[dev.typeparams] cmd/compile: record writer's stack at export data sync points

This CL extends the unified export data format's existing sync
mechanism to save writer stacks, controlled by the -d=syncframes debug
flag. This allows readers to provide more details when reporting
desync errors, which should simplify development of the data format
and the various reader/writer implementations.

For example, CL 328051 updated reader and writer, but missed making a
similar change to the linker (fix in CL 328054). Re-reviewing the CL
in isolation after the failure, it was not immediately obvious what
was going wrong. But the pair of stack traces below identifies exactly
what happened: it should have updated linker.relocFuncExt to write out
the new sync marker too.

```
data sync error: package "internal/abi", section 6, index 4, offset 536

found UseReloc, written at:
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/encoder.go:221: (*encoder).reloc +0x44
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/linker.go:214: (*linker).relocFuncExt +0x580
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/linker.go:233: (*linker).relocTypeExt +0x234
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/linker.go:161: (*linker).relocObj +0x2198
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/linker.go:64: (*linker).relocIdx +0x196

expected ImplicitTypes, reading at:
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/reader.go:796: (*reader).implicitTypes +0x36
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/reader.go:810: (*reader).addBody +0x81
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/reader.go:727: (*reader).funcExt +0x542
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/reader.go:651: (*reader).method +0x324
	/home/mdempsky/wd/go/src/cmd/compile/internal/noder/reader.go:557: (*pkgReader).objIdx +0x2704
```

Change-Id: I911193edd2a965f81b7459f15fb613a773584685
Reviewed-on: https://go-review.googlesource.com/c/go/+/328909
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/base/debug.go

@@ -44,6 +44,7 @@ type DebugFlags struct {
 	Panic                int    `help:"show all compiler panics"`
 	Slice                int    `help:"print information about slice compilation"`
 	SoftFloat            int    `help:"force compiler to emit soft-float code"`
+	SyncFrames           int    `help:"how many writer stack frames to include at sync points in unified export data"`
 	TypeAssert           int    `help:"print information about type assertion inlining"`
 	TypecheckInl         int    `help:"eager typechecking of inline function bodies"`
 	Unified              int    `help:"enable unified IR construction"`

src/cmd/compile/internal/noder/decoder.go

@@ -13,6 +13,7 @@ import (
 	"go/token"
 	"math/big"
 	"os"
+	"runtime"
 	"strings"
 
 	"cmd/compile/internal/base"
@@ -131,17 +132,82 @@ func (r *decoder) checkErr(err error) {
 	}
 }
 
-func (r *decoder) sync(m syncMarker) {
-	if debug {
-		pos, err0 := r.data.Seek(0, os.SEEK_CUR)
-		x, err := r.data.ReadByte()
-		r.checkErr(err)
-		if x != byte(m) {
-			// TODO(mdempsky): Revisit this error message, and make it more
-			// useful (e.g., include r.p.pkgPath).
-			base.Fatalf("data sync error: found %v at %v (%v) in (%v:%v), but expected %v", syncMarker(x), pos, err0, r.k, r.idx, m)
-		}
+func (r *decoder) rawUvarint() uint64 {
+	x, err := binary.ReadUvarint(&r.data)
+	r.checkErr(err)
+	return x
+}
+
+func (r *decoder) rawVarint() int64 {
+	ux := r.rawUvarint()
+
+	// Zig-zag decode.
+	x := int64(ux >> 1)
+	if ux&1 != 0 {
+		x = ^x
 	}
+	return x
+}
+
+func (r *decoder) rawReloc(k reloc, idx int) int {
+	e := r.relocs[idx]
+	assert(e.kind == k)
+	return e.idx
+}
+
+func (r *decoder) sync(mWant syncMarker) {
+	if !enableSync {
+		return
+	}
+
+	pos, _ := r.data.Seek(0, os.SEEK_CUR) // TODO(mdempsky): io.SeekCurrent after #44505 is resolved
+	mHave := syncMarker(r.rawUvarint())
+	writerPCs := make([]int, r.rawUvarint())
+	for i := range writerPCs {
+		writerPCs[i] = int(r.rawUvarint())
+	}
+
+	if mHave == mWant {
+		return
+	}
+
+	// There's some tension here between printing:
+	//
+	// (1) full file paths that tools can recognize (e.g., so emacs
+	//     hyperlinks the "file:line" text for easy navigation), or
+	//
+	// (2) short file paths that are easier for humans to read (e.g., by
+	//     omitting redundant or irrelevant details, so it's easier to
+	//     focus on the useful bits that remain).
+	//
+	// The current formatting favors the former, as it seems more
+	// helpful in practice. But perhaps the formatting could be improved
+	// to better address both concerns. For example, use relative file
+	// paths if they would be shorter, or rewrite file paths to contain
+	// "$GOROOT" (like objabi.AbsFile does) if tools can be taught how
+	// to reliably expand that again.
+
+	fmt.Printf("export data desync: package %q, section %v, index %v, offset %v\n", r.common.pkgPath, r.k, r.idx, pos)
+
+	fmt.Printf("\nfound %v, written at:\n", mHave)
+	if len(writerPCs) == 0 {
+		fmt.Printf("\t[stack trace unavailable; recompile package %q with -d=syncframes]\n", r.common.pkgPath)
+	}
+	for _, pc := range writerPCs {
+		fmt.Printf("\t%s\n", r.common.stringIdx(r.rawReloc(relocString, pc)))
+	}
+
+	fmt.Printf("\nexpected %v, reading at:\n", mWant)
+	var readerPCs [32]uintptr // TODO(mdempsky): Dynamically size?
+	n := runtime.Callers(2, readerPCs[:])
+	for _, pc := range fmtFrames(readerPCs[:n]...) {
+		fmt.Printf("\t%s\n", pc)
+	}
+
+	// We already printed a stack trace for the reader, so now we can
+	// simply exit. Printing a second one with panic or base.Fatalf
+	// would just be noise.
+	os.Exit(1)
 }
 
