cmd/compile, runtime: track argument stack slot liveness

Currently, for stack traces (e.g. at panic or when runtime.Stack
is called), we print argument values from the stack. With register
ABI, we may never store the argument to stack therefore the
argument value on stack may be meaningless. This causes confusion.

This CL makes the compiler keep trace of which argument stack
slots are meaningful. If it is meaningful, it will be printed in
stack traces as before. If it may not be meaningful, it will be
printed as the stack value with a question mark ("?"). In general,
the value could be meaningful on some code paths but not others
depending on the execution, and the compiler couldn't know
statically, so we still print the stack value, instead of not
printing it at all. Also note that if the argument variable is
updated in the function body the printed value may be stale (like
before register ABI) but still considered meaningful.

Arguments passed on stack are always meaningful therefore always
printed without a question mark. Results are never printed, as
before.

(Due to a bug in the compiler we sometimes don't spill args into
their dedicated spill slots (as we should), causing it having
fewer meaningful values than it should be.)

This increases binary sizes a bit:
            old       new
hello      1129760   1142080  +1.09%
cmd/go    13932320  14088016  +1.12%
cmd/link   6267696   6329168  +0.98%

Fixes #45728.

Change-Id: I308a0402e5c5ab94ca0953f8bd85a56acd28f58c
Reviewed-on: https://go-review.googlesource.com/c/go/+/352057
Trust: Cherry Mui <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: cherrymui

src/cmd/compile/internal/bitvec/bv.go

@@ -128,10 +128,21 @@ func (bv BitVec) IsEmpty() bool {
 	return true
 }
 
+func (bv BitVec) Count() int {
+	n := 0
+	for _, x := range bv.B {
+		n += bits.OnesCount32(x)
+	}
+	return n
+}
+
 func (bv BitVec) Not() {
 	for i, x := range bv.B {
 		bv.B[i] = ^x
 	}
+	if bv.N%wordBits != 0 {
+		bv.B[len(bv.B)-1] &= 1<<uint(bv.N%wordBits) - 1 // clear bits past N in the last word
+	}
 }
 
 // union

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

@@ -249,8 +249,7 @@ func addGCLocals() {
 			}
 		}
 		if x := fn.StackObjects; x != nil {
-			attr := int16(obj.RODATA)
-			objw.Global(x, int32(len(x.P)), attr)
+			objw.Global(x, int32(len(x.P)), obj.RODATA)
 			x.Set(obj.AttrStatic, true)
 		}
 		if x := fn.OpenCodedDeferInfo; x != nil {
@@ -260,6 +259,10 @@ func addGCLocals() {
 			objw.Global(x, int32(len(x.P)), obj.RODATA|obj.DUPOK)
 			x.Set(obj.AttrStatic, true)
 		}
+		if x := fn.ArgLiveInfo; x != nil {
+			objw.Global(x, int32(len(x.P)), obj.RODATA|obj.DUPOK)
+			x.Set(obj.AttrStatic, true)
+		}
 	}
 }
 

