[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>

3 files changed, 11 insertions, 99 deletions

diff --git a/src/cmd/compile/internal/typecheck/iimport.go b/src/cmd/compile/internal/typecheck/iimport.go
index 45a177951e..cafb18d7a8 100644
--- a/src/cmd/compile/internal/typecheck/iimport.go
+++ b/src/cmd/compile/internal/typecheck/iimport.go
@@ -12,7 +12,6 @@ import (
 	"encoding/binary"
 	"fmt"
 	"go/constant"
-	"io"
 	"math/big"
 	"os"
 	"strings"
@@ -20,8 +19,6 @@ import (
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/types"
-	"cmd/internal/bio"
-	"cmd/internal/goobj"
 	"cmd/internal/obj"
 	"cmd/internal/src"
 )
@@ -95,7 +92,7 @@ func importReaderFor(sym *types.Sym, importers map[*types.Sym]iimporterAndOffset
 }
 
 type intReader struct {
-	*bio.Reader
+	*strings.Reader
 	pkg *types.Pkg
 }
 
@@ -117,8 +114,8 @@ func (r *intReader) uint64() uint64 {
 	return i
 }
 
-func ReadImports(pkg *types.Pkg, in *bio.Reader) (fingerprint goobj.FingerprintType) {
-	ird := &intReader{in, pkg}
+func ReadImports(pkg *types.Pkg, data string) {
+	ird := &intReader{strings.NewReader(data), pkg}
 
 	version := ird.uint64()
 	switch version {
@@ -132,21 +129,15 @@ func ReadImports(pkg *types.Pkg, in *bio.Reader) (fingerprint goobj.FingerprintT
 		base.ErrorExit()
 	}
 
-	sLen := ird.uint64()
-	dLen := ird.uint64()
+	sLen := int64(ird.uint64())
+	dLen := int64(ird.uint64())
 
-	// Map string (and data) section into memory as a single large
-	// string. This reduces heap fragmentation and allows
-	// returning individual substrings very efficiently.
-	data, err := mapFile(in.File(), in.Offset(), int64(sLen+dLen))
-	if err != nil {
-		base.Errorf("import %q: mapping input: %v", pkg.Path, err)
-		base.ErrorExit()
-	}
-	stringData := data[:sLen]
-	declData := data[sLen:]
-
-	in.MustSeek(int64(sLen+dLen), os.SEEK_CUR)
+	// TODO(mdempsky): Replace os.SEEK_CUR with io.SeekCurrent after
+	// #44505 is fixed.
+	whence, _ := ird.Seek(0, os.SEEK_CUR)
+	stringData := data[whence : whence+sLen]
+	declData := data[whence+sLen : whence+sLen+dLen]
+	ird.Seek(sLen+dLen, os.SEEK_CUR)
 
 	p := &iimporter{
 		exportVersion: version,
@@ -208,14 +199,6 @@ func ReadImports(pkg *types.Pkg, in *bio.Reader) (fingerprint goobj.FingerprintT
 			}
 		}
 	}
-
-	// Fingerprint.
-	_, err = io.ReadFull(in, fingerprint[:])
-	if err != nil {
-		base.Errorf("import %s: error reading fingerprint", pkg.Path)
-		base.ErrorExit()
-	}
-	return fingerprint
 }
 
 type iimporter struct {
diff --git a/src/cmd/compile/internal/typecheck/mapfile_mmap.go b/src/cmd/compile/internal/typecheck/mapfile_mmap.go
deleted file mode 100644
index 298b385bcb..0000000000
--- a/src/cmd/compile/internal/typecheck/mapfile_mmap.go
+++ /dev/null
@@ -1,49 +0,0 @@
-// 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 typecheck
-
-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/typecheck/mapfile_read.go b/src/cmd/compile/internal/typecheck/mapfile_read.go
deleted file mode 100644
index 9637ab97ab..0000000000
--- a/src/cmd/compile/internal/typecheck/mapfile_read.go
+++ /dev/null
@@ -1,22 +0,0 @@
-// 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 typecheck
-
-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
-}