cmd/link: MapViewOfFile output file

CL 170738 used mmap for writing most of the output file content.

This change implements similar functionality for Windows.

The output of

compilebench -count=5

command before and after this change

name                      old time/op       new time/op       delta
Template                        254ms ±14%        239ms ±10%    ~     (p=0.222 n=5+5)
Unicode                         119ms ±14%        113ms ±12%    ~     (p=0.421 n=5+5)
GoTypes                         892ms ±23%        850ms ± 1%    ~     (p=0.841 n=5+5)
Compiler                        3.86s ± 2%        3.82s ± 1%    ~     (p=0.222 n=5+5)
SSA                             12.6s ± 1%        12.6s ± 1%    ~     (p=0.095 n=5+5)
Flate                           162ms ±18%        149ms ± 1%  -7.91%  (p=0.016 n=5+5)
GoParser                        199ms ±12%        184ms ± 1%    ~     (p=0.056 n=5+5)
Reflect                         524ms ±13%        507ms ± 3%    ~     (p=0.421 n=5+5)
Tar                             207ms ± 7%        198ms ± 0%  -4.58%  (p=0.016 n=5+4)
XML                             305ms ± 6%        299ms ± 5%    ~     (p=0.690 n=5+5)
LinkCompiler                    1.14s ±11%        1.14s ± 3%    ~     (p=0.222 n=5+5)
ExternalLinkCompiler            2.80s ± 5%        2.92s ±13%    ~     (p=0.222 n=5+5)
LinkWithoutDebugCompiler        727ms ± 2%        750ms ± 7%    ~     (p=0.151 n=5+5)
StdCmd                          44.0s ± 8%        43.3s ± 2%    ~     (p=1.000 n=5+5)

name                      old user-time/op  new user-time/op  delta
Template                        300ms ±27%        259ms ±34%    ~     (p=0.341 n=5+5)
Unicode                         134ms ±51%        144ms ±67%    ~     (p=0.548 n=5+5)
GoTypes                         1.05s ±10%        1.03s ± 6%    ~     (p=0.968 n=5+5)
Compiler                        5.01s ± 3%        4.88s ± 3%    ~     (p=0.286 n=5+5)
SSA                             16.8s ± 1%        16.7s ± 1%  -0.95%  (p=0.008 n=5+5)
Flate                           178ms ±67%        181ms ±38%    ~     (p=0.849 n=5+5)
GoParser                        231ms ±32%        219ms ±21%    ~     (p=0.810 n=5+5)
Reflect                         634ms ±33%        650ms ± 6%    ~     (p=0.135 n=5+5)
Tar                             219ms ±36%        231ms ±19%    ~     (p=0.905 n=5+5)
XML                             378ms ±20%        366ms ±23%    ~     (p=0.913 n=5+5)
LinkCompiler                    1.34s ±15%        1.32s ±10%    ~     (p=0.730 n=5+5)
ExternalLinkCompiler            1.22s ±13%        1.18s ±15%    ~     (p=0.873 n=5+5)
LinkWithoutDebugCompiler        847ms ±13%        841ms ±21%    ~     (p=0.667 n=5+5)

name                      old text-bytes    new text-bytes    delta
HelloSize                       767kB ± 0%        767kB ± 0%    ~     (all equal)
CmdGoSize                      10.6MB ± 0%       10.6MB ± 0%    ~     (all equal)

name                      old data-bytes    new data-bytes    delta
HelloSize                      10.1kB ± 0%       10.1kB ± 0%    ~     (all equal)
CmdGoSize                       310kB ± 0%        310kB ± 0%    ~     (all equal)

name                      old bss-bytes     new bss-bytes     delta
HelloSize                       0.00B             0.00B         ~     (all equal)
CmdGoSize                       0.00B             0.00B         ~     (all equal)

name                      old exe-bytes     new exe-bytes     delta
HelloSize                      1.10MB ± 0%       1.10MB ± 0%    ~     (all equal)
CmdGoSize                      14.7MB ± 0%       14.7MB ± 0%    ~     (all equal)

Change-Id: I653f63213b9cc8a4b05f71938e34b5d53b05e3f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/196846
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/link/internal/ld/outbuf_nommap.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !darwin,!dragonfly,!freebsd,!linux,!openbsd
+// +build !darwin,!dragonfly,!freebsd,!linux,!openbsd,!windows
 
 package ld
 

src/cmd/link/internal/ld/outbuf_windows.go

+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ld
+
+import (
+	"reflect"
+	"syscall"
+	"unsafe"
+)
+
+func (out *OutBuf) Mmap(filesize uint64) error {
+	err := out.f.Truncate(int64(filesize))
+	if err != nil {
+		Exitf("resize output file failed: %v", err)
+	}
+
+	low, high := uint32(filesize), uint32(filesize>>32)
+	fmap, err := syscall.CreateFileMapping(syscall.Handle(out.f.Fd()), nil, syscall.PAGE_READONLY, high, low, nil)
+	if err != nil {
+		return err
+	}
+	defer syscall.CloseHandle(fmap)
+
+	ptr, err := syscall.MapViewOfFile(fmap, syscall.FILE_MAP_READ|syscall.FILE_MAP_WRITE, 0, 0, uintptr(filesize))
+	if err != nil {
+		return err
+	}
+	*(*reflect.SliceHeader)(unsafe.Pointer(&out.buf)) = reflect.SliceHeader{Data: ptr, Len: int(filesize), Cap: int(filesize)}
+	return nil
+}
+
+func (out *OutBuf) Munmap() {
+	if out.buf == nil {
+		return
+	}
+	err := syscall.UnmapViewOfFile(uintptr(unsafe.Pointer(&out.buf[0])))
+	if err != nil {
+		Exitf("UnmapViewOfFile failed: %v", err)
+	}
+}
+
+func (out *OutBuf) Msync() error {
+	// does nothing on windows
+	return nil
+}