schedule.go

Documentation: cmd/compile/internal/ssa

     1  // Copyright 2015 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package ssa
     6  
     7  import (
     8  	"cmd/compile/internal/base"
     9  	"cmd/compile/internal/types"
    10  	"container/heap"
    11  	"sort"
    12  )
    13  
    14  const (
    15  	ScorePhi       = iota // towards top of block
    16  	ScoreArg              // must occur at the top of the entry block
    17  	ScoreInitMem          // after the args - used as mark by debug info generation
    18  	ScoreReadTuple        // must occur immediately after tuple-generating insn (or call)
    19  	ScoreNilCheck
    20  	ScoreMemory
    21  	ScoreReadFlags
    22  	ScoreDefault
    23  	ScoreFlags
    24  	ScoreControl // towards bottom of block
    25  )
    26  
    27  type ValHeap struct {
    28  	a           []*Value
    29  	score       []int8
    30  	inBlockUses []bool
    31  }
    32  
    33  func (h ValHeap) Len() int      { return len(h.a) }
    34  func (h ValHeap) Swap(i, j int) { a := h.a; a[i], a[j] = a[j], a[i] }
    35  
    36  func (h *ValHeap) Push(x interface{}) {
    37  	// Push and Pop use pointer receivers because they modify the slice's length,
    38  	// not just its contents.
    39  	v := x.(*Value)
    40  	h.a = append(h.a, v)
    41  }
    42  func (h *ValHeap) Pop() interface{} {
    43  	old := h.a
    44  	n := len(old)
    45  	x := old[n-1]
    46  	h.a = old[0 : n-1]
    47  	return x
    48  }
    49  func (h ValHeap) Less(i, j int) bool {
    50  	x := h.a[i]
    51  	y := h.a[j]
    52  	sx := h.score[x.ID]
    53  	sy := h.score[y.ID]
    54  	if c := sx - sy; c != 0 {
    55  		return c < 0 // lower scores come earlier.
    56  	}
    57  	// Note: only scores are required for correct scheduling.
    58  	// Everything else is just heuristics.
    59  
    60  	ix := h.inBlockUses[x.ID]
    61  	iy := h.inBlockUses[y.ID]
    62  	if ix != iy {
    63  		return ix // values with in-block uses come earlier
    64  	}
    65  
    66  	if x.Pos != y.Pos { // Favor in-order line stepping
    67  		return x.Pos.Before(y.Pos)
    68  	}
    69  	if x.Op != OpPhi {
    70  		if c := len(x.Args) - len(y.Args); c != 0 {
    71  			return c > 0 // smaller args come later
    72  		}
    73  	}
    74  	if c := x.Uses - y.Uses; c != 0 {
    75  		return c > 0 // smaller uses come later
    76  	}
    77  	// These comparisons are fairly arbitrary.
    78  	// The goal here is stability in the face
    79  	// of unrelated changes elsewhere in the compiler.
    80  	if c := x.AuxInt - y.AuxInt; c != 0 {
    81  		return c < 0
    82  	}
    83  	if cmp := x.Type.Compare(y.Type); cmp != types.CMPeq {
    84  		return cmp == types.CMPlt
    85  	}
    86  	return x.ID < y.ID
    87  }
    88  
    89  func (op Op) isLoweredGetClosurePtr() bool {
    90  	switch op {
    91  	case OpAMD64LoweredGetClosurePtr, OpPPC64LoweredGetClosurePtr, OpARMLoweredGetClosurePtr, OpARM64LoweredGetClosurePtr,
    92  		Op386LoweredGetClosurePtr, OpMIPS64LoweredGetClosurePtr, OpLOONG64LoweredGetClosurePtr, OpS390XLoweredGetClosurePtr, OpMIPSLoweredGetClosurePtr,
    93  		OpRISCV64LoweredGetClosurePtr, OpWasmLoweredGetClosurePtr:
    94  		return true
    95  	}
    96  	return false
    97  }
    98  
    99  // Schedule the Values in each Block. After this phase returns, the
   100  // order of b.Values matters and is the order in which those values
   101  // will appear in the assembly output. For now it generates a
   102  // reasonable valid schedule using a priority queue. TODO(khr):
   103  // schedule smarter.
   104  func schedule(f *Func) {
   105  	// reusable priority queue
   106  	priq := new(ValHeap)
   107  
   108  	// "priority" for a value
   109  	score := f.Cache.allocInt8Slice(f.NumValues())
   110  	defer f.Cache.freeInt8Slice(score)
   111  
   112  	// maps mem values to the next live memory value
   113  	nextMem := f.Cache.allocValueSlice(f.NumValues())
   114  	defer f.Cache.freeValueSlice(nextMem)
   115  
   116  	// inBlockUses records whether a value is used in the block
   117  	// in which it lives. (block control values don't count as uses.)