 func (r *decoder) bool() bool {
@@ -154,16 +220,12 @@ func (r *decoder) bool() bool {
 
 func (r *decoder) int64() int64 {
 	r.sync(syncInt64)
-	x, err := binary.ReadVarint(&r.data)
-	r.checkErr(err)
-	return x
+	return r.rawVarint()
 }
 
 func (r *decoder) uint64() uint64 {
 	r.sync(syncUint64)
-	x, err := binary.ReadUvarint(&r.data)
-	r.checkErr(err)
-	return x
+	return r.rawUvarint()
 }
 
 func (r *decoder) len() int   { x := r.uint64(); v := int(x); assert(uint64(v) == x); return v }
@@ -177,11 +239,7 @@ func (r *decoder) code(mark syncMarker) int {
 
 func (r *decoder) reloc(k reloc) int {
 	r.sync(syncUseReloc)
-	idx := r.len()
-
-	e := r.relocs[idx]
-	assert(e.kind == k)
-	return e.idx
+	return r.rawReloc(k, r.len())
 }
 
 func (r *decoder) string() string {

src/cmd/compile/internal/noder/encoder.go

@@ -13,6 +13,7 @@ import (
 	"go/constant"
 	"io"
 	"math/big"
+	"runtime"
 
 	"cmd/compile/internal/base"
 )
@@ -93,6 +94,8 @@ type encoder struct {
 	relocs []relocEnt
 	data   bytes.Buffer
 
+	encodingRelocHeader bool
+
 	k   reloc
 	idx int
 }
@@ -107,6 +110,10 @@ func (w *encoder) flush() int {
 	// TODO(mdempsky): Consider writing these out separately so they're
 	// easier to strip, along with function bodies, so that we can prune
 	// down to just the data that's relevant to go/types.
+	if w.encodingRelocHeader {
+		base.Fatalf("encodingRelocHeader already true; recursive flush?")
+	}
+	w.encodingRelocHeader = true
 	w.sync(syncRelocs)
 	w.len(len(w.relocs))
 	for _, rent := range w.relocs {
@@ -128,10 +135,58 @@ func (w *encoder) checkErr(err error) {
 	}
 }
 
+func (w *encoder) rawUvarint(x uint64) {
+	var buf [binary.MaxVarintLen64]byte
+	n := binary.PutUvarint(buf[:], x)
+	_, err := w.data.Write(buf[:n])
+	w.checkErr(err)
+}
+
+func (w *encoder) rawVarint(x int64) {
+	// Zig-zag encode.
+	ux := uint64(x) << 1
+	if x < 0 {
+		ux = ^ux
+	}
+
+	w.rawUvarint(ux)
+}
+
+func (w *encoder) rawReloc(r reloc, idx int) int {
+	// TODO(mdempsky): Use map for lookup.
+	for i, rent := range w.relocs {
+		if rent.kind == r && rent.idx == idx {
+			return i
+		}
+	}
+
+	i := len(w.relocs)
+	w.relocs = append(w.relocs, relocEnt{r, idx})
+	return i
+}
+
 func (w *encoder) sync(m syncMarker) {
-	if debug {
-		err := w.data.WriteByte(byte(m))
-		w.checkErr(err)
+	if !enableSync {
+		return
+	}
+
+	// Writing out stack frame string references requires working
+	// relocations, but writing out the relocations themselves involves
+	// sync markers. To prevent infinite recursion, we simply trim the
+	// stack frame for sync markers within the relocation header.
+	var frames []string
+	if !w.encodingRelocHeader && base.Debug.SyncFrames > 0 {
+		pcs := make([]uintptr, base.Debug.SyncFrames)
+		n := runtime.Callers(2, pcs)
+		frames = fmtFrames(pcs[:n]...)
+	}
+
+	// TODO(mdempsky): Save space by writing out stack frames as a
+	// linked list so we can share common stack frames.
+	w.rawUvarint(uint64(m))
+	w.rawUvarint(uint64(len(frames)))
+	for _, frame := range frames {
+		w.rawUvarint(uint64(w.rawReloc(relocString, w.p.stringIdx(frame))))
 	}
 }
 
