cmd/internal/src: cache prefixed filenames

CL 37234 introduced string concatenation into some hot code. 
This CL does that work earlier and caches the result.

Updates #19386

Performance impact vs master:

name       old time/op      new time/op      delta
Template        223ms ± 5%       216ms ± 5%   -2.98%  (p=0.001 n=20+20)
Unicode        98.7ms ± 4%      99.0ms ± 4%     ~     (p=0.749 n=20+19)
GoTypes         631ms ± 4%       626ms ± 4%     ~     (p=0.253 n=20+20)
Compiler        2.91s ± 1%       2.87s ± 3%   -1.11%  (p=0.005 n=18+20)
SSA             4.48s ± 2%       4.36s ± 2%   -2.77%  (p=0.000 n=20+20)
Flate           130ms ± 2%       129ms ± 6%     ~     (p=0.428 n=19+20)
GoParser        160ms ± 4%       157ms ± 3%   -1.62%  (p=0.005 n=20+18)
Reflect         395ms ± 2%       394ms ± 4%     ~     (p=0.445 n=20+20)
Tar             120ms ± 5%       118ms ± 6%     ~     (p=0.101 n=19+20)
XML             224ms ± 3%       223ms ± 3%     ~     (p=0.544 n=19+19)

name       old user-ns/op   new user-ns/op   delta
Template   291user-ms ± 5%  265user-ms ± 5%   -9.02%  (p=0.000 n=20+19)
Unicode    140user-ms ± 3%  139user-ms ± 8%     ~     (p=0.904 n=20+20)
GoTypes    844user-ms ± 3%  849user-ms ± 3%     ~     (p=0.251 n=20+18)
Compiler   4.06user-s ± 5%  3.98user-s ± 2%     ~     (p=0.056 n=20+20)
SSA        6.89user-s ± 5%  6.50user-s ± 3%   -5.61%  (p=0.000 n=20+20)
Flate      164user-ms ± 5%  163user-ms ± 4%     ~     (p=0.365 n=20+19)
GoParser   206user-ms ± 6%  204user-ms ± 4%     ~     (p=0.534 n=20+18)
Reflect    501user-ms ± 4%  505user-ms ± 5%     ~     (p=0.383 n=20+20)
Tar        151user-ms ± 3%  152user-ms ± 7%     ~     (p=0.798 n=17+20)
XML        283user-ms ± 7%  280user-ms ± 5%     ~     (p=0.301 n=20+20)

name       old alloc/op     new alloc/op     delta
Template       42.5MB ± 0%      40.2MB ± 0%   -5.59%  (p=0.000 n=20+20)
Unicode        31.7MB ± 0%      31.0MB ± 0%   -2.19%  (p=0.000 n=20+18)
GoTypes         124MB ± 0%       117MB ± 0%   -5.90%  (p=0.000 n=20+20)
Compiler        533MB ± 0%       490MB ± 0%   -8.07%  (p=0.000 n=20+20)
SSA             989MB ± 0%       893MB ± 0%   -9.74%  (p=0.000 n=20+20)
Flate          27.8MB ± 0%      26.1MB ± 0%   -5.92%  (p=0.000 n=20+20)
GoParser       34.3MB ± 0%      32.1MB ± 0%   -6.43%  (p=0.000 n=19+20)
Reflect        84.6MB ± 0%      81.4MB ± 0%   -3.84%  (p=0.000 n=20+20)
Tar            28.8MB ± 0%      27.7MB ± 0%   -3.89%  (p=0.000 n=20+20)
XML            47.2MB ± 0%      44.2MB ± 0%   -6.45%  (p=0.000 n=20+19)

name       old allocs/op    new allocs/op    delta
Template         420k ± 1%        381k ± 1%   -9.35%  (p=0.000 n=20+20)
Unicode          338k ± 1%        324k ± 1%   -4.29%  (p=0.000 n=20+19)
GoTypes         1.28M ± 0%       1.15M ± 0%  -10.30%  (p=0.000 n=20+20)
Compiler        5.06M ± 0%       4.41M ± 0%  -12.92%  (p=0.000 n=20+20)
SSA             9.14M ± 0%       7.91M ± 0%  -13.46%  (p=0.000 n=19+20)
Flate            267k ± 0%        241k ± 1%   -9.53%  (p=0.000 n=20+20)
GoParser         347k ± 1%        312k ± 0%  -10.15%  (p=0.000 n=19+20)
Reflect         1.07M ± 0%       1.00M ± 0%   -6.86%  (p=0.000 n=20+20)
Tar              274k ± 1%        256k ± 1%   -6.73%  (p=0.000 n=20+20)
XML              448k ± 0%        398k ± 0%  -11.17%  (p=0.000 n=20+18)


