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/ir"
    11  	"cmd/compile/internal/types"
    12  	"cmd/internal/src"
    13  	"fmt"
    14  )
    15  
    16  type stackAllocState struct {
    17  	f *Func
    18  
    19  	// live is the output of stackalloc.
    20  	// live[b.id] = live values at the end of block b.
    21  	live [][]ID
    22  
    23  	// The following slices are reused across multiple users
    24  	// of stackAllocState.
    25  	values    []stackValState
    26  	interfere [][]ID // interfere[v.id] = values that interfere with v.
    27  	names     []LocalSlot
    28  
    29  	nArgSlot, // Number of Values sourced to arg slot
    30  	nNotNeed, // Number of Values not needing a stack slot
    31  	nNamedSlot, // Number of Values using a named stack slot
    32  	nReuse, // Number of values reusing a stack slot
    33  	nAuto, // Number of autos allocated for stack slots.
    34  	nSelfInterfere int32 // Number of self-interferences
    35  }
    36  
    37  func newStackAllocState(f *Func) *stackAllocState {
    38  	s := f.Cache.stackAllocState
    39  	if s == nil {
    40  		return new(stackAllocState)
    41  	}
    42  	if s.f != nil {
    43  		f.fe.Fatalf(src.NoXPos, "newStackAllocState called without previous free")
    44  	}
    45  	return s
    46  }
    47  
    48  func putStackAllocState(s *stackAllocState) {
    49  	for i := range s.values {
    50  		s.values[i] = stackValState{}
    51  	}
    52  	for i := range s.interfere {
    53  		s.interfere[i] = nil
    54  	}
    55  	for i := range s.names {
    56  		s.names[i] = LocalSlot{}
    57  	}
    58  	s.f.Cache.stackAllocState = s
    59  	s.f = nil
    60  	s.live = nil
    61  	s.nArgSlot, s.nNotNeed, s.nNamedSlot, s.nReuse, s.nAuto, s.nSelfInterfere = 0, 0, 0, 0, 0, 0
    62  }
    63  
    64  type stackValState struct {
    65  	typ      *types.Type
    66  	spill    *Value
    67  	needSlot bool
    68  	isArg    bool
    69  }
    70  
    71  // stackalloc allocates storage in the stack frame for
    72  // all Values that did not get a register.
    73  // Returns a map from block ID to the stack values live at the end of that block.
    74  func stackalloc(f *Func, spillLive [][]ID) [][]ID {
    75  	if f.pass.debug > stackDebug {
    76  		fmt.Println("before stackalloc")
    77  		fmt.Println(f.String())
    78  	}
    79  	s := newStackAllocState(f)
    80  	s.init(f, spillLive)
    81  	defer putStackAllocState(s)
    82  
    83  	s.stackalloc()
    84  	if f.pass.stats > 0 {
    85  		f.LogStat("stack_alloc_stats",
    86  			s.nArgSlot, "arg_slots", s.nNotNeed, "slot_not_needed",
    87  			s.nNamedSlot, "named_slots", s.nAuto, "auto_slots",
    88  			s.nReuse, "reused_slots", s.nSelfInterfere, "self_interfering")
    89  	}
    90  
    91  	return s.live
    92  }
    93  
    94  func (s *stackAllocState) init(f *Func, spillLive [][]ID) {
    95  	s.f = f
    96  
    97  	// Initialize value information.
    98  	if n := f.NumValues(); cap(s.values) >= n {
    99  		s.values = s.values[:n]
   100  	} else {
   101  		s.values = make([]stackValState, n)
   102  	}
   103  	for _, b := range f.Blocks {
   104  		for _, v := range b.Values {
   105  			s.values[v.ID].typ = v.Type
   106  			s.values[v.ID].needSlot = !v.Type.IsMemory() && !v.Type.IsVoid() && !v.Type.IsFlags() && f.getHome(v.ID) == nil && !v.rematerializeable() && !v.OnWasmStack
   107  			s.values[v.ID].isArg = hasAnyArgOp(v)
   108  			if f.pass.debug > stackDebug && s.values[v.ID].needSlot {
   109  				fmt.Printf("%s needs a stack slot\n", v)
   110  			}
   111  			if v.Op == OpStoreReg {
   112  				s.values[v.Args[0].ID].spill = v
   113  			}
   114  		}
   115  	}
   116  
   117  	// Compute liveness info for values needing a slot.
   118  	s.computeLive(spillLive)
   119  
   120  	// Build interference graph among values needing a slot.
