一、什么是函数内联
内联优化是什么？
内联(inlining)是编程语言编译器常用的优化手段，其优化的对象为函数，也称为函数内联。如果某函数F支持内联，则意味着编译器可以用F的函数体/函数定义替换掉对函数F进行调用的代码，以消除函数调用带来的额外开销，这个过程如下图所示：
但是内联并不是只有优点而无缺点，因为内联，其实就是将一个函数调用原地展开，替换成这个函数的实现。当该函数被多次调用，就会被多次展开，这会增加编译后二进制文件的大小。而非内联函数，只需要保存一份函数体的代码，然后进行调用。所以，在空间上，一般来说使用内联函数会导致生成的可执行文件变大。
如何判断函数是否支持内联
我们先看下面的一段代码，testInline1、testInline2、testInline3是否能够被内联，我们猜想的是testInline1能够被内联，testInline2和testInline3不能够被内联。
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package main
import (
"fmt"
)
func main() {
fmt.Println(testInline1(1, 2))
fmt.Println(testInline2(1, 2))
fmt.Println(testInline3(1, 2))
}
func testInline1(a, b int) int {
if a > b {
return a
}
return b
}
//go:noinline
func testInline2(a, b int) int {
if a > b {
return a
}
return b
}
func testInline3(a, b int) int {
for i := 0; i < 10; i++ {
fmt.Println(i)
}
if a > b {
return a
}
return b
}
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我们可以使用go tool compile查看函数是否能够被内联：
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go tool compile -m=2 test.go
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- testInline1符合我们预期，是不能被内联的
- testInline2由于被打上注释“//go:noinline”，所以也不会被内联
- testInline3则是因为函数过于复杂，超过阈值80了，所以也不能被内联
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$ go tool compile -m=2 test.go
test.go:15:6: can inline testInline1 with cost 8 as: func(int, int) int { if a > b { return a }; return b }
test.go:23:6: cannot inline testInline2: marked go:noinline
test.go:30:6: cannot inline testInline3: function too complex: cost 96 exceeds budget 80
test.go:32:14: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
test.go:9:6: cannot inline main: function too complex: cost 359 exceeds budget 80
test.go:10:25: inlining call to testInline1 func(int, int) int { if a > b { return a }; return b }
test.go:10:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
test.go:11:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
test.go:12:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
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方法2：使用go build
由于go tool compile仅支持单个文件,使用go build编译一个目录查询函数是否被内联
执行如下命令：
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$ go build -gcflags="-m=2" .
.\test.go:15:6: can inline testInline1 with cost 8 as: func(int, int) int { if a > b { return a }; return b }
.\test.go:23:6: cannot inline testInline2: marked go:noinline
.\test.go:30:6: cannot inline testInline3: function too complex: cost 96 exceeds budget 80
.\test.go:32:14: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
.\test.go:9:6: cannot inline main: function too complex: cost 359 exceeds budget 80
.\test.go:10:25: inlining call to testInline1 func(int, int) int { if a > b { return a }; return b }
.\test.go:10:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
.\test.go:11:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
.\test.go:12:13: inlining call to fmt.Println func(...interface {}) (int, error) { var fmt..autotmp_3 int; fmt..autotmp_3 = <nil>; var fmt..autotmp_4 error; fmt..autotmp_4 = <nil>; fmt..autotmp_3, fmt..autotmp_4 = fmt.Fprintln(io.Writer(os.Stdout), fmt.a...); return fmt..autotmp_3, fmt..autotmp_4 }
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