block.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/internal/src"
     9  	"fmt"
    10  )
    11  
    12  // Block represents a basic block in the control flow graph of a function.
    13  type Block struct {
    14  	// A unique identifier for the block. The system will attempt to allocate
    15  	// these IDs densely, but no guarantees.
    16  	ID ID
    17  
    18  	// Source position for block's control operation
    19  	Pos src.XPos
    20  
    21  	// The kind of block this is.
    22  	Kind BlockKind
    23  
    24  	// Likely direction for branches.
    25  	// If BranchLikely, Succs[0] is the most likely branch taken.
    26  	// If BranchUnlikely, Succs[1] is the most likely branch taken.
    27  	// Ignored if len(Succs) < 2.
    28  	// Fatal if not BranchUnknown and len(Succs) > 2.
    29  	Likely BranchPrediction
    30  
    31  	// After flagalloc, records whether flags are live at the end of the block.
    32  	FlagsLiveAtEnd bool
    33  
    34  	// Subsequent blocks, if any. The number and order depend on the block kind.
    35  	Succs []Edge
    36  
    37  	// Inverse of successors.
    38  	// The order is significant to Phi nodes in the block.
    39  	// TODO: predecessors is a pain to maintain. Can we somehow order phi
    40  	// arguments by block id and have this field computed explicitly when needed?
    41  	Preds []Edge
    42  
    43  	// A list of values that determine how the block is exited. The number
    44  	// and type of control values depends on the Kind of the block. For
    45  	// instance, a BlockIf has a single boolean control value and BlockExit
    46  	// has a single memory control value.
    47  	//
    48  	// The ControlValues() method may be used to get a slice with the non-nil
    49  	// control values that can be ranged over.
    50  	//
    51  	// Controls[1] must be nil if Controls[0] is nil.
    52  	Controls [2]*Value
    53  
    54  	// Auxiliary info for the block. Its value depends on the Kind.
    55  	Aux    Aux
    56  	AuxInt int64
    57  
    58  	// The unordered set of Values that define the operation of this block.
    59  	// After the scheduling pass, this list is ordered.
    60  	Values []*Value
    61  
    62  	// The containing function
    63  	Func *Func
    64  
    65  	// Storage for Succs, Preds and Values.
    66  	succstorage [2]Edge
    67  	predstorage [4]Edge
    68  	valstorage  [9]*Value
    69  }
    70  
    71  // Edge represents a CFG edge.
    72  // Example edges for b branching to either c or d.
    73  // (c and d have other predecessors.)
    74  //
    75  //	b.Succs = [{c,3}, {d,1}]
    76  //	c.Preds = [?, ?, ?, {b,0}]
    77  //	d.Preds = [?, {b,1}, ?]
    78  //
    79  // These indexes allow us to edit the CFG in constant time.
    80  // In addition, it informs phi ops in degenerate cases like:
    81  //
    82  //	b:
    83  //	   if k then c else c
    84  //	c:
    85  //	   v = Phi(x, y)
    86  //
    87  // Then the indexes tell you whether x is chosen from
    88  // the if or else branch from b.
    89  //
    90  //	b.Succs = [{c,0},{c,1}]
    91  //	c.Preds = [{b,0},{b,1}]
    92  //
    93  // means x is chosen if k is true.
    94  type Edge struct {
    95  	// block edge goes to (in a Succs list) or from (in a Preds list)
    96  	b *Block
    97  	// index of reverse edge.  Invariant:
    98  	//   e := x.Succs[idx]
    99  	//   e.b.Preds[e.i] = Edge{x,idx}
   100  	// and similarly for predecessors.
   101  	i int
   102  }
   103  
   104  func (e Edge) Block() *Block {
   105  	return e.b
   106  }
   107  func (e Edge) Index() int {
   108  	return e.i
   109  }
   110  func (e Edge) String() string {
   111  	return fmt.Sprintf("{%v,%d}", e.b, e.i)
   112  }
   113  
   114  // BlockKind is the kind of SSA block.
   115  type BlockKind int16
   116  
   117  // short form print
   118  func (b *Block) String() string {
   119  	return fmt.Sprintf("b%d", b.ID)
   120  }
   121  
   122  // long form print
   123  func (b *Block) LongString() string {
   124  	s := b.Kind.String()
   125  	if b.Aux != nil {
   126  		s += fmt.Sprintf(" {%s}", b.Aux)
   127  	}
   128  	if t := b.AuxIntString(); t != "" {
   129  		s += fmt.Sprintf(" [%s]", t)
   130  	}
   131  	for _, c := range b.ControlValues() {
   132  		s += fmt.Sprintf(" %s", c)
   133  	}
   134  	if len(b.Succs) > 0 {
   135  		s += " ->"
   136  		for _, c := range b.Succs {
   137  			s += " " + c.b.String()
   138  		}
   139  	}
   140  	switch b.Likely {
   141  	case BranchUnlikely:
   142  		s += " (unlikely)"
   143  	case BranchLikely:
   144  		s += " (likely)"
   145  	}
   146  	return s
   147  }
   148  
   149  // NumControls returns the number of non-nil control values the
   150  // block has.