Performance impact when applied together with CL 37234
atop CL 37234's parent commit (i.e. as if it were
a part of CL 37234), to show that this commit
makes CL 37234 completely performance-neutral:

name       old time/op      new time/op      delta
Template        222ms ±14%       222ms ±14%    ~     (p=1.000 n=14+15)
Unicode         104ms ±18%       106ms ±18%    ~     (p=0.650 n=13+14)
GoTypes         653ms ± 7%       638ms ± 5%    ~     (p=0.145 n=14+12)
Compiler        3.10s ± 1%       3.13s ±10%    ~     (p=1.000 n=2+2)
SSA             4.73s ±11%       4.68s ±11%    ~     (p=0.567 n=15+15)
Flate           136ms ± 4%       133ms ± 7%    ~     (p=0.231 n=12+14)
GoParser        163ms ±11%       169ms ±10%    ~     (p=0.352 n=14+14)
Reflect         415ms ±15%       423ms ±20%    ~     (p=0.715 n=15+14)
Tar             133ms ±17%       130ms ±23%    ~     (p=0.252 n=14+15)
XML             236ms ±16%       235ms ±14%    ~     (p=0.874 n=14+14)

name       old user-ns/op   new user-ns/op   delta
Template   271user-ms ±10%  271user-ms ±10%    ~     (p=0.780 n=14+15)
Unicode    143user-ms ± 5%  146user-ms ±11%    ~     (p=0.432 n=12+14)
GoTypes    864user-ms ± 5%  866user-ms ± 9%    ~     (p=0.905 n=14+13)
Compiler   4.17user-s ± 1%  4.26user-s ± 7%    ~     (p=1.000 n=2+2)
SSA        6.79user-s ± 8%  6.79user-s ± 6%    ~     (p=0.902 n=15+15)
Flate      169user-ms ± 8%  164user-ms ± 5%  -3.13%  (p=0.014 n=14+14)
GoParser   212user-ms ± 7%  217user-ms ±22%    ~     (p=1.000 n=13+15)
Reflect    521user-ms ± 7%  533user-ms ±15%    ~     (p=0.511 n=14+14)
Tar        165user-ms ±17%  161user-ms ±15%    ~     (p=0.345 n=15+15)
XML        294user-ms ±11%  292user-ms ±10%    ~     (p=0.839 n=14+14)

name       old alloc/op     new alloc/op     delta
Template       39.9MB ± 0%      39.9MB ± 0%    ~     (p=0.621 n=15+14)
Unicode        31.0MB ± 0%      31.0MB ± 0%    ~     (p=0.098 n=13+15)
GoTypes         117MB ± 0%       117MB ± 0%    ~     (p=0.775 n=15+15)
Compiler        488MB ± 0%       488MB ± 0%    ~     (p=0.333 n=2+2)
SSA             892MB ± 0%       892MB ± 0%  +0.03%  (p=0.000 n=15+15)
Flate          26.1MB ± 0%      26.1MB ± 0%    ~     (p=0.098 n=15+15)
GoParser       31.8MB ± 0%      31.8MB ± 0%    ~     (p=0.525 n=15+13)
Reflect        81.2MB ± 0%      81.2MB ± 0%  +0.06%  (p=0.001 n=12+14)
Tar            27.5MB ± 0%      27.5MB ± 0%    ~     (p=0.595 n=15+15)
XML            44.1MB ± 0%      44.1MB ± 0%    ~     (p=0.486 n=15+15)

name       old allocs/op    new allocs/op    delta
Template         378k ± 1%        378k ± 0%    ~     (p=0.949 n=15+14)
Unicode          324k ± 0%        324k ± 1%    ~     (p=0.057 n=14+15)
GoTypes         1.15M ± 0%       1.15M ± 0%    ~     (p=0.461 n=15+15)
Compiler        4.39M ± 0%       4.39M ± 0%    ~     (p=0.333 n=2+2)
SSA             7.90M ± 0%       7.90M ± 0%  +0.06%  (p=0.008 n=15+15)
Flate            240k ± 1%        241k ± 0%    ~     (p=0.233 n=15+15)
GoParser         309k ± 1%        309k ± 0%    ~     (p=0.867 n=15+12)
Reflect         1.00M ± 0%       1.00M ± 0%    ~     (p=0.139 n=12+15)
Tar              254k ± 1%        253k ± 1%    ~     (p=0.345 n=15+15)
XML              398k ± 0%        397k ± 1%    ~     (p=0.267 n=15+15)


Change-Id: Ic999a0f456a371c99eebba0f9747263a13836e33
Reviewed-on: https://go-review.googlesource.com/37766
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/line.go

@@ -74,16 +74,13 @@ func (ctxt *Link) AddImport(pkg string) {
 	ctxt.Imports = append(ctxt.Imports, pkg)
 }
 