   118  	inBlockUses := f.Cache.allocBoolSlice(f.NumValues())
   119  	defer f.Cache.freeBoolSlice(inBlockUses)
   120  	if f.Config.optimize {
   121  		for _, b := range f.Blocks {
   122  			for _, v := range b.Values {
   123  				for _, a := range v.Args {
   124  					if a.Block == b {
   125  						inBlockUses[a.ID] = true
   126  					}
   127  				}
   128  			}
   129  		}
   130  	}
   131  	priq.inBlockUses = inBlockUses
   132  
   133  	for _, b := range f.Blocks {
   134  		// Compute score. Larger numbers are scheduled closer to the end of the block.
   135  		for _, v := range b.Values {
   136  			switch {
   137  			case v.Op.isLoweredGetClosurePtr():
   138  				// We also score GetLoweredClosurePtr as early as possible to ensure that the
   139  				// context register is not stomped. GetLoweredClosurePtr should only appear
   140  				// in the entry block where there are no phi functions, so there is no
   141  				// conflict or ambiguity here.
   142  				if b != f.Entry {
   143  					f.Fatalf("LoweredGetClosurePtr appeared outside of entry block, b=%s", b.String())
   144  				}
   145  				score[v.ID] = ScorePhi
   146  			case opcodeTable[v.Op].nilCheck:
   147  				// Nil checks must come before loads from the same address.
   148  				score[v.ID] = ScoreNilCheck
   149  			case v.Op == OpPhi:
   150  				// We want all the phis first.
   151  				score[v.ID] = ScorePhi
   152  			case v.Op == OpArgIntReg || v.Op == OpArgFloatReg:
   153  				// In-register args must be scheduled as early as possible to ensure that they
   154  				// are not stomped (similar to the closure pointer above).
   155  				// In particular, they need to come before regular OpArg operations because
   156  				// of how regalloc places spill code (see regalloc.go:placeSpills:mustBeFirst).
   157  				if b != f.Entry {
   158  					f.Fatalf("%s appeared outside of entry block, b=%s", v.Op, b.String())
   159  				}
   160  				score[v.ID] = ScorePhi
   161  			case v.Op == OpArg || v.Op == OpSP || v.Op == OpSB:
   162  				// We want all the args as early as possible, for better debugging.
   163  				score[v.ID] = ScoreArg
   164  			case v.Op == OpInitMem:
   165  				// Early, but after args. See debug.go:buildLocationLists
   166  				score[v.ID] = ScoreInitMem
   167  			case v.Type.IsMemory():
   168  				// Schedule stores as early as possible. This tends to
   169  				// reduce register pressure.
   170  				score[v.ID] = ScoreMemory
   171  			case v.Op == OpSelect0 || v.Op == OpSelect1 || v.Op == OpSelectN:
   172  				// Tuple selectors need to appear immediately after the instruction
   173  				// that generates the tuple.
   174  				score[v.ID] = ScoreReadTuple
   175  			case v.hasFlagInput():
   176  				// Schedule flag-reading ops earlier, to minimize the lifetime
   177  				// of flag values.
   178  				score[v.ID] = ScoreReadFlags
   179  			case v.isFlagOp():
   180  				// Schedule flag register generation as late as possible.
   181  				// This makes sure that we only have one live flags
   182  				// value at a time.
   183  				// Note that this case is after the case above, so values
   184  				// which both read and generate flags are given ScoreReadFlags.
   185  				score[v.ID] = ScoreFlags
   186  			default:
   187  				score[v.ID] = ScoreDefault
   188  				// If we're reading flags, schedule earlier to keep flag lifetime short.
   189  				for _, a := range v.Args {
   190  					if a.isFlagOp() {
   191  						score[v.ID] = ScoreReadFlags
   192  					}
   193  				}
   194  			}
   195  		}
   196  		for _, c := range b.ControlValues() {
   197  			// Force the control values to be scheduled at the end,
   198  			// unless they have other special priority.
   199  			if c.Block != b || score[c.ID] < ScoreReadTuple {
   200  				continue
   201  			}
   202  			if score[c.ID] == ScoreReadTuple {
   203  				score[c.Args[0].ID] = ScoreControl
   204  				continue
   205  			}
   206  			score[c.ID] = ScoreControl
   207  		}
   208  	}
   209  	priq.score = score
   210  
   211  	// An edge represents a scheduling constraint that x must appear before y in the schedule.