src/cmd/compile/internal/liveness/arg.go

@@ -0,0 +1,339 @@
+// 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.
+
+package liveness
+
+import (
+	"fmt"
+
+	"cmd/compile/internal/base"
+	"cmd/compile/internal/bitvec"
+	"cmd/compile/internal/ir"
+	"cmd/compile/internal/objw"
+	"cmd/compile/internal/ssa"
+	"cmd/internal/obj"
+	"cmd/internal/objabi"
+)
+
+// Argument liveness tracking.
+//
+// For arguments passed in registers, this file tracks if their spill slots
+// are live for runtime traceback. An argument spill slot is live at a PC
+// if we know that an actual value has stored into it at or before this point.
+//
+// Stack args are always live and not tracked in this code. Stack args are
+// laid out before register spill slots, so we emit the smallest offset that
+// needs tracking. Slots before that offset are always live. That offset is
+// usually the offset of the first spill slot. But if the first spill slot is
+// always live (e.g. if it is address-taken), it will be the offset of a later
+// one.
+//
+// The liveness information is emitted as a FUNCDATA and a PCDATA.
+//
+// FUNCDATA format:
+// - start (smallest) offset that needs tracking (1 byte)
+// - a list of bitmaps.
+//   In a bitmap bit i is set if the i-th spill slot is live.
+//
+// At a PC where the liveness info changes, a PCDATA indicates the
+// byte offset of the liveness map in the FUNCDATA. PCDATA -1 is a
+// special case indicating all slots are live (for binary size
+// saving).
+
+const allLiveIdx = -1
+
+// name and offset
+type nameOff struct {
+	n   *ir.Name
+	off int64
+}
+
+func (a nameOff) FrameOffset() int64 { return a.n.FrameOffset() + a.off }
+func (a nameOff) String() string     { return fmt.Sprintf("%v+%d", a.n, a.off) }
+
+type blockArgEffects struct {
+	livein  bitvec.BitVec // variables live at block entry
+	liveout bitvec.BitVec // variables live at block exit
+}
+
+type argLiveness struct {
+	fn   *ir.Func
+	f    *ssa.Func
+	args []nameOff         // name and offset of spill slots
+	idx  map[nameOff]int32 // index in args
+
+	be []blockArgEffects // indexed by block ID
+
+	bvset bvecSet // Set of liveness bitmaps, used for uniquifying.
+
+	// Liveness map indices at each Value (where it changes) and Block entry.
+	// During the computation the indices are temporarily index to bvset.
+	// At the end they will be index (offset) to the output funcdata (changed
+	// in (*argLiveness).emit).
+	blockIdx map[ssa.ID]int
+	valueIdx map[ssa.ID]int
+}
+
+// ArgLiveness computes the liveness information of register argument spill slots.
+// An argument's spill slot is "live" if we know it contains a meaningful value,
+// that is, we have stored the register value to it.
+// Returns the liveness map indices at each Block entry and at each Value (where
+// it changes).
+func ArgLiveness(fn *ir.Func, f *ssa.Func, pp *objw.Progs) (blockIdx, valueIdx map[ssa.ID]int) {
+	if f.OwnAux.ABIInfo().InRegistersUsed() == 0 || base.Flag.N != 0 {
+		// No register args. Nothing to emit.
+		// Or if -N is used we spill everything upfront so it is always live.
+		return nil, nil
+	}
+
+	lv := &argLiveness{
+		fn:       fn,
+		f:        f,
+		idx:      make(map[nameOff]int32),
+		be:       make([]blockArgEffects, f.NumBlocks()),
+		blockIdx: make(map[ssa.ID]int),
+		valueIdx: make(map[ssa.ID]int),
+	}
+	// Gather all register arg spill slots.
+	for _, a := range f.OwnAux.ABIInfo().InParams() {
+		n, ok := a.Name.(*ir.Name)
+		if !ok || len(a.Registers) == 0 {
+			continue
+		}
+		_, offs := a.RegisterTypesAndOffsets()
+		for _, off := range offs {
+			if n.FrameOffset()+off > 0xff {
+				// We only print a limited number of args, with stack
+				// offsets no larger than 255.
+				continue
+			}
+			lv.args = append(lv.args, nameOff{n, off})
+		}
+	}
+	if len(lv.args) > 10 {
+		lv.args = lv.args[:10] // We print no more than 10 args.
+	}
+
+	// We spill address-taken or non-SSA-able value upfront, so they are always live.
+	alwaysLive := func(n *ir.Name) bool { return n.Addrtaken() || !f.Frontend().CanSSA(n.Type()) }
+
+	// We'll emit the smallest offset for the slots that need liveness info.
+	// No need to include a slot with a lower offset if it is always live.
+	for len(lv.args) > 0 && alwaysLive(lv.args[0].n) {
+		lv.args = lv.args[1:]
+	}
+	if len(lv.args) == 0 {
+		return // everything is always live
+	}
+
+	for i, a := range lv.args {
+		lv.idx[a] = int32(i)
+	}
+
+	nargs := int32(len(lv.args))
+	bulk := bitvec.NewBulk(nargs, int32(len(f.Blocks)*2))
+	for _, b := range f.Blocks {
+		be := &lv.be[b.ID]
+		be.livein = bulk.Next()
+		be.liveout = bulk.Next()
+
+		// initialize to all 1s, so we can AND them
+		be.livein.Not()
+		be.liveout.Not()
+	}
+
+	entrybe := &lv.be[f.Entry.ID]
+	entrybe.livein.Clear()
+	for i, a := range lv.args {
+		if alwaysLive(a.n) {