   121  	s.buildInterferenceGraph()
   122  }
   123  
   124  func (s *stackAllocState) stackalloc() {
   125  	f := s.f
   126  
   127  	// Build map from values to their names, if any.
   128  	// A value may be associated with more than one name (e.g. after
   129  	// the assignment i=j). This step picks one name per value arbitrarily.
   130  	if n := f.NumValues(); cap(s.names) >= n {
   131  		s.names = s.names[:n]
   132  	} else {
   133  		s.names = make([]LocalSlot, n)
   134  	}
   135  	names := s.names
   136  	empty := LocalSlot{}
   137  	for _, name := range f.Names {
   138  		// Note: not "range f.NamedValues" above, because
   139  		// that would be nondeterministic.
   140  		for _, v := range f.NamedValues[*name] {
   141  			if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
   142  				aux := v.Aux.(*AuxNameOffset)
   143  				// Never let an arg be bound to a differently named thing.
   144  				if name.N != aux.Name || name.Off != aux.Offset {
   145  					if f.pass.debug > stackDebug {
   146  						fmt.Printf("stackalloc register arg %s skipping name %s\n", v, name)
   147  					}
   148  					continue
   149  				}
   150  			} else if name.N.Class == ir.PPARAM && v.Op != OpArg {
   151  				// PPARAM's only bind to OpArg
   152  				if f.pass.debug > stackDebug {
   153  					fmt.Printf("stackalloc PPARAM name %s skipping non-Arg %s\n", name, v)
   154  				}
   155  				continue
   156  			}
   157  
   158  			if names[v.ID] == empty {
   159  				if f.pass.debug > stackDebug {
   160  					fmt.Printf("stackalloc value %s to name %s\n", v, *name)
   161  				}
   162  				names[v.ID] = *name
   163  			}
   164  		}
   165  	}
   166  
   167  	// Allocate args to their assigned locations.
   168  	for _, v := range f.Entry.Values {
   169  		if !hasAnyArgOp(v) {
   170  			continue
   171  		}
   172  		if v.Aux == nil {
   173  			f.Fatalf("%s has nil Aux\n", v.LongString())
   174  		}
   175  		if v.Op == OpArg {
   176  			loc := LocalSlot{N: v.Aux.(*ir.Name), Type: v.Type, Off: v.AuxInt}
   177  			if f.pass.debug > stackDebug {
   178  				fmt.Printf("stackalloc OpArg %s to %s\n", v, loc)
   179  			}
   180  			f.setHome(v, loc)
   181  			continue
   182  		}
   183  		// You might think this below would be the right idea, but you would be wrong.
   184  		// It almost works; as of 105a6e9518 - 2021-04-23,
   185  		// GOSSAHASH=11011011001011111 == cmd/compile/internal/noder.(*noder).embedded
   186  		// is compiled incorrectly.  I believe the cause is one of those SSA-to-registers
   187  		// puzzles that the register allocator untangles; in the event that a register
   188  		// parameter does not end up bound to a name, "fixing" it is a bad idea.
   189  		//
   190  		//if f.DebugTest {
   191  		//	if v.Op == OpArgIntReg || v.Op == OpArgFloatReg {
   192  		//		aux := v.Aux.(*AuxNameOffset)
   193  		//		loc := LocalSlot{N: aux.Name, Type: v.Type, Off: aux.Offset}
   194  		//		if f.pass.debug > stackDebug {
   195  		//			fmt.Printf("stackalloc Op%s %s to %s\n", v.Op, v, loc)
   196  		//		}
   197  		//		names[v.ID] = loc
   198  		//		continue
   199  		//	}
   200  		//}
   201  
   202  	}
   203  
   204  	// For each type, we keep track of all the stack slots we
   205  	// have allocated for that type.
   206  	// TODO: share slots among equivalent types. We would need to
   207  	// only share among types with the same GC signature. See the
   208  	// type.Equal calls below for where this matters.
   209  	locations := map[*types.Type][]LocalSlot{}
   210  
   211  	// Each time we assign a stack slot to a value v, we remember
   212  	// the slot we used via an index into locations[v.Type].
   213  	slots := f.Cache.allocIntSlice(f.NumValues())
   214  	defer f.Cache.freeIntSlice(slots)
   215  	for i := range slots {
   216  		slots[i] = -1
   217  	}
   218  
   219  	// Pick a stack slot for each value needing one.
