prealloc
1.0.0
PREALLOC是一个GO静态分析工具，可查找有可能预先分配的切片声明。
go install github.com/alexkohler/prealloc@latest
与其他GO静态分析工具（例如Golint，Go Vet）类似，可以使用一个或多个以其导入路径命名的文件名，目录或软件包来调用Prealloc。 Prealloc还支持...通配符。
prealloc [flags] files/directories/packages
尽管GO确实试图通过提前增强容量来避免重新分配，但有时这不足以用于更长的切片。如果在创建时已知切片的大小，则应指定它。
考虑以下基准：（可以在此存储库中找到prealloc_test.go）
import "testing"
func BenchmarkNoPreallocate ( b * testing. B ) {
existing := make ([] int64 , 10 , 10 )
b . ResetTimer ()
for i := 0 ; i < b . N ; i ++ {
// Don't preallocate our initial slice
var init [] int64
for _ , element := range existing {
init = append ( init , element )
}
}
}
func BenchmarkPreallocate ( b * testing. B ) {
existing := make ([] int64 , 10 , 10 )
b . ResetTimer ()
for i := 0 ; i < b . N ; i ++ {
// Preallocate our initial slice
init := make ([] int64 , 0 , len ( existing ))
for _ , element := range existing {
init = append ( init , element )
}
}
}$ go test -bench=. -benchmem
goos: linux
goarch: amd64
BenchmarkNoPreallocate-4 3000000 510 ns/op 248 B/op 5 allocs/op
BenchmarkPreallocate-4 20000000 111 ns/op 80 B/op 1 allocs/op如您所见，不进行预先关注会导致性能命中，这主要是由于必须重新分配基础阵列。上面的模式基准在GO中很常见：声明切片，然后写入某种范围或将其附加或索引中的循环编写。该工具的目的是标记slice/loop声明，例如BenchmarkNoPreallocate中的一个声明。
GO 1.9.2来源的一些示例：
$ prealloc go/src/....
archive/tar/reader_test.go:854 Consider preallocating ss
archive/zip/zip_test.go:201 Consider preallocating all
cmd/api/goapi.go:301 Consider preallocating missing
cmd/api/goapi.go:476 Consider preallocating files
cmd/asm/internal/asm/endtoend_test.go:345 Consider preallocating extra
cmd/cgo/main.go:60 Consider preallocating ks
cmd/cgo/ast.go:149 Consider preallocating pieces
cmd/compile/internal/ssa/flagalloc.go:64 Consider preallocating oldSched
cmd/compile/internal/ssa/regalloc.go:719 Consider preallocating phis
cmd/compile/internal/ssa/regalloc.go:718 Consider preallocating oldSched
cmd/compile/internal/ssa/regalloc.go:1674 Consider preallocating oldSched
cmd/compile/internal/ssa/gen/rulegen.go:145 Consider preallocating ops
cmd/compile/internal/ssa/gen/rulegen.go:145 Consider preallocating ops
cmd/dist/build.go:893 Consider preallocating all
cmd/dist/build.go:1246 Consider preallocating plats
cmd/dist/build.go:1264 Consider preallocating results
cmd/dist/buildgo.go:59 Consider preallocating list
cmd/doc/pkg.go:363 Consider preallocating names
cmd/fix/typecheck.go:219 Consider preallocating b
cmd/go/internal/base/path.go:34 Consider preallocating out
cmd/go/internal/get/get.go:175 Consider preallocating out
cmd/go/internal/load/pkg.go:1894 Consider preallocating dirent
cmd/go/internal/work/build.go:2402 Consider preallocating absOfiles
cmd/go/internal/work/build.go:2731 Consider preallocating absOfiles
cmd/internal/objfile/pe.go:48 Consider preallocating syms
cmd/internal/objfile/pe.go:38 Consider preallocating addrs
cmd/internal/objfile/goobj.go:43 Consider preallocating syms
cmd/internal/objfile/elf.go:35 Consider preallocating syms
cmd/link/internal/ld/lib.go:1070 Consider preallocating argv
cmd/vet/all/main.go:91 Consider preallocating pp
database/sql/sql.go:66 Consider preallocating list
debug/macho/file.go:506 Consider preallocating all
internal/trace/order.go:55 Consider preallocating batches
mime/quotedprintable/reader_test.go:191 Consider preallocating outcomes
net/dnsclient_unix_test.go:954 Consider preallocating confLines
net/interface_solaris.go:85 Consider preallocating ifat
net/interface_linux_test.go:91 Consider preallocating ifmat4
net/interface_linux_test.go:100 Consider preallocating ifmat6
net/internal/socktest/switch.go:34 Consider preallocating st
os/os_windows_test.go:766 Consider preallocating args
runtime/pprof/internal/profile/filter.go:77 Consider preallocating lines
runtime/pprof/internal/profile/profile.go:554 Consider preallocating names
text/template/parse/node.go:189 Consider preallocating decl // cmd/api/goapi.go:301
var missing [] string
for feature := range optionalSet {
missing = append ( missing , feature )
}
// cmd/fix/typecheck.go:219
var b []ast. Expr
for _ , x := range a {
b = append ( b , x )
}
// net/internal/socktest/switch.go:34
var st [] Stat
sw . smu . RLock ()
for _ , s := range sw . stats {
ns := * s
st = append ( st , ns )
}
sw . smu . RUnlock ()
// cmd/api/goapi.go:301
var missing [] string
for feature := range optionalSet {
missing = append ( missing , feature )
}即使将切片的大小预先分配到很小，仍然可以显式指定容量，而不是让它append以发现其需要进行预先分配。当然，在任何地方都不需要进行预先安排。该工具的工作只是为了帮助建议人们考虑进行预列的地方。
在声明切片时，不要使用var的零值零值，而是使用GO的内置make功能初始化它，并传递适当的类型和长度。这个长度通常是您所范围的。修复上面的示例看起来像：
// cmd/api/goapi.go:301
missing := make ([] string , 0 , len ( optionalSet ))
for feature := range optionalSet {
missing = append ( missing , feature )
}
// cmd/fix/typecheck.go:219
b := make ([]ast. Expr , 0 , len ( a ))
for _ , x := range a {
b = append ( b , x )
}
// net/internal/socktest/switch.go:34
st := make ([] Stat , 0 , len ( sw . stats ))
sw . smu . RLock ()
for _ , s := range sw . stats {
ns := * s
st = append ( st , ns )
}
sw . smu . RUnlock ()
// cmd/api/goapi.go:301
missing := make ([] string , 0 , len ( optionalSet ))
for feature := range optionalSet {
missing = append ( missing , feature )
}注意：如果性能绝对至关重要，则使用copy而不是append较大切片可能更有效。作为参考，请参见以下基准：
func BenchmarkSize200PreallocateCopy ( b * testing. B ) {
existing := make ([] int64 , 200 , 200 )
b . ResetTimer ()
for i := 0 ; i < b . N ; i ++ {
// Preallocate our initial slice
init := make ([] int64 , len ( existing ))
copy ( init , existing )
}
} $ go test -bench=. -benchmem
goos: linux
goarch: amd64
BenchmarkSize200NoPreallocate-4 500000 3080 ns/op 4088 B/op 9 allocs/op
BenchmarkSize200Preallocate-4 1000000 1163 ns/op 1792 B/op 1 allocs/op
BenchmarkSize200PreallocateCopy-4 2000000 807 ns/op 1792 B/op 1 allocs/op
欢迎拉请求！
如果您喜欢Prealloc，请查看我的其他静态分析工具！