+			entrybe.livein.Set(int32(i))
+		}
+	}
+
+	// Visit blocks in reverse-postorder, compute block effects.
+	po := f.Postorder()
+	for i := len(po) - 1; i >= 0; i-- {
+		b := po[i]
+		be := &lv.be[b.ID]
+
+		// A slot is live at block entry if it is live in all predecessors.
+		for _, pred := range b.Preds {
+			pb := pred.Block()
+			be.livein.And(be.livein, lv.be[pb.ID].liveout)
+		}
+
+		be.liveout.Copy(be.livein)
+		for _, v := range b.Values {
+			lv.valueEffect(v, be.liveout)
+		}
+	}
+
+	// Coalesce identical live vectors. Compute liveness indices at each PC
+	// where it changes.
+	live := bitvec.New(nargs)
+	addToSet := func(bv bitvec.BitVec) (int, bool) {
+		if bv.Count() == int(nargs) { // special case for all live
+			return allLiveIdx, false
+		}
+		return lv.bvset.add(bv)
+	}
+	for _, b := range lv.f.Blocks {
+		be := &lv.be[b.ID]
+		lv.blockIdx[b.ID], _ = addToSet(be.livein)
+
+		live.Copy(be.livein)
+		var lastv *ssa.Value
+		for i, v := range b.Values {
+			if lv.valueEffect(v, live) {
+				// Record that liveness changes but not emit a map now.
+				// For a sequence of StoreRegs we only need to emit one
+				// at last.
+				lastv = v
+			}
+			if lastv != nil && (mayFault(v) || i == len(b.Values)-1) {
+				// Emit the liveness map if it may fault or at the end of
+				// the block. We may need a traceback if the instruction
+				// may cause a panic.
+				var added bool
+				lv.valueIdx[lastv.ID], added = addToSet(live)
+				if added {
+					// live is added to bvset and we cannot modify it now.
+					// Make a copy.
+					t := live
+					live = bitvec.New(nargs)
+					live.Copy(t)
+				}
+				lastv = nil
+			}
+		}
+
+		// Sanity check.
+		if !live.Eq(be.liveout) {
+			panic("wrong arg liveness map at block end")
+		}
+	}
+
+	// Emit funcdata symbol, update indices to offsets in the symbol data.
+	lsym := lv.emit()
+	fn.LSym.Func().ArgLiveInfo = lsym
+
+	//lv.print()
+
+	p := pp.Prog(obj.AFUNCDATA)
+	p.From.SetConst(objabi.FUNCDATA_ArgLiveInfo)
+	p.To.Type = obj.TYPE_MEM
+	p.To.Name = obj.NAME_EXTERN
+	p.To.Sym = lsym
+
+	return lv.blockIdx, lv.valueIdx
+}
+
+// valueEffect applies the effect of v to live, return whether it is changed.
+func (lv *argLiveness) valueEffect(v *ssa.Value, live bitvec.BitVec) bool {
+	if v.Op != ssa.OpStoreReg { // TODO: include other store instructions?
+		return false
+	}
+	n, off := ssa.AutoVar(v)
+	if n.Class != ir.PPARAM {
+		return false
+	}
+	i, ok := lv.idx[nameOff{n, off}]
+	if !ok || live.Get(i) {
+		return false
+	}
+	live.Set(i)
+	return true
+}
+
+func mayFault(v *ssa.Value) bool {
+	switch v.Op {
+	case ssa.OpLoadReg, ssa.OpStoreReg, ssa.OpCopy, ssa.OpPhi,
+		ssa.OpVarDef, ssa.OpVarKill, ssa.OpVarLive, ssa.OpKeepAlive,
+		ssa.OpSelect0, ssa.OpSelect1, ssa.OpSelectN, ssa.OpMakeResult,
+		ssa.OpConvert, ssa.OpInlMark, ssa.OpGetG:
+		return false
+	}
+	if len(v.Args) == 0 {
+		return false // assume constant op cannot fault
+	}
+	return true // conservatively assume all other ops could fault
+}
+
+func (lv *argLiveness) print() {
+	fmt.Println("argument liveness:", lv.f.Name)
+	live := bitvec.New(int32(len(lv.args)))
+	for _, b := range lv.f.Blocks {
+		be := &lv.be[b.ID]
+
+		fmt.Printf("%v: live in: ", b)
+		lv.printLivenessVec(be.livein)
+		if idx, ok := lv.blockIdx[b.ID]; ok {
+			fmt.Printf("   #%d", idx)
+		}
+		fmt.Println()
+
+		for _, v := range b.Values {
+			if lv.valueEffect(v, live) {
+				fmt.Printf("  %v: ", v)
+				lv.printLivenessVec(live)
+				if idx, ok := lv.valueIdx[v.ID]; ok {
+					fmt.Printf("   #%d", idx)
+				}
+				fmt.Println()
+			}
+		}
+
+		fmt.Printf("%v: live out: ", b)
+		lv.printLivenessVec(be.liveout)
+		fmt.Println()
+	}
+	fmt.Println("liveness maps data:", lv.fn.LSym.Func().ArgLiveInfo.P)
+}
+
+func (lv *argLiveness) printLivenessVec(bv bitvec.BitVec) {
+	for i, a := range lv.args {
+		if bv.Get(int32(i)) {
+			fmt.Printf("%v ", a)
+		}
+	}
+}
+
+func (lv *argLiveness) emit() *obj.LSym {
+	livenessMaps := lv.bvset.extractUnique()
+
+	// stack offsets of register arg spill slots
+	argOffsets := make([]uint8, len(lv.args))
+	for i, a := range lv.args {
+		off := a.FrameOffset()
+		if off > 0xff {
+			panic("offset too large")
+		}
+		argOffsets[i] = uint8(off)
+	}
+
+	idx2off := make([]int, len(livenessMaps))
+
+	lsym := base.Ctxt.Lookup(lv.fn.LSym.Name + ".argliveinfo")
+	lsym.Set(obj.AttrContentAddressable, true)
+
+	off := objw.Uint8(lsym, 0, argOffsets[0]) // smallest offset that needs liveness info.
+	for idx, live := range livenessMaps {
+		idx2off[idx] = off
+		off = objw.BitVec(lsym, off, live)
+	}
+
+	// Update liveness indices to offsets.
+	for i, x := range lv.blockIdx {
+		if x != allLiveIdx {
+			lv.blockIdx[i] = idx2off[x]
+		}
+	}
+	for i, x := range lv.valueIdx {
+		if x != allLiveIdx {
+			lv.valueIdx[i] = idx2off[x]
+		}
+	}
+
+	return lsym
+}