Source file src/cmd/compile/internal/ssa/stackalloc.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  // TODO: live at start of block instead?
     6  
     7  package ssa
     8  
     9  import (
    10  	"cmd/compile/internal/types"
    11  	"cmd/internal/src"
    12  	"fmt"
    13  )
    14  
    15  type stackAllocState struct {
    16  	f *Func
    17  
    18  	// live is the output of stackalloc.
    19  	// live[b.id] = live values at the end of block b.
    20  	live [][]ID
    21  
    22  	// The following slices are reused across multiple users
    23  	// of stackAllocState.
    24  	values    []stackValState
    25  	interfere [][]ID // interfere[v.id] = values that interfere with v.
    26  	names     []LocalSlot
    27  	slots     []int
    28  	used      []bool
    29  
    30  	nArgSlot, // Number of Values sourced to arg slot
    31  	nNotNeed, // Number of Values not needing a stack slot
    32  	nNamedSlot, // Number of Values using a named stack slot
    33  	nReuse, // Number of values reusing a stack slot
    34  	nAuto, // Number of autos allocated for stack slots.
    35  	nSelfInterfere int32 // Number of self-interferences
    36  }
    37  
    38  func newStackAllocState(f *Func) *stackAllocState {
    39  	s := f.Cache.stackAllocState
    40  	if s == nil {
    41  		return new(stackAllocState)
    42  	}
    43  	if s.f != nil {
    44  		f.fe.Fatalf(src.NoXPos, "newStackAllocState called without previous free")
    45  	}
    46  	return s
    47  }
    48  
    49  func putStackAllocState(s *stackAllocState) {
    50  	for i := range s.values {
    51  		s.values[i] = stackValState{}
    52  	}
    53  	for i := range s.interfere {
    54  		s.interfere[i] = nil
    55  	}
    56  	for i := range s.names {
    57  		s.names[i] = LocalSlot{}
    58  	}
    59  	for i := range s.slots {
    60  		s.slots[i] = 0
    61  	}
    62  	for i := range s.used {
    63  		s.used[i] = false
    64  	}
    65  	s.f.Cache.stackAllocState = s
    66  	s.f = nil
    67  	s.live = nil
    68  	s.nArgSlot, s.nNotNeed, s.nNamedSlot, s.nReuse, s.nAuto, s.nSelfInterfere = 0, 0, 0, 0, 0, 0
    69  }
    70  
    71  type stackValState struct {
    72  	typ      *types.Type
    73  	spill    *Value
    74  	needSlot bool
    75  	isArg    bool
    76  }
    77  
    78  // stackalloc allocates storage in the stack frame for
    79  // all Values that did not get a register.
    80  // Returns a map from block ID to the stack values live at the end of that block.
    81  func stackalloc(f *Func, spillLive [][]ID) [][]ID {
    82  	if f.pass.debug > stackDebug {
    83  		fmt.Println("before stackalloc")
    84  		fmt.Println(f.String())
    85  	}
    86  	s := newStackAllocState(f)
    87  	s.init(f, spillLive)
    88  	defer putStackAllocState(s)
    89  
    90  	s.stackalloc()
    91  	if f.pass.stats > 0 {
    92  		f.LogStat("stack_alloc_stats",
    93  			s.nArgSlot, "arg_slots", s.nNotNeed, "slot_not_needed",
    94  			s.nNamedSlot, "named_slots", s.nAuto, "auto_slots",
    95  			s.nReuse, "reused_slots", s.nSelfInterfere, "self_interfering")
    96  	}
    97  
    98  	return s.live
    99  }
   100  
   101  func (s *stackAllocState) init(f *Func, spillLive [][]ID) {
   102  	s.f = f
   103  
   104  	// Initialize value information.
