copyfighter

Copyfighter statically analyzes Go code and reports functions that are passing large structs by value. It helps you help your code.

Every Go function call copies the values given to it, including structs. When large structs are passed around without using a pointer to them, the copying of new data in memory causes more allocations and more work for your garbage collector.

Copyfighter's static analysis will identify where large structs, without pointers, are being used as method receivers, function parameters and return values.

Install with go get or similar.

Example output

$ copyfighter /gopath/path/to/pkg
# parameter 'f' at index 0 should be made into a pointer
func CallsFoo(f Foo)

# receiver, and parameter 'o' at index 0 should be made into pointers
func (Foo).OnOtherToo(o other)

# receiver should be made into a pointer
func (other).OnStruct()

# receiver should be made into a pointer
func (other).OnStruct2()

$ copyfighter -max 32 path/to/pkg
# parameter 'f' at index 0 should be made into a pointer
func CallsFoo(f Foo)

# receiver should be made into a pointer
func (Foo).OnOtherToo(o other)

Defaults And Flags

By default, copyfighter assumes structs wider than 16 bytes (two words on x86_64) should not be copied. This can be adjusted with the -max flag. max should typically be set to some multiple of the word size. You can also adjust the word size and alignment offset for your preferred architecture with -wordSize and -maxAlign.

Flags like -max have to go before the package name.

FAQ

Why not just use Go's heap profiler to fix these problems when they show up?

Because copyfighter can find problems before you put your code in production. It's nice to prevent issues before they matter.