cmd/compile: reduce debugger-worsening line number churn

Reuse block head or preceding instruction's line number for
register allocator's spill, fill, copy, rematerialization
instructionsl; and also for phi, and for no-src-pos
instructions.  Assembler creates same line number tables
for copy-predecessor-line and for no-src-pos,
but copy-predecessor produces better-looking assembly
language output with -S and with GOSSAFUNC, and does not
require changes to tests of existing assembly language.

Split "copyInto" into two cases, one for register allocation,
one for otherwise.  This caused the test score line change
count to increase by one, which may reflect legitimately
useful information preserved.  Without any special treatment
for copyInto, the change count increases by 21 more, from
51 to 72 (i.e., quite a lot).

There is a test; using two naive "scores" for line number
churn, the old numbering is 2x or 4x worse.

Fixes #18902.

Change-Id: I0a0a69659d30ee4e5d10116a0dd2b8c5df8457b1
Reviewed-on: https://go-review.googlesource.com/36207
Run-TryBot: David Chase <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: dr2chase

src/cmd/compile/internal/gc/ssa.go

@@ -4346,6 +4346,29 @@ func (s *SSAGenState) SetPos(pos src.XPos) {
 	s.pp.pos = pos
 }
 
+// DebugFriendlySetPos sets the position subject to heuristics
+// that reduce "jumpy" line number churn when debugging.
+// Spill/fill/copy instructions from the register allocator,
+// phi functions, and instructions with a no-pos position
+// are examples of instructions that can cause churn.
+func (s *SSAGenState) DebugFriendlySetPosFrom(v *ssa.Value) {
+	// The two choices here are either to leave lineno unchanged,
+	// or to explicitly set it to src.NoXPos.  Leaving it unchanged
+	// (reusing the preceding line number) produces slightly better-
+	// looking assembly language output from the compiler, and is
+	// expected by some already-existing tests.
+	// The debug information appears to be the same in either case
+	switch v.Op {
+	case ssa.OpPhi, ssa.OpCopy, ssa.OpLoadReg, ssa.OpStoreReg:
+		// leave the position unchanged from beginning of block
+		// or previous line number.
+	default:
+		if v.Pos != src.NoXPos {
+			s.SetPos(v.Pos)
+		}
+	}
+}
+
 // genssa appends entries to pp for each instruction in f.
 func genssa(f *ssa.Func, pp *Progs) {
 	var s SSAGenState
@@ -4381,8 +4404,7 @@ func genssa(f *ssa.Func, pp *Progs) {
 		thearch.SSAMarkMoves(&s, b)
 		for _, v := range b.Values {
 			x := s.pp.next
-			s.SetPos(v.Pos)
-
+			s.DebugFriendlySetPosFrom(v)
 			switch v.Op {
 			case ssa.OpInitMem:
 				// memory arg needs no code

src/cmd/compile/internal/ssa/regalloc.go

@@ -468,7 +468,7 @@ func (s *regAllocState) allocValToReg(v *Value, mask regMask, nospill bool, pos
 		c = s.curBlock.NewValue1(pos, OpCopy, v.Type, s.regs[r2].c)
 	} else if v.rematerializeable() {
 		// Rematerialize instead of loading from the spill location.
-		c = v.copyInto(s.curBlock)
+		c = v.copyIntoNoXPos(s.curBlock)
 	} else {
 		// Load v from its spill location.
 		spill := s.makeSpill(v, s.curBlock)
@@ -1949,13 +1949,13 @@ func (e *edgeState) processDest(loc Location, vid ID, splice **Value, pos src.XP
 			e.s.f.Fatalf("can't find source for %s->%s: %s\n", e.p, e.b, v.LongString())
 		}
 		if dstReg {
-			x = v.copyInto(e.p)
+			x = v.copyIntoNoXPos(e.p)
 		} else {
 			// Rematerialize into stack slot. Need a free
 			// register to accomplish this.
 			e.erase(loc) // see pre-clobber comment below
 			r := e.findRegFor(v.Type)
-			x = v.copyInto(e.p)
+			x = v.copyIntoNoXPos(e.p)
 			e.set(r, vid, x, false, pos)
 			// Make sure we spill with the size of the slot, not the
 			// size of x (which might be wider due to our dropping

src/cmd/compile/internal/ssa/value.go

@@ -212,7 +212,21 @@ func (v *Value) reset(op Op) {
 
 // copyInto makes a new value identical to v and adds it to the end of b.
 func (v *Value) copyInto(b *Block) *Value {
-	c := b.NewValue0(v.Pos, v.Op, v.Type)
+	c := b.NewValue0(v.Pos, v.Op, v.Type) // Lose the position, this causes line number churn otherwise.
+	c.Aux = v.Aux
+	c.AuxInt = v.AuxInt
+	c.AddArgs(v.Args...)
+	for _, a := range v.Args {
+		if a.Type.IsMemory() {
+			v.Fatalf("can't move a value with a memory arg %s", v.LongString())
+		}
+	}
+	return c
+}
+
+// copyInto makes a new value identical to v and adds it to the end of b.
+func (v *Value) copyIntoNoXPos(b *Block) *Value {
+	c := b.NewValue0(src.NoXPos, v.Op, v.Type) // Lose the position, this causes line number churn otherwise.
 	c.Aux = v.Aux
 	c.AuxInt = v.AuxInt
 	c.AddArgs(v.Args...)

