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cmd/compile: arm64 Broadcast* generates dead VMOVI/VMOV before VDUP instead of single DUP/LD1R #81352
Description
Go version
go version go1.27.1 darwin/arm64
Output of go env in your module/workspace:
AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN='/Users/drgo/local/bin'
GOCACHE='/Users/drgo/ramdisk/gobuild'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/drgo/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT='greenteagc,newinliner,jsonv2,simd'
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1174145987=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/drgo/local/code/contrib/gosmidarm/reports/broadcast/go.mod'
GOMODCACHE='/Users/drgo/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPACKAGESDRIVER=''
GOPATH='/Users/drgo/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.27.1/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/drgo/Library/Application Support/go/telemetry'
GOTMPDIR='/tmp'
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.27.1/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.27.1'
GOWORK=''
PKG_CONFIG='pkg-config'What did you do?
BroadcastFloat64x2 is generated as zero, insert lane 0, dup lane 0:
Same shape for every Broadcast*.
func BroadcastFloat64x2(x float64) Float64x2 {
var z Float64x2
return z.SetElem(0, x).broadcast1To2()
}
### What did you see happen?
VMOVI $0, V1.B16
VMOV V0.D[0], V1.D[0]
VDUP V1.D[0], V0.D2DUP overwrites both lanes, so the zero is dead.
What did you expect to see?
One-instruction:
- scalar already in an FPR/GPR:
DUP Vd.2D, Vn.D[0]orDUP Vd.2D, Xn - scalar in memory:
LD1R {Vd.2D}, [Xn]
Activity
gabyhelp commentedon Sep 4, 2026
Related Issues
Related Code Changes
(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)
prattmic commentedon Sep 4, 2026
cc @golang/compiler @dr2chase @JunyangShao
gopherbot commentedon Sep 5, 2026
Change https://go.dev/cl/827904 mentions this issue:
cmd/compile: fold arm64 broadcasts of inserted scalars#128: draft the simd experience report (post-acceptance field report)
docs: truth-maintain the public face against the shipped arm64 path
docs: the upstream watch read Gerrit only, and a question sat five we…