, it can be done soon in each case of optab.

It is not where to make the check. The question is among the many variations of MOVW, which should accept the .S bit. For example, I don't think the MVN case you mentioned above should.

This is not a critical bug, nor a regression. Let's wait for Go 1.10.

Now in "func checkBits", the PWS suffixes are checked via (instruction, bit flags), how about change to (Optab.type_, bit flags). For example,

Case 1 MOVW accepts .S, but case 12 does not. So the
var mayHaveSuffix = map[obj.As]uint8{
AMOVW: T_SBIT | T_WBIT | T_PBIT,
...
} is changed to
var mayHaveSuffix = map[int8]uint8{
1: T_SBIT | T_WBIT | T_PBIT,
12: T_WBIT | T_PBIT,
...
}

Change https://golang.org/cl/70910 mentions this issue: cmd/internal/obj/arm: better solution of .S/.P/.U/.W suffix check