From: Robert Findley <rfindley@google.com>
Date: Wed, 20 Oct 2021 18:54:13 +0000 (-0400)
Subject: go/internal/gcimporter: avoid setting unnecessary lines in fakeFileSet
X-Git-Tag: go1.18beta1~825
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=4320949f36f5f523a08ce8da6bdcd35ef51b8941;p=gostls13.git

go/internal/gcimporter: avoid setting unnecessary lines in fakeFileSet

This is a clean port of CL 357291 from x/tools.

For #46586

Change-Id: Ib22087ae7fe8477d368acd230b263b83cdad4d36
Reviewed-on: https://go-review.googlesource.com/c/go/+/357429
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
---

diff --git a/src/go/internal/gcimporter/iimport.go b/src/go/internal/gcimporter/iimport.go
index 9aae2a31f3..0771fa3c26 100644
--- a/src/go/internal/gcimporter/iimport.go
+++ b/src/go/internal/gcimporter/iimport.go
@@ -135,9 +135,10 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, dataRea
 
 		fake: fakeFileSet{
 			fset:  fset,
-			files: make(map[string]*token.File),
+			files: make(map[string]*fileInfo),
 		},
 	}
+	defer p.fake.setLines() // set lines for files in fset
 
 	for i, pt := range predeclared {
 		p.typCache[uint64(i)] = pt
diff --git a/src/go/internal/gcimporter/support.go b/src/go/internal/gcimporter/support.go
index 09810dd85b..5aef63ec1e 100644
--- a/src/go/internal/gcimporter/support.go
+++ b/src/go/internal/gcimporter/support.go
@@ -25,37 +25,50 @@ const deltaNewFile = -64
 // Synthesize a token.Pos
 type fakeFileSet struct {
 	fset  *token.FileSet
-	files map[string]*token.File
+	files map[string]*fileInfo
 }
 
+type fileInfo struct {
+	file     *token.File
+	lastline int
+}
+
+const maxlines = 64 * 1024
+
 func (s *fakeFileSet) pos(file string, line, column int) token.Pos {
 	// TODO(mdempsky): Make use of column.
 
-	// Since we don't know the set of needed file positions, we
-	// reserve maxlines positions per file.
-	const maxlines = 64 * 1024
+	// Since we don't know the set of needed file positions, we reserve
+	// maxlines positions per file. We delay calling token.File.SetLines until
+	// all positions have been calculated (by way of fakeFileSet.setLines), so
+	// that we can avoid setting unnecessary lines. See also golang/go#46586.
 	f := s.files[file]
 	if f == nil {
-		f = s.fset.AddFile(file, -1, maxlines)
+		f = &fileInfo{file: s.fset.AddFile(file, -1, maxlines)}
 		s.files[file] = f
-		// Allocate the fake linebreak indices on first use.
-		// TODO(adonovan): opt: save ~512KB using a more complex scheme?
-		fakeLinesOnce.Do(func() {
-			fakeLines = make([]int, maxlines)
-			for i := range fakeLines {
-				fakeLines[i] = i
-			}
-		})
-		f.SetLines(fakeLines)
 	}
 
 	if line > maxlines {
 		line = 1
 	}
+	if line > f.lastline {
+		f.lastline = line
+	}
 
-	// Treat the file as if it contained only newlines
-	// and column=1: use the line number as the offset.
-	return f.Pos(line - 1)
+	// Return a fake position assuming that f.file consists only of newlines.
+	return token.Pos(f.file.Base() + line - 1)
+}
+
+func (s *fakeFileSet) setLines() {
+	fakeLinesOnce.Do(func() {
+		fakeLines = make([]int, maxlines)
+		for i := range fakeLines {
+			fakeLines[i] = i
+		}
+	})
+	for _, f := range s.files {
+		f.file.SetLines(fakeLines[:f.lastline])
+	}
 }
 
 var (