   212  	type edge struct {
   213  		x, y *Value
   214  	}
   215  	edges := make([]edge, 0, 64)
   216  
   217  	// inEdges is the number of scheduling edges incoming from values that haven't been scheduled yet.
   218  	// i.e. inEdges[y.ID] = |e in edges where e.y == y and e.x is not in the schedule yet|.
   219  	inEdges := f.Cache.allocInt32Slice(f.NumValues())
   220  	defer f.Cache.freeInt32Slice(inEdges)
   221  
   222  	for _, b := range f.Blocks {
   223  		edges = edges[:0]
   224  		// Standard edges: from the argument of a value to that value.
   225  		for _, v := range b.Values {
   226  			if v.Op == OpPhi {
   227  				// If a value is used by a phi, it does not induce
   228  				// a scheduling edge because that use is from the
   229  				// previous iteration.
   230  				continue
   231  			}
   232  			for _, a := range v.Args {
   233  				if a.Block == b {
   234  					edges = append(edges, edge{a, v})
   235  				}
   236  			}
   237  		}
   238  
   239  		// Find store chain for block.
   240  		// Store chains for different blocks overwrite each other, so
   241  		// the calculated store chain is good only for this block.
   242  		for _, v := range b.Values {
   243  			if v.Op != OpPhi && v.Op != OpInitMem && v.Type.IsMemory() {
   244  				nextMem[v.MemoryArg().ID] = v
   245  			}
   246  		}
   247  
   248  		// Add edges to enforce that any load must come before the following store.
   249  		for _, v := range b.Values {
   250  			if v.Op == OpPhi || v.Type.IsMemory() {
   251  				continue
   252  			}
   253  			w := v.MemoryArg()
   254  			if w == nil {
   255  				continue
   256  			}
   257  			if s := nextMem[w.ID]; s != nil && s.Block == b {
   258  				edges = append(edges, edge{v, s})
   259  			}
   260  		}
   261  
   262  		// Sort all the edges by source Value ID.
   263  		sort.Slice(edges, func(i, j int) bool {
   264  			return edges[i].x.ID < edges[j].x.ID
   265  		})
   266  		// Compute inEdges for values in this block.
   267  		for _, e := range edges {
   268  			inEdges[e.y.ID]++
   269  		}
   270  
   271  		// Initialize priority queue with schedulable values.
   272  		priq.a = priq.a[:0]
   273  		for _, v := range b.Values {
   274  			if inEdges[v.ID] == 0 {
   275  				heap.Push(priq, v)
   276  			}
   277  		}
   278  
   279  		// Produce the schedule. Pick the highest priority scheduleable value,
   280  		// add it to the schedule, add any of its uses that are now scheduleable
   281  		// to the queue, and repeat.
   282  		nv := len(b.Values)
   283  		b.Values = b.Values[:0]
   284  		for priq.Len() > 0 {
   285  			// Schedule the next schedulable value in priority order.
   286  			v := heap.Pop(priq).(*Value)
   287  			b.Values = append(b.Values, v)
   288  
   289  			// Find all the scheduling edges out from this value.
   290  			i := sort.Search(len(edges), func(i int) bool {
   291  				return edges[i].x.ID >= v.ID
   292  			})
   293  			j := sort.Search(len(edges), func(i int) bool {
   294  				return edges[i].x.ID > v.ID
   295  			})
   296  			// Decrement inEdges for each target of edges from v.
   297  			for _, e := range edges[i:j] {
   298  				inEdges[e.y.ID]--
   299  				if inEdges[e.y.ID] == 0 {
   300  					heap.Push(priq, e.y)
   301  				}
   302  			}
   303  		}
   304  		if len(b.Values) != nv {
   305  			f.Fatalf("schedule does not include all values in block %s", b)
   306  		}
   307  	}
   308  
   309  	// Remove SPanchored now that we've scheduled.
   310  	// Also unlink nil checks now that ordering is assured
   311  	// between the nil check and the uses of the nil-checked pointer.
   312  	for _, b := range f.Blocks {
   313  		for _, v := range b.Values {
   314  			for i, a := range v.Args {
   315  				if a.Op == OpSPanchored || opcodeTable[a.Op].nilCheck {
   316  					v.SetArg(i, a.Args[0])
   317  				}
   318  			}
   319  		}
   320  		for i, c := range b.ControlValues() {
   321  			if c.Op == OpSPanchored || opcodeTable[c.Op].nilCheck {
   322  				b.ReplaceControl(i, c.Args[0])
   323  			}
   324  		}
   325  	}
   326  	for _, b := range f.Blocks {
   327  		i := 0
   328  		for _, v := range b.Values {
   329  			if v.Op == OpSPanchored {
   330  				// Free this value
   331  				if v.Uses != 0 {
   332  					base.Fatalf("SPAnchored still has %d uses", v.Uses)
   333  				}
   334  				v.resetArgs()
   335  				f.freeValue(v)
   336  			} else {
   337  				if opcodeTable[v.Op].nilCheck {
   338  					if v.Uses != 0 {
   339  						base.Fatalf("nilcheck still has %d uses", v.Uses)
   340  					}
   341  					// We can't delete the nil check, but we mark
   342  					// it as having void type so regalloc won't
   343  					// try to allocate a register for it.