   151  func (b *Block) NumControls() int {
   152  	if b.Controls[0] == nil {
   153  		return 0
   154  	}
   155  	if b.Controls[1] == nil {
   156  		return 1
   157  	}
   158  	return 2
   159  }
   160  
   161  // ControlValues returns a slice containing the non-nil control
   162  // values of the block. The index of each control value will be
   163  // the same as it is in the Controls property and can be used
   164  // in ReplaceControl calls.
   165  func (b *Block) ControlValues() []*Value {
   166  	if b.Controls[0] == nil {
   167  		return b.Controls[:0]
   168  	}
   169  	if b.Controls[1] == nil {
   170  		return b.Controls[:1]
   171  	}
   172  	return b.Controls[:2]
   173  }
   174  
   175  // SetControl removes all existing control values and then adds
   176  // the control value provided. The number of control values after
   177  // a call to SetControl will always be 1.
   178  func (b *Block) SetControl(v *Value) {
   179  	b.ResetControls()
   180  	b.Controls[0] = v
   181  	v.Uses++
   182  }
   183  
   184  // ResetControls sets the number of controls for the block to 0.
   185  func (b *Block) ResetControls() {
   186  	if b.Controls[0] != nil {
   187  		b.Controls[0].Uses--
   188  	}
   189  	if b.Controls[1] != nil {
   190  		b.Controls[1].Uses--
   191  	}
   192  	b.Controls = [2]*Value{} // reset both controls to nil
   193  }
   194  
   195  // AddControl appends a control value to the existing list of control values.
   196  func (b *Block) AddControl(v *Value) {
   197  	i := b.NumControls()
   198  	b.Controls[i] = v // panics if array is full
   199  	v.Uses++
   200  }
   201  
   202  // ReplaceControl exchanges the existing control value at the index provided
   203  // for the new value. The index must refer to a valid control value.
   204  func (b *Block) ReplaceControl(i int, v *Value) {
   205  	b.Controls[i].Uses--
   206  	b.Controls[i] = v
   207  	v.Uses++
   208  }
   209  
   210  // CopyControls replaces the controls for this block with those from the
   211  // provided block. The provided block is not modified.
   212  func (b *Block) CopyControls(from *Block) {
   213  	if b == from {
   214  		return
   215  	}
   216  	b.ResetControls()
   217  	for _, c := range from.ControlValues() {
   218  		b.AddControl(c)
   219  	}
   220  }
   221  
   222  // Reset sets the block to the provided kind and clears all the blocks control
   223  // and auxiliary values. Other properties of the block, such as its successors,
   224  // predecessors and values are left unmodified.
   225  func (b *Block) Reset(kind BlockKind) {
   226  	b.Kind = kind
   227  	b.ResetControls()
   228  	b.Aux = nil
   229  	b.AuxInt = 0
   230  }
   231  
   232  // resetWithControl resets b and adds control v.
   233  // It is equivalent to b.Reset(kind); b.AddControl(v),
   234  // except that it is one call instead of two and avoids a bounds check.
   235  // It is intended for use by rewrite rules, where this matters.
   236  func (b *Block) resetWithControl(kind BlockKind, v *Value) {
   237  	b.Kind = kind
   238  	b.ResetControls()
   239  	b.Aux = nil
   240  	b.AuxInt = 0
   241  	b.Controls[0] = v
   242  	v.Uses++
   243  }
   244  
   245  // resetWithControl2 resets b and adds controls v and w.
   246  // It is equivalent to b.Reset(kind); b.AddControl(v); b.AddControl(w),
   247  // except that it is one call instead of three and avoids two bounds checks.
   248  // It is intended for use by rewrite rules, where this matters.
   249  func (b *Block) resetWithControl2(kind BlockKind, v, w *Value) {
   250  	b.Kind = kind
   251  	b.ResetControls()
   252  	b.Aux = nil
   253  	b.AuxInt = 0
   254  	b.Controls[0] = v
   255  	b.Controls[1] = w
   256  	v.Uses++
   257  	w.Uses++
   258  }
   259  
   260  // truncateValues truncates b.Values at the ith element, zeroing subsequent elements.
   261  // The values in b.Values after i must already have had their args reset,
   262  // to maintain correct value uses counts.
   263  func (b *Block) truncateValues(i int) {
   264  	tail := b.Values[i:]
   265  	for j := range tail {
   266  		tail[j] = nil
   267  	}
   268  	b.Values = b.Values[:i]
   269  }
   270  
   271  // AddEdgeTo adds an edge from block b to block c.
