x/tools/go/analysis/passes/fieldalignment: "struct with X pointer bytes could be Y" linting errors for (what appears to be) aligned structs

[atc0005]

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

$ go version
go version go1.16.3 windows/amd64

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\atc0005\AppData\Local\go-build
set GOENV=C:\Users\atc0005\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\atc0005\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\atc0005\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.16.3
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=T:\github\help-requests\go\struct-alignment\go.mod
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\atc0005\AppData\Local\Temp\go-build4294469141=/tmp/go-build -gno-record-gcc-switches

What did you do?

Example structs:

	type emailConfigAscending struct {
		timeout                time.Duration      // 8 bytes
		notificationRateLimit  time.Duration      // 8 bytes
		template               *template.Template // 8 bytes
		serverPort             int                // 8 bytes
		notificationRetries    int                // 8 bytes
		notificationRetryDelay int                // 8 bytes
		server                 string             // 16 bytes
		senderAddress          string             // 16 bytes
		clientIdentity         string             // 16 bytes
		recipientAddresses     []string           // 24 bytes
	}

	type emailConfigDescending struct {
		recipientAddresses     []string           // 24 bytes
		server                 string             // 16 bytes
		senderAddress          string             // 16 bytes
		clientIdentity         string             // 16 bytes
		timeout                time.Duration      // 8 bytes
		notificationRateLimit  time.Duration      // 8 bytes
		template               *template.Template // 8 bytes
		serverPort             int                // 8 bytes
		notificationRetries    int                // 8 bytes
		notificationRetryDelay int                // 8 bytes
	}

I first encountered the linting errors via CI Docker container (govet fieldalignment golangci-lint linter), then reproduced by enabling fieldalignment vscode Go plugin analyzer.

• https://play.golang.org/p/DxWIJd_OgPM
• https://github.com/atc0005/help-requests/tree/92e9e770ff4fcb8634677dea20e6253f8b39a737/go/struct-alignment
  • includes golangci-lint configuration which enables fieldalignment linter

What did you expect to see?

No linting errors.

What did you see instead?

These linting errors for two variations of the struct:

• fieldalignment: struct with 104 pointer bytes could be 64 (govet)
• fieldalignment: struct with 96 pointer bytes could be 64 (govet)

[seankhliao]

cc @leitzler

[leitzler]

The linting errors you see is about pointer bytes, not about size. As described in #44877 (comment) that is how many bytes of the object that the GC has to potentially scan for pointers.

We should probably clarify the documentation to reflect that there are two different diagnostics reported, struct of size x could be y and struct with x pointer bytes could be y, and how they differ.

I suggest closing this issue as a duplicate of #44877.

[stamblerre]

@leitzler: Do you want to send a CL to clarify the documentation before we close this issue (since #44877 is closed)?

[gopherbot]

Change https://golang.org/cl/314469 mentions this issue: go/analysis/passes/fieldalignment: clarify reported diagnostics in docs

[leitzler]

Apologies for the delay, sent a CL that hopefully clarifies the docs!

[atc0005]

@leitzler is there support for enabling only one of the diagnostics for that analyzer? For example, enabling just the struct size check and leaving the "pointer bytes" used disabled?