   105  	if n := f.NumValues(); cap(s.values) >= n {
   106  		s.values = s.values[:n]
   107  	} else {
   108  		s.values = make([]stackValState, n)
   109  	}
   110  	for _, b := range f.Blocks {
   111  		for _, v := range b.Values {
   112  			s.values[v.ID].typ = v.Type
   113  			s.values[v.ID].needSlot = !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && f.getHome(v.ID) == nil && !v.rematerializeable() && !v.OnWasmStack
   114  			s.values[v.ID].isArg = v.Op == OpArg
   115  			if f.pass.debug > stackDebug && s.values[v.ID].needSlot {
   116  				fmt.Printf("%s needs a stack slot\n", v)
   117  			}
   118  			if v.Op == OpStoreReg {
   119  				s.values[v.Args[0].ID].spill = v
   120  			}
   121  		}
   122  	}
   123  
   124  	// Compute liveness info for values needing a slot.
   125  	s.computeLive(spillLive)
   126  
   127  	// Build interference graph among values needing a slot.
   128  	s.buildInterferenceGraph()
   129  }
   130  
   131  func (s *stackAllocState) stackalloc() {
   132  	f := s.f
   133  
   134  	// Build map from values to their names, if any.
   135  	// A value may be associated with more than one name (e.g. after
   136  	// the assignment i=j). This step picks one name per value arbitrarily.
   137  	if n := f.NumValues(); cap(s.names) >= n {
   138  		s.names = s.names[:n]
   139  	} else {
   140  		s.names = make([]LocalSlot, n)
   141  	}
   142  	names := s.names
   143  	for _, name := range f.Names {
   144  		// Note: not "range f.NamedValues" above, because
   145  		// that would be nondeterministic.
   146  		for _, v := range f.NamedValues[name] {
   147  			names[v.ID] = name
   148  		}
   149  	}
   150  
   151  	// Allocate args to their assigned locations.
   152  	for _, v := range f.Entry.Values {
   153  		if v.Op != OpArg {
   154  			continue
   155  		}
   156  		loc := LocalSlot{N: v.Aux.(GCNode), Type: v.Type, Off: v.AuxInt}
   157  		if f.pass.debug > stackDebug {
   158  			fmt.Printf("stackalloc %s to %s\n", v, loc)
   159  		}
   160  		f.setHome(v, loc)
   161  	}
   162  
   163  	// For each type, we keep track of all the stack slots we
   164  	// have allocated for that type.
   165  	// TODO: share slots among equivalent types. We would need to
   166  	// only share among types with the same GC signature. See the
   167  	// type.Equal calls below for where this matters.
   168  	locations := map[*types.Type][]LocalSlot{}
   169  
   170  	// Each time we assign a stack slot to a value v, we remember
   171  	// the slot we used via an index into locations[v.Type].
   172  	slots := s.slots
   173  	if n := f.NumValues(); cap(slots) >= n {
   174  		slots = slots[:n]
   175  	} else {
   176  		slots = make([]int, n)
   177  		s.slots = slots
   178  	}
   179  	for i := range slots {
   180  		slots[i] = -1
   181  	}
   182  
   183  	// Pick a stack slot for each value needing one.
   184  	var used []bool
   185  	if n := f.NumValues(); cap(s.used) >= n {
   186  		used = s.used[:n]
   187  	} else {
   188  		used = make([]bool, n)
   189  		s.used = used
   190  	}
   191  	for _, b := range f.Blocks {
   192  		for _, v := range b.Values {
   193  			if !s.values[v.ID].needSlot {
   194  				s.nNotNeed++
   195  				continue
   196  			}
   197  			if v.Op == OpArg {
   198  				s.nArgSlot++
   199  				continue // already picked
   200  			}
   201  
   202  			// If this is a named value, try to use the name as
   203  			// the spill location.
   204  			var name LocalSlot
   205  			if v.Op == OpStoreReg {
   206  				name = names[v.Args[0].ID]
   207  			} else {
   208  				name = names[v.ID]
   209  			}
   210  			if name.N != nil && v.Type.Compare(name.Type) == types.CMPeq {
   211  				for _, id := range s.interfere[v.ID] {
   212  					h := f.getHome(id)
   213  					if h != nil && h.(LocalSlot).N == name.N && h.(LocalSlot).Off == name.Off {
   214  						// A variable can interfere with itself.