test/fixedbugs/issue18902.go

@@ -0,0 +1,137 @@
+// run
+// +build !nacl
+
+// Copyright 2016 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.
+
+// Runs a build -S to capture the assembly language
+// output, checks that the line numbers associated with
+// the stream of instructions do not change "too much".
+// The changes that fixes this (that reduces the amount
+// of change) does so by treating register spill, reload,
+// copy, and rematerializations as being "unimportant" and
+// just assigns them the line numbers of whatever "real"
+// instructions preceded them.
+
+// nacl is excluded because this runs a compiler.
+
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+)
+
+// updateEnv modifies env to ensure that key=val
+func updateEnv(env *[]string, key, val string) {
+	if val != "" {
+		var found bool
+		key = key + "="
+		for i, kv := range *env {
+			if strings.HasPrefix(kv, key) {
+				(*env)[i] = key + val
+				found = true
+				break
+			}
+		}
+		if !found {
+			*env = append(*env, key+val)
+		}
+	}
+}
+
+func main() {
+	testarch := os.Getenv("TESTARCH")     // Targets other platform in test compilation.
+	debug := os.Getenv("TESTDEBUG") != "" // Output the relevant assembly language.
+
+	cmd := exec.Command("go", "build", "-gcflags", "-S", "fixedbugs/issue18902b.go")
+	var buf bytes.Buffer
+	cmd.Stdout = &buf
+	cmd.Stderr = &buf
+	cmd.Env = os.Environ()
+
+	updateEnv(&cmd.Env, "GOARCH", testarch)
+
+	err := cmd.Run()
+	if err != nil {
+		fmt.Printf("%s\n%s", err, buf.Bytes())
+		return
+	}
+	begin := "\"\".(*gcSortBuf).flush" // Text at beginning of relevant dissassembly.
+	s := buf.String()
+	i := strings.Index(s, begin)
+	if i < 0 {
+		fmt.Printf("Failed to find expected symbol %s in output\n%s\n", begin, s)
+		return
+	}
+	s = s[i:]
+	r := strings.NewReader(s)
+	scanner := bufio.NewScanner(r)
+	first := true                         // The first line after the begin text will be skipped
+	beforeLineNumber := "issue18902b.go:" // Text preceding line number in each line.
+	lbln := len(beforeLineNumber)
+
+	var scannedCount, changes, sumdiffs float64
+
+	prevVal := 0
+	for scanner.Scan() {
+		line := scanner.Text()
+		if first {
+			first = false
+			continue
+		}
+		i = strings.Index(line, beforeLineNumber)
+		if i < 0 {
+			// Done reading lines
+			if scannedCount < 200 { // When test was written, 251 lines observed on amd64
+				fmt.Printf("Scanned only %d lines, was expecting more than 200", scannedCount)
+				return
+			}
+			// Note: when test was written, before changes=92, after=50 (was 62 w/o rematerialization NoXPos in *Value.copyInto())
+			// and before sumdiffs=784, after=180 (was 446 w/o rematerialization NoXPos in *Value.copyInto())
+			// Set the dividing line between pass and fail at the midpoint.
+			// Normalize against instruction count in case we unroll loops, etc.
+			if changes/scannedCount >= (50+92)/(2*scannedCount) || sumdiffs/scannedCount >= (180+784)/(2*scannedCount) {
+				fmt.Printf("Line numbers change too much, # of changes=%.f, sumdiffs=%.f, # of instructions=%.f\n", changes, sumdiffs, scannedCount)
+			}
+			return
+		}
+		scannedCount++
+		i += lbln
+		lineVal, err := strconv.Atoi(line[i : i+3])
+		if err != nil {
+			fmt.Printf("Expected 3-digit line number after %s in %s\n", beforeLineNumber, line)
+		}
+		if prevVal == 0 {
+			prevVal = lineVal
+		}
+		diff := lineVal - prevVal
+		if diff < 0 {
+			diff = -diff
+		}
+		if diff != 0 {
+			changes++
+			sumdiffs += float64(diff)
+		}
+		// If things change too much, set environment variable TESTDEBUG to help figure out what's up.
+		// The "before" behavior can be recreated in DebugFriendlySetPosFrom (currently in gc/ssa.go)
+		// by inserting unconditional
+		//   	s.SetPos(v.Pos)
+		// at the top of the function.
+
+		if debug {
+			fmt.Printf("%d %.f %.f %s\n", lineVal, changes, sumdiffs, line)
+		}
+		prevVal = lineVal
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Println("Reading standard input:", err)
+		return
+	}
+}

