cmd/asm: No way to use the dup instruction in ARM64 assembly

[kovidgoyal]

Go version

go version go1.21.6 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/kovid/.cache/go-build'
GOENV='/home/kovid/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/kovid/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/kovid/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.6'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/kovid/work/simdstring/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3570585833=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Tried to find a way to use the the dup arm64 assembly instruction via go assembly. It doesnt seem to exist. The closest I can find is VDUP but that seems to take something known a C_ELEM not sure what that is, if its intended to map to dup it should take general purpose registers as operands. Here is a simple function to set all bytes in a register:

TEXT ·test_set1_epi8_asm_128(SB), NOSPLIT, $0-16
	MOVBU b+0(FP), R0
	VDUP R0, V0
	MOVD ans+8(FP), R0
	VST1 [V0.B8], (R0)
	RET

Try to build it with go gives the error
asm: illegal combination: 00004 VDUP R0, V0 REG NONE NONE VREG

Grepping for VDUP in the go assembler package gives:

502:	{AVDUP, C_ELEM, C_NONE, C_NONE, C_ARNG, 79, 4, 0, 0, 0},
503:	{AVDUP, C_ELEM, C_NONE, C_NONE, C_VREG, 80, 4, 0, 0, 0},
504:	{AVDUP, C_ZREG, C_NONE, C_NONE, C_ARNG, 82, 4, 0, 0, 0},

Showing that the assembler thinks it doesnt take regular registers as operands.

ARM64 reference: https://developer.arm.com/documentation/ddi0596/2021-12/SIMD-FP-Instructions/DUP--general---Duplicate-general-purpose-register-to-vector-?lang=en

What did you see happen?

Got the error:

asm: illegal combination: 00004 	VDUP	R0, V0 REG NONE NONE VREG

What did you expect to see?

Expected it to generate instructions similar to

dup.8b	v0, w0

Maybe there is some other way to do this in Go assembly? If so would appreciate being enlightened.

[mauri870]

The obj docs for arm64 states:

Go adds a V prefix for most floating-point and SIMD instructions, except cryptographic extension instructions and floating-point(scalar) instructions. 

[cherrymui]

It does support integer registers.

504: {AVDUP, C_ZREG, C_NONE, C_NONE, C_ARNG, 82, 4, 0, 0, 0},

C_ZREG is an integer registers or ZR, C_ARNG is an arrangement. So you need an arrangement as the destination, like VDUP R0, V1.B8.

Thanks.

[kovidgoyal]

You are correct, my mistake. However, there are a couple of other useful instructions that are missing:

1. VCGT (> on vector registers) (sometimes called CMGT neither name is recognized by Go)
2. SHRN (shift right and narrow)

[cherrymui]

It is true that we don't yet support all vector instructions. You could write WORD directives for the bit encoding. Also feel free to send a CL to add the support. See also #44734 for a more general approach of adding new instructions. Thanks.

[kovidgoyal]

Yes, I have already implemented it in my code using WORD, I am just
pointing out that these are rather useful/common instructions that are
missing. CMGT is useful when searching for bytes in strings in a range
and SHRN is useful to speed up finding matching positions in the output
of CMEQ and CMGT and friends. Indeed, the ARM64 implementations in the
stdlib bytealg could probably be simplified/sped up by using SHRN.