   215  						// It is rare, but it can happen.
   216  						s.nSelfInterfere++
   217  						goto noname
   218  					}
   219  				}
   220  				if f.pass.debug > stackDebug {
   221  					fmt.Printf("stackalloc %s to %s\n", v, name)
   222  				}
   223  				s.nNamedSlot++
   224  				f.setHome(v, name)
   225  				continue
   226  			}
   227  
   228  		noname:
   229  			// Set of stack slots we could reuse.
   230  			locs := locations[v.Type]
   231  			// Mark all positions in locs used by interfering values.
   232  			for i := 0; i < len(locs); i++ {
   233  				used[i] = false
   234  			}
   235  			for _, xid := range s.interfere[v.ID] {
   236  				slot := slots[xid]
   237  				if slot >= 0 {
   238  					used[slot] = true
   239  				}
   240  			}
   241  			// Find an unused stack slot.
   242  			var i int
   243  			for i = 0; i < len(locs); i++ {
   244  				if !used[i] {
   245  					s.nReuse++
   246  					break
   247  				}
   248  			}
   249  			// If there is no unused stack slot, allocate a new one.
   250  			if i == len(locs) {
   251  				s.nAuto++
   252  				locs = append(locs, LocalSlot{N: f.fe.Auto(v.Pos, v.Type), Type: v.Type, Off: 0})
   253  				locations[v.Type] = locs
   254  			}
   255  			// Use the stack variable at that index for v.
   256  			loc := locs[i]
   257  			if f.pass.debug > stackDebug {
   258  				fmt.Printf("stackalloc %s to %s\n", v, loc)
   259  			}
   260  			f.setHome(v, loc)
   261  			slots[v.ID] = i
   262  		}
   263  	}
   264  }
   265  
   266  // computeLive computes a map from block ID to a list of
   267  // stack-slot-needing value IDs live at the end of that block.
   268  // TODO: this could be quadratic if lots of variables are live across lots of
   269  // basic blocks. Figure out a way to make this function (or, more precisely, the user
   270  // of this function) require only linear size & time.
   271  func (s *stackAllocState) computeLive(spillLive [][]ID) {
   272  	s.live = make([][]ID, s.f.NumBlocks())
   273  	var phis []*Value
   274  	live := s.f.newSparseSet(s.f.NumValues())
   275  	defer s.f.retSparseSet(live)
   276  	t := s.f.newSparseSet(s.f.NumValues())
   277  	defer s.f.retSparseSet(t)
   278  
   279  	// Instead of iterating over f.Blocks, iterate over their postordering.
   280  	// Liveness information flows backward, so starting at the end
   281  	// increases the probability that we will stabilize quickly.
   282  	po := s.f.postorder()
   283  	for {
   284  		changed := false
   285  		for _, b := range po {
   286  			// Start with known live values at the end of the block
   287  			live.clear()
   288  			live.addAll(s.live[b.ID])
   289  
   290  			// Propagate backwards to the start of the block
   291  			phis = phis[:0]
   292  			for i := len(b.Values) - 1; i >= 0; i-- {
   293  				v := b.Values[i]
   294  				live.remove(v.ID)
   295  				if v.Op == OpPhi {
   296  					// Save phi for later.
   297  					// Note: its args might need a stack slot even though
   298  					// the phi itself doesn't. So don't use needSlot.
   299  					if !v.Type.IsMemory() && !v.Type.IsVoid() {
   300  						phis = append(phis, v)
   301  					}
   302  					continue
   303  				}
   304  				for _, a := range v.Args {
   305  					if s.values[a.ID].needSlot {
   306  						live.add(a.ID)
   307  					}
   308  				}
   309  			}
   310  
   311  			// for each predecessor of b, expand its list of live-at-end values
   312  			// invariant: s contains the values live at the start of b (excluding phi inputs)
   313  			for i, e := range b.Preds {
   314  				p := e.b
   315  				t.clear()
   316  				t.addAll(s.live[p.ID])
   317  				t.addAll(live.contents())
   318  				t.addAll(spillLive[p.ID])
   319  				for _, v := range phis {
   320  					a := v.Args[i]
   321  					if s.values[a.ID].needSlot {
   322  						t.add(a.ID)
   323  					}
   324  					if spill := s.values[a.ID].spill; spill != nil {
   325  						//TODO: remove?  Subsumed by SpillUse?
