cmd/internal/dwarf: remove global encbuf

The global encbuf helped avoid allocations.
It is incompatible with a concurrent backend.
To avoid a performance regression while removing it,
introduce two optimizations.
First, re-use a buffer in dwarf.PutFunc.
Second, avoid a buffer entirely when the int
being encoded fits in seven bits, which is about 75%
of the time.

Passes toolstash-check.

Updates #15756


name       old alloc/op    new alloc/op    delta
Template      40.6MB ± 0%     40.6MB ± 0%  -0.08%  (p=0.001 n=8+9)
Unicode       29.9MB ± 0%     29.9MB ± 0%    ~     (p=0.068 n=8+10)
GoTypes        116MB ± 0%      116MB ± 0%  +0.05%  (p=0.043 n=10+9)
SSA            864MB ± 0%      864MB ± 0%  +0.01%  (p=0.010 n=10+9)
Flate         25.8MB ± 0%     25.8MB ± 0%    ~     (p=0.353 n=10+10)
GoParser      32.2MB ± 0%     32.2MB ± 0%    ~     (p=0.353 n=10+10)
Reflect       80.2MB ± 0%     80.2MB ± 0%    ~     (p=0.165 n=10+10)
Tar           27.0MB ± 0%     26.9MB ± 0%    ~     (p=0.143 n=10+10)
XML           42.8MB ± 0%     42.8MB ± 0%    ~     (p=0.400 n=10+9)

name       old allocs/op   new allocs/op   delta
Template        398k ± 0%       397k ± 0%  -0.20%  (p=0.002 n=8+9)
Unicode         320k ± 0%       321k ± 1%    ~     (p=0.122 n=8+10)
GoTypes        1.16M ± 0%      1.17M ± 0%    ~     (p=0.053 n=10+9)
SSA            7.65M ± 0%      7.65M ± 0%    ~     (p=0.122 n=10+8)
Flate           240k ± 1%       240k ± 1%    ~     (p=0.243 n=10+9)
GoParser        322k ± 1%       322k ± 1%    ~     (p=0.481 n=10+10)
Reflect        1.00M ± 0%      1.00M ± 0%    ~     (p=0.211 n=9+10)
Tar             256k ± 0%       255k ± 1%    ~     (p=0.052 n=10+10)
XML             400k ± 1%       400k ± 0%    ~     (p=0.631 n=10+10)


Change-Id: Ia39d9de09232fdbfc9c9cec14587bbf6939c9492
Reviewed-on: https://go-review.googlesource.com/38713
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/dwarf/dwarf.go

@@ -7,7 +7,9 @@
 // this package contains the shared code.
 package dwarf
 
-import "fmt"
+import (
+	"fmt"
+)
 
 // InfoPrefix is the prefix for all the symbols containing DWARF info entries.
 const InfoPrefix = "go.info."
@@ -68,17 +70,56 @@ func AppendSleb128(b []byte, v int64) []byte {
 	return b
 }
 
-var encbuf [20]byte
+// sevenbits contains all unsigned seven bit numbers, indexed by their value.
+var sevenbits = [...]byte{
+	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
+	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
+	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
+	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
+	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f,
+}
+
+// sevenBitU returns the unsigned LEB128 encoding of v if v is seven bits and nil otherwise.
+// The contents of the returned slice must not be modified.
+func sevenBitU(v int64) []byte {
+	if uint64(v) < uint64(len(sevenbits)) {
+		return sevenbits[v : v+1]
+	}
+	return nil
+}
 
-// AppendUleb128 appends v to s using DWARF's unsigned LEB128 encoding.
+// sevenBitS returns the signed LEB128 encoding of v if v is seven bits and nil otherwise.
+// The contents of the returned slice must not be modified.
+func sevenBitS(v int64) []byte {
+	if uint64(v) <= 63 {
+		return sevenbits[v : v+1]
+	}
+	if uint64(-v) <= 64 {
+		return sevenbits[128+v : 128+v+1]
+	}
+	return nil
+}
+
+// Uleb128put appends v to s using DWARF's unsigned LEB128 encoding.
 func Uleb128put(ctxt Context, s Sym, v int64) {
-	b := AppendUleb128(encbuf[:0], uint64(v))
+	b := sevenBitU(v)
+	if b == nil {
+		var encbuf [20]byte
+		b = AppendUleb128(encbuf[:0], uint64(v))
+	}
 	ctxt.AddBytes(s, b)
 }
 
-// AppendUleb128 appends v to s using DWARF's signed LEB128 encoding.
+// Sleb128put appends v to s using DWARF's signed LEB128 encoding.
 func Sleb128put(ctxt Context, s Sym, v int64) {
-	b := AppendSleb128(encbuf[:0], v)
+	b := sevenBitS(v)
+	if b == nil {
+		var encbuf [20]byte
+		b = AppendSleb128(encbuf[:0], v)
+	}
 	ctxt.AddBytes(s, b)
 }
 
@@ -396,7 +437,6 @@ func GetAbbrev() []byte {
 	for i := 1; i < DW_NABRV; i++ {
 		// See section 7.5.3
 		buf = AppendUleb128(buf, uint64(i))
-
 		buf = AppendUleb128(buf, uint64(abbrevs[i].tag))
 		buf = append(buf, byte(abbrevs[i].children))
 		for _, f := range abbrevs[i].attr {
@@ -572,6 +612,7 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 	}
 	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
 	names := make(map[string]bool)
+	var encbuf [20]byte
 	for _, v := range vars {
 		var n string
 		if names[v.Name] {
@@ -591,7 +632,6 @@ func PutFunc(ctxt Context, s Sym, name string, external bool, startPC Sym, size
 		}
 		putattr(ctxt, s, v.Abbrev, DW_FORM_block1, DW_CLS_BLOCK, int64(len(loc)), loc)
 		putattr(ctxt, s, v.Abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
-
 	}
 	Uleb128put(ctxt, s, 0)
 }

src/cmd/internal/dwarf/dwarf_test.go

+// Copyright 2017 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 dwarf
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestSevenBitEnc128(t *testing.T) {
+	t.Run("unsigned", func(t *testing.T) {
+		for v := int64(-255); v < 255; v++ {
+			s := sevenBitU(v)
+			if s == nil {
+				continue
+			}
+			b := AppendUleb128(nil, uint64(v))
+			if !reflect.DeepEqual(b, s) {
+				t.Errorf("sevenBitU(%d) = %v but AppendUleb128(%d) = %v", v, s, v, b)
+			}
+		}
+	})
+
+	t.Run("signed", func(t *testing.T) {
+		for v := int64(-255); v < 255; v++ {
+			s := sevenBitS(v)
+			if s == nil {
+				continue
+			}
+			b := AppendSleb128(nil, v)
+			if !reflect.DeepEqual(b, s) {
+				t.Errorf("sevenBitS(%d) = %v but AppendSleb128(%d) = %v", v, s, v, b)
+			}
+		}
+	})
+}