   220  	used := f.Cache.allocBoolSlice(f.NumValues())
   221  	defer f.Cache.freeBoolSlice(used)
   222  	for _, b := range f.Blocks {
   223  		for _, v := range b.Values {
   224  			if !s.values[v.ID].needSlot {
   225  				s.nNotNeed++
   226  				continue
   227  			}
   228  			if hasAnyArgOp(v) {
   229  				s.nArgSlot++
   230  				continue // already picked
   231  			}
   232  
   233  			// If this is a named value, try to use the name as
   234  			// the spill location.
   235  			var name LocalSlot
   236  			if v.Op == OpStoreReg {
   237  				name = names[v.Args[0].ID]
   238  			} else {
   239  				name = names[v.ID]
   240  			}
   241  			if name.N != nil && v.Type.Compare(name.Type) == types.CMPeq {
   242  				for _, id := range s.interfere[v.ID] {
   243  					h := f.getHome(id)
   244  					if h != nil && h.(LocalSlot).N == name.N && h.(LocalSlot).Off == name.Off {
   245  						// A variable can interfere with itself.
   246  						// It is rare, but it can happen.
   247  						s.nSelfInterfere++
   248  						goto noname
   249  					}
   250  				}
   251  				if f.pass.debug > stackDebug {
   252  					fmt.Printf("stackalloc %s to %s\n", v, name)
   253  				}
   254  				s.nNamedSlot++
   255  				f.setHome(v, name)
   256  				continue
   257  			}
   258  
   259  		noname:
   260  			// Set of stack slots we could reuse.
   261  			locs := locations[v.Type]
   262  			// Mark all positions in locs used by interfering values.
   263  			for i := 0; i < len(locs); i++ {
   264  				used[i] = false
   265  			}
   266  			for _, xid := range s.interfere[v.ID] {
   267  				slot := slots[xid]
   268  				if slot >= 0 {
   269  					used[slot] = true
   270  				}
   271  			}
   272  			// Find an unused stack slot.
   273  			var i int
   274  			for i = 0; i < len(locs); i++ {
   275  				if !used[i] {
   276  					s.nReuse++
   277  					break
   278  				}
   279  			}
   280  			// If there is no unused stack slot, allocate a new one.
   281  			if i == len(locs) {
   282  				s.nAuto++
   283  				locs = append(locs, LocalSlot{N: f.NewLocal(v.Pos, v.Type), Type: v.Type, Off: 0})
   284  				locations[v.Type] = locs
   285  			}
   286  			// Use the stack variable at that index for v.
   287  			loc := locs[i]
   288  			if f.pass.debug > stackDebug {
   289  				fmt.Printf("stackalloc %s to %s\n", v, loc)
   290  			}
   291  			f.setHome(v, loc)
   292  			slots[v.ID] = i
   293  		}
   294  	}
   295  }
   296  
   297  // computeLive computes a map from block ID to a list of
   298  // stack-slot-needing value IDs live at the end of that block.
   299  // TODO: this could be quadratic if lots of variables are live across lots of
   300  // basic blocks. Figure out a way to make this function (or, more precisely, the user
   301  // of this function) require only linear size & time.
   302  func (s *stackAllocState) computeLive(spillLive [][]ID) {
   303  	s.live = make([][]ID, s.f.NumBlocks())
   304  	var phis []*Value
   305  	live := s.f.newSparseSet(s.f.NumValues())
   306  	defer s.f.retSparseSet(live)
   307  	t := s.f.newSparseSet(s.f.NumValues())
   308  	defer s.f.retSparseSet(t)
   309  
   310  	// Instead of iterating over f.Blocks, iterate over their postordering.
   311  	// Liveness information flows backward, so starting at the end
   312  	// increases the probability that we will stabilize quickly.
   313  	po := s.f.postorder()
   314  	for {
   315  		changed := false
   316  		for _, b := range po {
   317  			// Start with known live values at the end of the block
   318  			live.clear()
   319  			live.addAll(s.live[b.ID])
   320  
   321  			// Propagate backwards to the start of the block
   322  			phis = phis[:0]
   323  			for i := len(b.Values) - 1; i >= 0; i-- {
   324  				v := b.Values[i]
   325  				live.remove(v.ID)
   326  				if v.Op == OpPhi {
   327  					// Save phi for later.
   328  					// Note: its args might need a stack slot even though
   329  					// the phi itself doesn't. So don't use needSlot.
