structslop

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

Installation

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

Usage

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.

Development

Go 1.20+

Running test

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

go test

Contributing