-const FileSymPrefix = "gofile.."
-
 func linkgetlineFromPos(ctxt *Link, xpos src.XPos) (f *LSym, l int32) {
 	pos := ctxt.PosTable.Pos(xpos)
-	filename := pos.AbsFilename()
-	if !pos.IsKnown() || filename == "" {
-		return Linklookup(ctxt, FileSymPrefix+"??", 0), 0
+	if !pos.IsKnown() {
+		pos = src.Pos{}
 	}
 	// TODO(gri) Should this use relative or absolute line number?
-	return Linklookup(ctxt, FileSymPrefix+filename, 0), int32(pos.RelLine())
+	return Linklookup(ctxt, pos.SymFilename(), 0), int32(pos.RelLine())
 }
 
 func fieldtrack(ctxt *Link, cursym *LSym) {

src/cmd/internal/obj/line_test.go

@@ -32,7 +32,7 @@ func TestLinkgetlineFromPos(t *testing.T) {
 	for _, test := range tests {
 		f, l := linkgetlineFromPos(ctxt, ctxt.PosTable.XPos(test.pos))
 		got := fmt.Sprintf("%s:%d", f.Name, l)
-		if got != FileSymPrefix+test.want {
+		if got != src.FileSymPrefix+test.want {
 			t.Errorf("linkgetline(%v) = %q, want %q", test.pos, got, test.want)
 		}
 	}

src/cmd/internal/src/pos.go

@@ -75,6 +75,10 @@ func (p Pos) RelLine() uint { b := p.base; return b.Line() + p.Line() - b.Pos().
 // AbsFilename() returns the absolute filename recorded with the position's base.
 func (p Pos) AbsFilename() string { return p.base.AbsFilename() }
 
+// SymFilename() returns the absolute filename recorded with the position's base,
+// prefixed by FileSymPrefix to make it appropriate for use as a linker symbol.
+func (p Pos) SymFilename() string { return p.base.SymFilename() }
+
 func (p Pos) String() string {
 	if !p.IsKnown() {
 		return "<unknown line number>"
@@ -117,6 +121,7 @@ type PosBase struct {
 	pos         Pos
 	filename    string // file name used to open source file, for error messages
 	absFilename string // absolute file name, for PC-Line tables
+	symFilename string // cached symbol file name, to avoid repeated string concatenation
 	line        uint   // relative line number at pos
 	inl         int    // inlining index (see cmd/internal/obj/inl.go)
 }
@@ -125,7 +130,12 @@ type PosBase struct {
 // absolute) filenames.
 func NewFileBase(filename, absFilename string) *PosBase {
 	if filename != "" {
-		base := &PosBase{filename: filename, absFilename: absFilename, inl: -1}
+		base := &PosBase{
+			filename:    filename,
+			absFilename: absFilename,
+			symFilename: FileSymPrefix + absFilename,
+			inl:         -1,
+		}
 		base.pos = MakePos(base, 0, 0)
 		return base
 	}
@@ -136,7 +146,7 @@ func NewFileBase(filename, absFilename string) *PosBase {
 //      //line filename:line
 // at position pos.
 func NewLinePragmaBase(pos Pos, filename string, line uint) *PosBase {
-	return &PosBase{pos, filename, filename, line - 1, -1}
+	return &PosBase{pos, filename, filename, FileSymPrefix + filename, line - 1, -1}
 }
 
 // NewInliningBase returns a copy of the old PosBase with the given inlining
@@ -185,6 +195,18 @@ func (b *PosBase) AbsFilename() string {
 	return ""
 }
 
+const FileSymPrefix = "gofile.."
+
+// SymFilename returns the absolute filename recorded with the base,
+// prefixed by FileSymPrefix to make it appropriate for use as a linker symbol.
+// If b is nil, SymFilename returns FileSymPrefix + "??".
+func (b *PosBase) SymFilename() string {
+	if b != nil {
+		return b.symFilename
+	}
+	return FileSymPrefix + "??"
+}
+
 // Line returns the line number recorded with the base.
 // If b == nil, the result is 0.
 func (b *PosBase) Line() uint {

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

@@ -6,6 +6,7 @@ package ld
 
 import (
 	"cmd/internal/obj"
+	"cmd/internal/src"
 	"log"
 	"os"
 	"path/filepath"
@@ -114,7 +115,7 @@ func numberfile(ctxt *Link, file *Symbol) {
 		ctxt.Filesyms = append(ctxt.Filesyms, file)
 		file.Value = int64(len(ctxt.Filesyms))
 		file.Type = obj.SFILEPATH
-		path := file.Name[len(obj.FileSymPrefix):]
+		path := file.Name[len(src.FileSymPrefix):]
 		file.Name = expandGoroot(path)
 	}
 }