@@ -148,18 +203,12 @@ func (w *encoder) bool(b bool) bool {
 
 func (w *encoder) int64(x int64) {
 	w.sync(syncInt64)
-	var buf [binary.MaxVarintLen64]byte
-	n := binary.PutVarint(buf[:], x)
-	_, err := w.data.Write(buf[:n])
-	w.checkErr(err)
+	w.rawVarint(x)
 }
 
 func (w *encoder) uint64(x uint64) {
 	w.sync(syncUint64)
-	var buf [binary.MaxVarintLen64]byte
-	n := binary.PutUvarint(buf[:], x)
-	_, err := w.data.Write(buf[:n])
-	w.checkErr(err)
+	w.rawUvarint(x)
 }
 
 func (w *encoder) len(x int)   { assert(x >= 0); w.uint64(uint64(x)) }
@@ -168,17 +217,7 @@ func (w *encoder) uint(x uint) { w.uint64(uint64(x)) }
 
 func (w *encoder) reloc(r reloc, idx int) {
 	w.sync(syncUseReloc)
-
-	// TODO(mdempsky): Use map for lookup.
-	for i, rent := range w.relocs {
-		if rent.kind == r && rent.idx == idx {
-			w.len(i)
-			return
-		}
-	}
-
-	w.len(len(w.relocs))
-	w.relocs = append(w.relocs, relocEnt{r, idx})
+	w.len(w.rawReloc(r, idx))
 }
 
 func (w *encoder) code(c code) {

src/cmd/compile/internal/noder/frames_go1.go

@@ -0,0 +1,20 @@
+// Copyright 2021 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.
+
+// +build !go1.7
+
+// TODO(mdempsky): Remove after #44505 is resolved
+
+package noder
+
+import "runtime"
+
+func walkFrames(pcs []uintptr, visit frameVisitor) {
+	for _, pc := range pcs {
+		fn := runtime.FuncForPC(pc)
+		file, line := fn.FileLine(pc)
+
+		visit(file, line, fn.Name(), pc-fn.Entry())
+	}
+}

src/cmd/compile/internal/noder/frames_go17.go

@@ -0,0 +1,24 @@
+// Copyright 2021 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.
+
+// +build go1.7
+
+package noder
+
+import "runtime"
+
+func walkFrames(pcs []uintptr, visit frameVisitor) {
+	if len(pcs) == 0 {
+		return
+	}
+
+	frames := runtime.CallersFrames(pcs)
+	for {
+		frame, more := frames.Next()
+		visit(frame.File, frame.Line, frame.Function, frame.PC-frame.Entry)
+		if !more {
+			return
+		}
+	}
+}

src/cmd/compile/internal/noder/reader.go

@@ -910,7 +910,7 @@ func (r *reader) addLocal(name *ir.Name, ctxt ir.Class) {
 	assert(ctxt == ir.PAUTO || ctxt == ir.PPARAM || ctxt == ir.PPARAMOUT)
 
 	r.sync(syncAddLocal)
-	if debug {
+	if enableSync {
 		want := r.int()
 		if have := len(r.locals); have != want {
 			base.FatalfAt(name.Pos(), "locals table has desynced")

src/cmd/compile/internal/noder/sync.go

@@ -6,8 +6,39 @@
 
 package noder
 
-const debug = true
+import (
+	"fmt"
+	"strings"
+)
 
+// enableSync controls whether sync markers are written into unified
+// IR's export data format and also whether they're expected when
+// reading them back in. They're inessential to the correct
+// functioning of unified IR, but are helpful during development to
+// detect mistakes.
+//
+// When sync is enabled, writer stack frames will also be included in
+// the export data. Currently, a fixed number of frames are included,
+// controlled by -d=syncframes (default 0).
+const enableSync = true
+
+// fmtFrames formats a backtrace for reporting reader/writer desyncs.
+func fmtFrames(pcs ...uintptr) []string {
+	res := make([]string, 0, len(pcs))
+	walkFrames(pcs, func(file string, line int, name string, offset uintptr) {
+		// Trim package from function name. It's just redundant noise.
+		name = strings.TrimPrefix(name, "cmd/compile/internal/noder.")
+
+		res = append(res, fmt.Sprintf("%s:%v: %s +0x%v", file, line, name, offset))
+	})
+	return res
+}
+
+type frameVisitor func(file string, line int, name string, offset uintptr)
+
+// syncMarker is an enum type that represents markers that may be
+// written to export data to ensure the reader and writer stay
+// synchronized.
 type syncMarker int
 
 //go:generate stringer -type=syncMarker -trimprefix=sync

src/cmd/compile/internal/noder/writer.go

@@ -733,7 +733,7 @@ func (w *writer) funcarg(param *types2.Var, result bool) {
 func (w *writer) addLocal(obj types2.Object) {
 	w.sync(syncAddLocal)
 	idx := len(w.localsIdx)
-	if debug {
+	if enableSync {
 		w.int(idx)
 	}
 	w.localsIdx[obj] = idx