cmd/compile: small struct initialization code is suboptimal because of redundant zeroing

[tdakkota]

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

$ go version
go version go1.20.1 windows/amd64

Also tested: go1.21-f5c7416511, go1.19

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\tdakkota\AppData\Local\go-build
set GOENV=C:\Users\tdakkota\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\tdakkota\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\tdakkota\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=G:\scoop\apps\go\current
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=G:\scoop\apps\go\current\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.20.1
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=C:\Users\tdakkota\Desktop\tmp\go.mod
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config

What did you do?

https://go.dev/play/p/KYZ1WC9j4VP
https://go.godbolt.org/z/1KMEo5nP8 (note: uses Go 1.19)

What did you expect to see?

Compiler should generate the same code for new(T) and &T{}.

What did you see instead?

new(T) compiles to

        MOVUPS  X15, main..autotmp_1+8(SP)

but &T{} compiles to

        MOVUPS  X15, main..autotmp_2+8(SP) 
        MOVUPS  X15, main..autotmp_2+8(SP) // overwrite zeroed value with zero

Big structures like this

type T struct {
    A, B, C, D, E int
}

are not affected.

[randall77]

This is a failure of dead store elimination. It comes about because at that point in compilation, it doesn't realize that the two stores are actually to the same address, because they are different LocalAddr ops at different memory states.

v6 (8) = LocalAddr <*T> {.autotmp_2} v2 v1
v3 (8) = OffPtr <*int> [8] v6
v22 (8) = OffPtr <*int> [0] v6
v4 (8) = Store <mem> {int} v22 v5 v1
v26 (8) = Store <mem> {int} v3 v5 v4
v8 (8) = LocalAddr <*T> {.autotmp_2} v2 v26 (a[*T])
v23 (8) = OffPtr <*int> [8] v8
v25 (8) = OffPtr <*int> [0] v8
v27 (+8) = Store <mem> {int} v25 v5 v26
v28 (8) = Store <mem> {int} v23 v5 v27

We could teach DSE that two LocalAddrs of the same variable are equal, even if they are from different memory states.

We could also get rid of the memory dependence of LocalAddr for types with no pointers in them. We really only need to be that picky when there is a GC implication of seeing a pointer early. (And even then, with conservative top frame scanning, we only need to worry about early pointers across calls.)