test/fixedbugs/issue18902b.go

@@ -0,0 +1,161 @@
+// skip
+
+// Copyright 2016 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 foo
+
+import (
+	"unsafe"
+)
+
+type gcMaxTreeNodeVal uint64
+
+var work struct {
+	full         uint64    // lock-free list of full blocks workbuf
+	empty        uint64    // lock-free list of empty blocks workbuf
+	pad0         [64]uint8 // prevents false-sharing between full/empty and nproc/nwait
+	bytesMarked  uint64
+	markrootNext uint32 // next markroot job
+	markrootJobs uint32 // number of markroot jobs
+	nproc        uint32
+	tstart       int64
+	nwait        uint32
+	ndone        uint32
+}
+
+type gcShardQueue1 struct {
+	partial *workbuf
+	full    *workbuf
+	n       uintptr
+	maxTree gcMaxTreeNodeVal
+}
+type gcShardQueue struct {
+	gcShardQueue1
+	pad [64 - unsafe.Sizeof(gcShardQueue1{})]byte
+}
+
+const gcSortBufPointers = (64 << 10) / 8
+
+type gcSortBuf struct {
+	buf *gcSortArray
+	tmp *gcSortArray
+	n   uintptr
+}
+
+//go:notinheap
+type gcSortArray [gcSortBufPointers]uintptr
+
+const (
+	_DebugGC             = 0
+	_ConcurrentSweep     = true
+	_FinBlockSize        = 4 * 1024
+	sweepMinHeapDistance = 1024 * 1024
+	gcShardShift         = 2 + 20
+	gcShardBytes         = 1 << gcShardShift
+)
+
+//go:notinheap
+type mheap struct {
+	shardQueues       []gcShardQueue
+	_                 uint32     // align uint64 fields on 32-bit for atomics
+	pagesInUse        uint64     // pages of spans in stats _MSpanInUse; R/W with mheap.lock
+	spanBytesAlloc    uint64     // bytes of spans allocated this cycle; updated atomically
+	pagesSwept        uint64     // pages swept this cycle; updated atomically
+	sweepPagesPerByte float64    // proportional sweep ratio; written with lock, read without
+	largefree         uint64     // bytes freed for large objects (>maxsmallsize)
+	nlargefree        uint64     // number of frees for large objects (>maxsmallsize)
+	nsmallfree        [67]uint64 // number of frees for small objects (<=maxsmallsize)
+	bitmap            uintptr    // Points to one byte past the end of the bitmap
+	bitmap_mapped     uintptr
+	arena_start       uintptr
+	arena_used        uintptr // always mHeap_Map{Bits,Spans} before updating
+	arena_end         uintptr
+	arena_reserved    bool
+}
+
+var mheap_ mheap
+
+type lfnode struct {
+	next    uint64
+	pushcnt uintptr
+}
+type workbufhdr struct {
+	node lfnode // must be first
+	next *workbuf
+	nobj int
+}
+
+//go:notinheap
+type workbuf struct {
+	workbufhdr
+	obj [(2048 - unsafe.Sizeof(workbufhdr{})) / 8]uintptr
+}
+
+//go:noinline
+func (b *workbuf) checkempty() {
+	if b.nobj != 0 {
+		b.nobj = 0
+	}
+}
+func putempty(b *workbuf) {
+	b.checkempty()
+	lfstackpush(&work.empty, &b.node)
+}
+
+//go:noinline
+func lfstackpush(head *uint64, node *lfnode) {
+}
+
+//go:noinline
+func (q *gcShardQueue) add(qidx uintptr, ptrs []uintptr, spare *workbuf) *workbuf {
+	return spare
+}
+
+func (b *gcSortBuf) flush() {
+	if b.n == 0 {
+		return
+	}
+	const sortDigitBits = 11
+	buf, tmp := b.buf[:b.n], b.tmp[:b.n]
+	moreBits := true
+	for shift := uint(gcShardShift); moreBits; shift += sortDigitBits {
+		const k = 1 << sortDigitBits
+		var pos [k]uint16
+		nshift := shift + sortDigitBits
+		nbits := buf[0] >> nshift
+		moreBits = false
+		for _, v := range buf {
+			pos[(v>>shift)%k]++
+			moreBits = moreBits || v>>nshift != nbits
+		}
+		var sum uint16
+		for i, count := range &pos {
+			pos[i] = sum
+			sum += count
+		}
+		for _, v := range buf {
+			digit := (v >> shift) % k
+			tmp[pos[digit]] = v
+			pos[digit]++
+		}
+		buf, tmp = tmp, buf
+	}
+	start := mheap_.arena_start
+	i0 := 0
+	shard0 := (buf[0] - start) / gcShardBytes
+	var spare *workbuf
+	for i, p := range buf {
+		shard := (p - start) / gcShardBytes
+		if shard != shard0 {
+			spare = mheap_.shardQueues[shard0].add(shard0, buf[i0:i], spare)
+			i0, shard0 = i, shard
+		}
+	}
+	spare = mheap_.shardQueues[shard0].add(shard0, buf[i0:], spare)
+	b.n = 0
+	if spare != nil {
+		putempty(spare)
+	}
+}