   326  						t.add(spill.ID)
   327  					}
   328  				}
   329  				if t.size() == len(s.live[p.ID]) {
   330  					continue
   331  				}
   332  				// grow p's live set
   333  				s.live[p.ID] = append(s.live[p.ID][:0], t.contents()...)
   334  				changed = true
   335  			}
   336  		}
   337  
   338  		if !changed {
   339  			break
   340  		}
   341  	}
   342  	if s.f.pass.debug > stackDebug {
   343  		for _, b := range s.f.Blocks {
   344  			fmt.Printf("stacklive %s %v\n", b, s.live[b.ID])
   345  		}
   346  	}
   347  }
   348  
   349  func (f *Func) getHome(vid ID) Location {
   350  	if int(vid) >= len(f.RegAlloc) {
   351  		return nil
   352  	}
   353  	return f.RegAlloc[vid]
   354  }
   355  
   356  func (f *Func) setHome(v *Value, loc Location) {
   357  	for v.ID >= ID(len(f.RegAlloc)) {
   358  		f.RegAlloc = append(f.RegAlloc, nil)
   359  	}
   360  	f.RegAlloc[v.ID] = loc
   361  }
   362  
   363  func (s *stackAllocState) buildInterferenceGraph() {
   364  	f := s.f
   365  	if n := f.NumValues(); cap(s.interfere) >= n {
   366  		s.interfere = s.interfere[:n]
   367  	} else {
   368  		s.interfere = make([][]ID, n)
   369  	}
   370  	live := f.newSparseSet(f.NumValues())
   371  	defer f.retSparseSet(live)
   372  	for _, b := range f.Blocks {
   373  		// Propagate liveness backwards to the start of the block.
   374  		// Two values interfere if one is defined while the other is live.
   375  		live.clear()
   376  		live.addAll(s.live[b.ID])
   377  		for i := len(b.Values) - 1; i >= 0; i-- {
   378  			v := b.Values[i]
   379  			if s.values[v.ID].needSlot {
   380  				live.remove(v.ID)
   381  				for _, id := range live.contents() {
   382  					// Note: args can have different types and still interfere
   383  					// (with each other or with other values). See issue 23522.
   384  					if s.values[v.ID].typ.Compare(s.values[id].typ) == types.CMPeq || v.Op == OpArg || s.values[id].isArg {
   385  						s.interfere[v.ID] = append(s.interfere[v.ID], id)
   386  						s.interfere[id] = append(s.interfere[id], v.ID)
   387  					}
   388  				}
   389  			}
   390  			for _, a := range v.Args {
   391  				if s.values[a.ID].needSlot {
   392  					live.add(a.ID)
   393  				}
   394  			}
   395  			if v.Op == OpArg && s.values[v.ID].needSlot {
   396  				// OpArg is an input argument which is pre-spilled.
   397  				// We add back v.ID here because we want this value
   398  				// to appear live even before this point. Being live
   399  				// all the way to the start of the entry block prevents other
   400  				// values from being allocated to the same slot and clobbering
   401  				// the input value before we have a chance to load it.
   402  				live.add(v.ID)
   403  			}
   404  		}
   405  	}
   406  	if f.pass.debug > stackDebug {
   407  		for vid, i := range s.interfere {
   408  			if len(i) > 0 {
   409  				fmt.Printf("v%d interferes with", vid)
   410  				for _, x := range i {
   411  					fmt.Printf(" v%d", x)
   412  				}
   413  				fmt.Println()
   414  			}
   415  		}
   416  	}
   417  }
   418  

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