   344  					v.Type = types.TypeVoid
   345  				}
   346  				b.Values[i] = v
   347  				i++
   348  			}
   349  		}
   350  		b.truncateValues(i)
   351  	}
   352  
   353  	f.scheduled = true
   354  }
   355  
   356  // storeOrder orders values with respect to stores. That is,
   357  // if v transitively depends on store s, v is ordered after s,
   358  // otherwise v is ordered before s.
   359  // Specifically, values are ordered like
   360  //
   361  //	store1
   362  //	NilCheck that depends on store1
   363  //	other values that depends on store1
   364  //	store2
   365  //	NilCheck that depends on store2
   366  //	other values that depends on store2
   367  //	...
   368  //
   369  // The order of non-store and non-NilCheck values are undefined
   370  // (not necessarily dependency order). This should be cheaper
   371  // than a full scheduling as done above.
   372  // Note that simple dependency order won't work: there is no
   373  // dependency between NilChecks and values like IsNonNil.
   374  // Auxiliary data structures are passed in as arguments, so
   375  // that they can be allocated in the caller and be reused.
   376  // This function takes care of reset them.
   377  func storeOrder(values []*Value, sset *sparseSet, storeNumber []int32) []*Value {
   378  	if len(values) == 0 {
   379  		return values
   380  	}
   381  
   382  	f := values[0].Block.Func
   383  
   384  	// find all stores
   385  
   386  	// Members of values that are store values.
   387  	// A constant bound allows this to be stack-allocated. 64 is
   388  	// enough to cover almost every storeOrder call.
   389  	stores := make([]*Value, 0, 64)
   390  	hasNilCheck := false
   391  	sset.clear() // sset is the set of stores that are used in other values
   392  	for _, v := range values {
   393  		if v.Type.IsMemory() {
   394  			stores = append(stores, v)
   395  			if v.Op == OpInitMem || v.Op == OpPhi {
   396  				continue
   397  			}
   398  			sset.add(v.MemoryArg().ID) // record that v's memory arg is used
   399  		}
   400  		if v.Op == OpNilCheck {
   401  			hasNilCheck = true
   402  		}
   403  	}
   404  	if len(stores) == 0 || !hasNilCheck && f.pass.name == "nilcheckelim" {
   405  		// there is no store, the order does not matter
   406  		return values
   407  	}
   408  
   409  	// find last store, which is the one that is not used by other stores
   410  	var last *Value
   411  	for _, v := range stores {
   412  		if !sset.contains(v.ID) {
   413  			if last != nil {
   414  				f.Fatalf("two stores live simultaneously: %v and %v", v, last)
   415  			}
   416  			last = v
   417  		}
   418  	}
   419  
   420  	// We assign a store number to each value. Store number is the
   421  	// index of the latest store that this value transitively depends.
   422  	// The i-th store in the current block gets store number 3*i. A nil
   423  	// check that depends on the i-th store gets store number 3*i+1.
   424  	// Other values that depends on the i-th store gets store number 3*i+2.
   425  	// Special case: 0 -- unassigned, 1 or 2 -- the latest store it depends
   426  	// is in the previous block (or no store at all, e.g. value is Const).
   427  	// First we assign the number to all stores by walking back the store chain,
   428  	// then assign the number to other values in DFS order.
   429  	count := make([]int32, 3*(len(stores)+1))
   430  	sset.clear() // reuse sparse set to ensure that a value is pushed to stack only once
   431  	for n, w := len(stores), last; n > 0; n-- {
   432  		storeNumber[w.ID] = int32(3 * n)
   433  		count[3*n]++
   434  		sset.add(w.ID)
   435  		if w.Op == OpInitMem || w.Op == OpPhi {
   436  			if n != 1 {
   437  				f.Fatalf("store order is wrong: there are stores before %v", w)
   438  			}
   439  			break
   440  		}
   441  		w = w.MemoryArg()
   442  	}
   443  	var stack []*Value
   444  	for _, v := range values {
   445  		if sset.contains(v.ID) {
   446  			// in sset means v is a store, or already pushed to stack, or already assigned a store number
   447  			continue
   448  		}
   449  		stack = append(stack, v)
   450  		sset.add(v.ID)
   451  
   452  		for len(stack) > 0 {
   453  			w := stack[len(stack)-1]
   454  			if storeNumber[w.ID] != 0 {
   455  				stack = stack[:len(stack)-1]
   456  				continue
   457  			}
   458  			if w.Op == OpPhi {
   459  				// Phi value doesn't depend on store in the current block.
