Inlining

Learn how the compiler inlines functions.

Inlining

Inlining replaces a call to a small function with its body; the call overhead disappears and more optimizations open up in the caller. The Go compiler gives each function a cost; those under the budget (80 nodes) are inlined.

func kare(x int) int { return x * x }

y := kare(5)  // after compiling: y := 5 * 5 → 25

The -gcflags=-m output shows the lines can inline kare and inlining call to kare.

Mid-stack inlining and PGO

Since Go 1.12, functions that call other functions (mid-stack) can also be inlined. Functions with loops, defer, recover or a large switch may exceed the cost. For calls marked "hot" in a PGO profile the budget is raised, so the most-run paths are optimized more aggressively.

Exercise Effect of noinline

Write topla(a, b int) int and toplaNI(a, b int) int marked //go:noinline. Measure both with testing.Benchmark and print whether both results ran more than 0 iterations (N > 0).

Show example solution

package main

import (
    "fmt"
    "testing"
)

var sink int

func topla(a, b int) int { return a + b }

//go:noinline
func toplaNI(a, b int) int { return a + b }

func main() {
    r1 := testing.Benchmark(func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            sink = topla(i, 1)
        }
    })
    r2 := testing.Benchmark(func(b *testing.B) {
        for i := 0; i < b.N; i++ {
            sink = toplaNI(i, 1)
        }
    })
    fmt.Println(r1.N > 0 && r2.N > 0)
}

Test yourself

Which directive prevents a function from being inlined?
- //go:noinline
- //go:inline
- //go:nosplit
- //go:embed
Show answer

//go:noinline — //go:noinline is usually used in benchmarks and tests to measure real call cost; Go has no //go:inline directive.

Disable inlining

go build -gcflags=____ .

Show answer

go build -gcflags=-l .

Flag and meaning

-gcflags=-m

Show decisions
-gcflags=-l

Inlining off
-gcflags=-N

Optimizations off
//go:noinline

Block one function

Next lesson: Compiler directives

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