prealloc下載 - prealloc源代碼下載

prealloc

其他類別

1.0.0

下載

prealloc

PREALLOC是一個GO靜態分析工具，可查找有可能預先分配的切片聲明。

安裝

 go install github.com/alexkohler/prealloc@latest

用法

與其他GO靜態分析工具（例如Golint，Go Vet）類似，可以使用一個或多個以其導入路徑命名的文件名，目錄或軟件包來調用Prealloc。 Prealloc還支持...通配符。

 prealloc [flags] files/directories/packages

標誌

- 簡單（默認為true） - 僅在沒有返回/斷裂/持續/gotos的簡單循環上報告預定建議。將其設置為假可能會增加誤報。
-rangeloops （默認為true） - 報告範圍循環的預訂建議。
- forloops （默認為false） - 報告循環的預訂建議。默認情況下，這是錯誤的，因為通常會發生怪異的事情以供循環（至少來自我在標準庫中觀察到的內容）。
-set_exit_status （默認錯誤） - 如果找到任何問題，將退出狀態設置為1。

目的

儘管GO確實試圖通過提前增強容量來避免重新分配，但有時這不足以用於更長的切片。如果在創建時已知切片的大小，則應指定它。

考慮以下基準：（可以在此存儲庫中找到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

如您所見，不進行預先關注會導致性能命中，這主要是由於必須重新分配基礎陣列。上面的模式基准在GO中很常見：聲明切片，然後寫入某種範圍或將其附加或索引中的循環編寫。該工具的目的是標記slice/loop聲明，例如BenchmarkNoPreallocate中的一個聲明。

例子

GO 1.9.2來源的一些示例：

$ 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 )
}

即使將切片的大小預先分配到很小，仍然可以顯式指定容量，而不是讓它append以發現其需要進行預先分配。當然，在任何地方都不需要進行預先安排。該工具的工作只是為了幫助建議人們考慮進行預列的地方。

如何修復Prealloc的建議？

在聲明切片時，不要使用var的零值零值，而是使用GO的內置make功能初始化它，並傳遞適當的類型和長度。這個長度通常是您所範圍的。修復上面的示例看起來像：

 // 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：如果性能絕對至關重要，則使用copy而不是append較大切片可能更有效。作為參考，請參見以下基準：

 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