Package structslop defines an Analyzer that checks if struct fields can be re-arranged to optimize size.

With Go modules:

go get github.com/orijtech/structslop/cmd/structslop

Without Go modules:

$ cd $GOPATH/src/github.com/orijtech/structslop
$ git checkout v0.0.6
$ go get
$ install ./cmd/structslop

You can run structslop either on a Go package or Go files, the same way as other Go tools work.

Example:

$ structslop github.com/orijtech/structslop/testdata/src/struct

or:

$ structslop ./testdata/src/struct/p.go

Sample output:

/go/src/github.com/orijtech/structslop/testdata/struct/p.go:30:9: struct has size 24 (size class 32), could be 16 (size class 16), you'll save 50.00% if you rearrange it to:
struct {
	y uint64
	x uint32
	z uint32
}

/go/src/github.com/orijtech/structslop/testdata/struct/p.go:36:9: struct has size 40 (size class 48), could be 24 (size class 32), you'll save 33.33% if you rearrange it to:
struct {
	_  [0]func()
	i1 int
	i2 int
	a3 [3]bool
	b  bool
}

/go/src/github.com/orijtech/structslop/testdata/struct/p.go:59:9: struct has size 40 (size class 48), could be 32 (size class 32), you'll save 33.33% if you rearrange it to:
struct {
	y uint64
	t *httptest.Server
	w uint64
	x uint32
	z uint32
}

/go/src/github.com/orijtech/structslop/testdata/struct/p.go:67:9: struct has size 40 (size class 48), could be 32 (size class 32), you'll save 33.33% if you rearrange it to:
struct {
	y uint64
	t *s
	w uint64
	x uint32
	z uint32
}

Example, for the first report above, the output meaning:

• The current struct size is 24, the size that the Go runtime will allocate for that struct is 32.
• The optimal struct size is 16, the size that the Go runtime will allocate for that struct is 16.
• The layout of optimal struct.
• The percentage savings with new struct layout.

That said, some structs may have a smaller size, but for efficiency, the Go runtime will allocate them in the same size class, then those structs are not considered sloppy:

type s1 struct {
	x uint32
	y uint64
	z *s
	t uint32
}

and:

type s2 struct {
	y uint64
	z *s
	x uint32
	t uint32
}

have the same size class 32, though s2 layout is only 24 byte in size.

However, you can still get this information when you want, using -verbose flag:

$ structslop -verbose ./testdata/src/verbose/p.go
/go/src/github.com/orijtech/structslop/testdata/src/verbose/p.go:17:8: struct has size 0 (size class 0)
/go/src/github.com/orijtech/structslop/testdata/src/verbose/p.go:19:9: struct has size 1 (size class 8)
/go/src/github.com/orijtech/structslop/testdata/src/verbose/p.go:23:9: struct has size 32 (size class 32), could be 24 (size class 32), optimal fields order:
struct {
	y uint64
	z *s
	x uint32
	t uint32
}

Note

For applying suggested fix, use -apply flag, instead of -fix.

Go 1.20+

Add test case to testdata/src/struct directory, then run:

go test

TODO