[dev.typeparams] cmd/compile: refactor import reading

This CL restructures the gcimports importer to mmap the export data
into memory as a string, and then pass that same string to both the
typecheck and types2 importers.

This is primarily motivated by preparation for unified IR; but it
should also improve performance (fewer string copies) and reduces
divergance between the two importers.

Passes toolstash -cmp.

Change-Id: I397f720693e9e6360bfcb5acb12609ab339d251f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325210
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

2 files changed, 71 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/base/mapfile_mmap.go b/src/cmd/compile/internal/base/mapfile_mmap.go
new file mode 100644
index 0000000000..c1616db8e9
--- /dev/null
+++ b/src/cmd/compile/internal/base/mapfile_mmap.go
@@ -0,0 +1,49 @@
+// Copyright 2018 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.
+
+//go:build darwin || dragonfly || freebsd || linux || netbsd || openbsd
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package base
+
+import (
+	"os"
+	"reflect"
+	"syscall"
+	"unsafe"
+)
+
+// TODO(mdempsky): Is there a higher-level abstraction that still
+// works well for iimport?
+
+// mapFile returns length bytes from the file starting at the
+// specified offset as a string.
+func MapFile(f *os.File, offset, length int64) (string, error) {
+	// POSIX mmap: "The implementation may require that off is a
+	// multiple of the page size."
+	x := offset & int64(os.Getpagesize()-1)
+	offset -= x
+	length += x
+
+	buf, err := syscall.Mmap(int(f.Fd()), offset, int(length), syscall.PROT_READ, syscall.MAP_SHARED)
+	keepAlive(f)
+	if err != nil {
+		return "", err
+	}
+
+	buf = buf[x:]
+	pSlice := (*reflect.SliceHeader)(unsafe.Pointer(&buf))
+
+	var res string
+	pString := (*reflect.StringHeader)(unsafe.Pointer(&res))
+
+	pString.Data = pSlice.Data
+	pString.Len = pSlice.Len
+
+	return res, nil
+}
+
+// keepAlive is a reimplementation of runtime.KeepAlive, which wasn't
+// added until Go 1.7, whereas we need to compile with Go 1.4.
+var keepAlive = func(interface{}) {}
diff --git a/src/cmd/compile/internal/base/mapfile_read.go b/src/cmd/compile/internal/base/mapfile_read.go
new file mode 100644
index 0000000000..01796a9bab
--- /dev/null
+++ b/src/cmd/compile/internal/base/mapfile_read.go
@@ -0,0 +1,22 @@
+// Copyright 2018 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.
+
+//go:build !darwin && !dragonfly && !freebsd && !linux && !netbsd && !openbsd
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd
+
+package base
+
+import (
+	"io"
+	"os"
+)
+
+func MapFile(f *os.File, offset, length int64) (string, error) {
+	buf := make([]byte, length)
+	_, err := io.ReadFull(io.NewSectionReader(f, offset, length), buf)
+	if err != nil {
+		return "", err
+	}
+	return string(buf), nil
+}