   460  				// Do this early to avoid dependency cycle.
   461  				storeNumber[w.ID] = 2
   462  				count[2]++
   463  				stack = stack[:len(stack)-1]
   464  				continue
   465  			}
   466  
   467  			max := int32(0) // latest store dependency
   468  			argsdone := true
   469  			for _, a := range w.Args {
   470  				if a.Block != w.Block {
   471  					continue
   472  				}
   473  				if !sset.contains(a.ID) {
   474  					stack = append(stack, a)
   475  					sset.add(a.ID)
   476  					argsdone = false
   477  					break
   478  				}
   479  				if storeNumber[a.ID]/3 > max {
   480  					max = storeNumber[a.ID] / 3
   481  				}
   482  			}
   483  			if !argsdone {
   484  				continue
   485  			}
   486  
   487  			n := 3*max + 2
   488  			if w.Op == OpNilCheck {
   489  				n = 3*max + 1
   490  			}
   491  			storeNumber[w.ID] = n
   492  			count[n]++
   493  			stack = stack[:len(stack)-1]
   494  		}
   495  	}
   496  
   497  	// convert count to prefix sum of counts: count'[i] = sum_{j<=i} count[i]
   498  	for i := range count {
   499  		if i == 0 {
   500  			continue
   501  		}
   502  		count[i] += count[i-1]
   503  	}
   504  	if count[len(count)-1] != int32(len(values)) {
   505  		f.Fatalf("storeOrder: value is missing, total count = %d, values = %v", count[len(count)-1], values)
   506  	}
   507  
   508  	// place values in count-indexed bins, which are in the desired store order
   509  	order := make([]*Value, len(values))
   510  	for _, v := range values {
   511  		s := storeNumber[v.ID]
   512  		order[count[s-1]] = v
   513  		count[s-1]++
   514  	}
   515  
   516  	// Order nil checks in source order. We want the first in source order to trigger.
   517  	// If two are on the same line, we don't really care which happens first.
   518  	// See issue 18169.
   519  	if hasNilCheck {
   520  		start := -1
   521  		for i, v := range order {
   522  			if v.Op == OpNilCheck {
   523  				if start == -1 {
   524  					start = i
   525  				}
   526  			} else {
   527  				if start != -1 {
   528  					sort.Sort(bySourcePos(order[start:i]))
   529  					start = -1
   530  				}
   531  			}
   532  		}
   533  		if start != -1 {
   534  			sort.Sort(bySourcePos(order[start:]))
   535  		}
   536  	}
   537  
   538  	return order
   539  }
   540  
   541  // isFlagOp reports if v is an OP with the flag type.
   542  func (v *Value) isFlagOp() bool {
   543  	if v.Type.IsFlags() || v.Type.IsTuple() && v.Type.FieldType(1).IsFlags() {
   544  		return true
   545  	}
   546  	// PPC64 carry generators put their carry in a non-flag-typed register
   547  	// in their output.
   548  	switch v.Op {
   549  	case OpPPC64SUBC, OpPPC64ADDC, OpPPC64SUBCconst, OpPPC64ADDCconst:
   550  		return true
   551  	}
   552  	return false
   553  }
   554  
   555  // hasFlagInput reports whether v has a flag value as any of its inputs.
   556  func (v *Value) hasFlagInput() bool {
   557  	for _, a := range v.Args {
   558  		if a.isFlagOp() {
   559  			return true
   560  		}
   561  	}
   562  	// PPC64 carry dependencies are conveyed through their final argument,
   563  	// so we treat those operations as taking flags as well.
   564  	switch v.Op {
   565  	case OpPPC64SUBE, OpPPC64ADDE, OpPPC64SUBZEzero, OpPPC64ADDZE, OpPPC64ADDZEzero:
   566  		return true
   567  	}
   568  	return false
   569  }
   570  
   571  type bySourcePos []*Value
   572  
   573  func (s bySourcePos) Len() int           { return len(s) }
   574  func (s bySourcePos) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
   575  func (s bySourcePos) Less(i, j int) bool { return s[i].Pos.Before(s[j].Pos) }
   576
View as plain text