cmd/asm: amd64 MOVBQZX instructions referring to upper-byte registers are not rejected

[WojciechMula]

What version of Go are you using (go version)?

$ go version
go version go1.17.3 linux/amd64

Does this issue reproduce with the latest release?

Haven't checked

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/wojtek/.cache/go-build"
GOENV="/home/wojtek/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/wojtek/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/wojtek/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go-1.17"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.17/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.3"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2132970980=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Consider this simple program

main.go

package main

// go:noescape
func asmfun()
func main() { asmfun() }

and the asm implementation --- asmfun_amd64.s

#include "textflag.h"

TEXT ·asmfun(SB), NOSPLIT, $0
    // wrong code
    MOVBQZX AH, AX
    MOVBQZX BH, AX
    MOVBQZX CH, AX
    MOVBQZX DH, AX
    RET   

What did you expect to see?

I'd expect the assembler to reject invalid code, as referencing the upper-byte
registers is impossible.

What did you see instead?

$ go build
$ go tool objdump -s asmfun main
TEXT main.asmfun.abi0(SB) /home/wojtek/t/bug/asmfun_amd64.s
  asmfun_amd64.s:5  0x455420        480fb6c4        MOVZX SP, AX        
  asmfun_amd64.s:6  0x455424        480fb6c7        MOVZX DI, AX        
  asmfun_amd64.s:7  0x455428        480fb6c5        MOVZX BP, AX        
  asmfun_amd64.s:8  0x45542c        480fb6c6        MOVZX SI, AX        
  asmfun_amd64.s:9  0x455430        c3              RET         

[randall77]

Related, but probably not identical: #14288