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Commit 71446e9

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os: optimise cgo clearenv
For programs with very large environments, calling unsetenv(3) for each environment variable can be fairly expensive because of CGo overhead, but clearenv(3) is much faster. The only thing we have to track is whether GODEBUG is being unset by the operation, which can be done very quickly without resorting to doing unsetenv(3) for every variable. This change makes os.Clearenv() ~30% faster when run an environment with 1000 environment variable set (and gets better with more environment variables): cpu: AMD Ryzen 7 7840U w/ Radeon 780M Graphics │ old.txt │ new.txt │ │ sec/op │ sec/op vs base │ Env-16 213.6μ ± 1% 185.1μ ± 3% -13.32% (p=0.000 n=10) (The time taken to set up 1000 variables in this benchmark is 117.2μs, so while the naive timing is 13% the speedup for os.Clearenv() itself the speedup is from 96.4μs to 67.9μs which is ~30%.) Change-Id: I589794845854e1721e6fc5badc671474c2bbd037
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6 files changed

+87
-3
lines changed

6 files changed

+87
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lines changed

‎src/runtime/cgo/clearenv.go

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// Copyright 2024 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build linux
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package cgo
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import _ "unsafe" // for go:linkname
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//go:cgo_import_static x_cgo_clearenv
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//go:linkname x_cgo_clearenv x_cgo_clearenv
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//go:linkname _cgo_clearenv runtime._cgo_clearenv
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var x_cgo_clearenv byte
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var _cgo_clearenv = &x_cgo_clearenv

‎src/runtime/cgo/gcc_clearenv.c

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// Copyright 2024 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build linux
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#include "libcgo.h"
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#include <stdlib.h>
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/* Stub for calling clearenv */
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void
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x_cgo_clearenv(void **_unused)
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{
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_cgo_tsan_acquire();
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clearenv();
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_cgo_tsan_release();
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}
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‎src/runtime/runtime_clearenv.go

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// Copyright 2024 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build linux
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package runtime
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import "unsafe"
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var _cgo_clearenv unsafe.Pointer // pointer to C function
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// Clear the C environment if cgo is loaded.
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func clearenv_c() {
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if _cgo_clearenv == nil {
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return
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}
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asmcgocall(_cgo_clearenv, nil)
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}
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//go:linkname syscall_runtimeClearenv syscall.runtimeClearenv
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func syscall_runtimeClearenv(env map[string]int) {
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clearenv_c()
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// Did we just unset GODEBUG?
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if _, ok := env["GODEBUG"]; ok {
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godebugEnv.Store(nil)
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godebugNotify(true)
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}
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}

‎src/runtime/runtime_noclearenv.go

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// Copyright 2024 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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//go:build !linux
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package runtime
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import _ "unsafe" // for go:linkname
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//go:linkname syscall_runtimeClearenv syscall.runtimeClearenv
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func syscall_runtimeClearenv(env map[string]int) {
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// The system doesn't have clearenv(3) so emulate it by unsetting all of
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// the variables manually.
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for k := range env {
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syscall_runtimeUnsetenv(k)
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}
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}

‎src/syscall/env_unix.go

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@@ -129,9 +129,8 @@ func Clearenv() {
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envLock.Lock()
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defer envLock.Unlock()
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for k := range env {
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runtimeUnsetenv(k)
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}
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runtimeClearenv(env)
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env = make(map[string]int)
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envs = []string{}
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}

‎src/syscall/syscall.go

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// runtimeSetenv and runtimeUnsetenv are provided by the runtime.
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func runtimeSetenv(k, v string)
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func runtimeUnsetenv(k string)
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// runtimeClearenv is provided by the runtime (on platforms without
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// clearenv(3), it is just a wrapper around runtimeUnsetenv).
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func runtimeClearenv(env map[string]int)

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