   330  					if !v.Type.IsMemory() && !v.Type.IsVoid() {
   331  						phis = append(phis, v)
   332  					}
   333  					continue
   334  				}
   335  				for _, a := range v.Args {
   336  					if s.values[a.ID].needSlot {
   337  						live.add(a.ID)
   338  					}
   339  				}
   340  			}
   341  
   342  			// for each predecessor of b, expand its list of live-at-end values
   343  			// invariant: s contains the values live at the start of b (excluding phi inputs)
   344  			for i, e := range b.Preds {
   345  				p := e.b
   346  				t.clear()
   347  				t.addAll(s.live[p.ID])
   348  				t.addAll(live.contents())
   349  				t.addAll(spillLive[p.ID])
   350  				for _, v := range phis {
   351  					a := v.Args[i]
   352  					if s.values[a.ID].needSlot {
   353  						t.add(a.ID)
   354  					}
   355  					if spill := s.values[a.ID].spill; spill != nil {
   356  						//TODO: remove?  Subsumed by SpillUse?
   357  						t.add(spill.ID)
   358  					}
   359  				}
   360  				if t.size() == len(s.live[p.ID]) {
   361  					continue
   362  				}
   363  				// grow p's live set
   364  				s.live[p.ID] = append(s.live[p.ID][:0], t.contents()...)
   365  				changed = true
   366  			}
   367  		}
   368  
   369  		if !changed {
   370  			break
   371  		}
   372  	}
   373  	if s.f.pass.debug > stackDebug {
   374  		for _, b := range s.f.Blocks {
   375  			fmt.Printf("stacklive %s %v\n", b, s.live[b.ID])
   376  		}
   377  	}
   378  }
   379  
   380  func (f *Func) getHome(vid ID) Location {
   381  	if int(vid) >= len(f.RegAlloc) {
   382  		return nil
   383  	}
   384  	return f.RegAlloc[vid]
   385  }
   386  
   387  func (f *Func) setHome(v *Value, loc Location) {
   388  	for v.ID >= ID(len(f.RegAlloc)) {
   389  		f.RegAlloc = append(f.RegAlloc, nil)
   390  	}
   391  	f.RegAlloc[v.ID] = loc
   392  }
   393  
   394  func (s *stackAllocState) buildInterferenceGraph() {
   395  	f := s.f
   396  	if n := f.NumValues(); cap(s.interfere) >= n {
   397  		s.interfere = s.interfere[:n]
   398  	} else {
   399  		s.interfere = make([][]ID, n)
   400  	}
   401  	live := f.newSparseSet(f.NumValues())
   402  	defer f.retSparseSet(live)
   403  	for _, b := range f.Blocks {
   404  		// Propagate liveness backwards to the start of the block.
   405  		// Two values interfere if one is defined while the other is live.
   406  		live.clear()
   407  		live.addAll(s.live[b.ID])
   408  		for i := len(b.Values) - 1; i >= 0; i-- {
   409  			v := b.Values[i]
   410  			if s.values[v.ID].needSlot {
   411  				live.remove(v.ID)
   412  				for _, id := range live.contents() {
   413  					// Note: args can have different types and still interfere
   414  					// (with each other or with other values). See issue 23522.
   415  					if s.values[v.ID].typ.Compare(s.values[id].typ) == types.CMPeq || hasAnyArgOp(v) || s.values[id].isArg {
   416  						s.interfere[v.ID] = append(s.interfere[v.ID], id)
   417  						s.interfere[id] = append(s.interfere[id], v.ID)
   418  					}
   419  				}
   420  			}
   421  			for _, a := range v.Args {
   422  				if s.values[a.ID].needSlot {
   423  					live.add(a.ID)
   424  				}
   425  			}
   426  			if hasAnyArgOp(v) && s.values[v.ID].needSlot {
   427  				// OpArg is an input argument which is pre-spilled.
   428  				// We add back v.ID here because we want this value
   429  				// to appear live even before this point. Being live
   430  				// all the way to the start of the entry block prevents other
   431  				// values from being allocated to the same slot and clobbering
   432  				// the input value before we have a chance to load it.
   433  
   434  				// TODO(register args) this is apparently not wrong for register args -- is it necessary?
   435  				live.add(v.ID)
   436  			}
   437  		}
   438  	}
   439  	if f.pass.debug > stackDebug {
   440  		for vid, i := range s.interfere {
   441  			if len(i) > 0 {
   442  				fmt.Printf("v%d interferes with", vid)
   443  				for _, x := range i {
   444  					fmt.Printf(" v%d", x)
   445  				}
   446  				fmt.Println()
   447  			}
   448  		}
   449  	}
   450  }
   451  
   452  func hasAnyArgOp(v *Value) bool {
   453  	return v.Op == OpArg || v.Op == OpArgIntReg || v.Op == OpArgFloatReg
   454  }
   455
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