   272  func (b *Block) AddEdgeTo(c *Block) {
   273  	i := len(b.Succs)
   274  	j := len(c.Preds)
   275  	b.Succs = append(b.Succs, Edge{c, j})
   276  	c.Preds = append(c.Preds, Edge{b, i})
   277  	b.Func.invalidateCFG()
   278  }
   279  
   280  // removePred removes the ith input edge from b.
   281  // It is the responsibility of the caller to remove
   282  // the corresponding successor edge, and adjust any
   283  // phi values by calling b.removePhiArg(v, i).
   284  func (b *Block) removePred(i int) {
   285  	n := len(b.Preds) - 1
   286  	if i != n {
   287  		e := b.Preds[n]
   288  		b.Preds[i] = e
   289  		// Update the other end of the edge we moved.
   290  		e.b.Succs[e.i].i = i
   291  	}
   292  	b.Preds[n] = Edge{}
   293  	b.Preds = b.Preds[:n]
   294  	b.Func.invalidateCFG()
   295  }
   296  
   297  // removeSucc removes the ith output edge from b.
   298  // It is the responsibility of the caller to remove
   299  // the corresponding predecessor edge.
   300  // Note that this potentially reorders successors of b, so it
   301  // must be used very carefully.
   302  func (b *Block) removeSucc(i int) {
   303  	n := len(b.Succs) - 1
   304  	if i != n {
   305  		e := b.Succs[n]
   306  		b.Succs[i] = e
   307  		// Update the other end of the edge we moved.
   308  		e.b.Preds[e.i].i = i
   309  	}
   310  	b.Succs[n] = Edge{}
   311  	b.Succs = b.Succs[:n]
   312  	b.Func.invalidateCFG()
   313  }
   314  
   315  func (b *Block) swapSuccessors() {
   316  	if len(b.Succs) != 2 {
   317  		b.Fatalf("swapSuccessors with len(Succs)=%d", len(b.Succs))
   318  	}
   319  	e0 := b.Succs[0]
   320  	e1 := b.Succs[1]
   321  	b.Succs[0] = e1
   322  	b.Succs[1] = e0
   323  	e0.b.Preds[e0.i].i = 1
   324  	e1.b.Preds[e1.i].i = 0
   325  	b.Likely *= -1
   326  }
   327  
   328  // Swaps b.Succs[x] and b.Succs[y].
   329  func (b *Block) swapSuccessorsByIdx(x, y int) {
   330  	if x == y {
   331  		return
   332  	}
   333  	ex := b.Succs[x]
   334  	ey := b.Succs[y]
   335  	b.Succs[x] = ey
   336  	b.Succs[y] = ex
   337  	ex.b.Preds[ex.i].i = y
   338  	ey.b.Preds[ey.i].i = x
   339  }
   340  
   341  // removePhiArg removes the ith arg from phi.
   342  // It must be called after calling b.removePred(i) to
   343  // adjust the corresponding phi value of the block:
   344  //
   345  // b.removePred(i)
   346  // for _, v := range b.Values {
   347  //
   348  //	if v.Op != OpPhi {
   349  //	    continue
   350  //	}
   351  //	b.removePhiArg(v, i)
   352  //
   353  // }
   354  func (b *Block) removePhiArg(phi *Value, i int) {
   355  	n := len(b.Preds)
   356  	if numPhiArgs := len(phi.Args); numPhiArgs-1 != n {
   357  		b.Fatalf("inconsistent state for %v, num predecessors: %d, num phi args: %d", phi, n, numPhiArgs)
   358  	}
   359  	phi.Args[i].Uses--
   360  	phi.Args[i] = phi.Args[n]
   361  	phi.Args[n] = nil
   362  	phi.Args = phi.Args[:n]
   363  	phielimValue(phi)
   364  }
   365  
   366  // LackingPos indicates whether b is a block whose position should be inherited
   367  // from its successors.  This is true if all the values within it have unreliable positions
   368  // and if it is "plain", meaning that there is no control flow that is also very likely
   369  // to correspond to a well-understood source position.
   370  func (b *Block) LackingPos() bool {
   371  	// Non-plain predecessors are If or Defer, which both (1) have two successors,
   372  	// which might have different line numbers and (2) correspond to statements
   373  	// in the source code that have positions, so this case ought not occur anyway.
   374  	if b.Kind != BlockPlain {
   375  		return false
   376  	}
   377  	if b.Pos != src.NoXPos {
   378  		return false
   379  	}
   380  	for _, v := range b.Values {
   381  		if v.LackingPos() {
   382  			continue
   383  		}
   384  		return false
   385  	}
   386  	return true
   387  }
   388  
   389  func (b *Block) AuxIntString() string {
   390  	switch b.Kind.AuxIntType() {
   391  	case "int8":
   392  		return fmt.Sprintf("%v", int8(b.AuxInt))
   393  	case "uint8":
   394  		return fmt.Sprintf("%v", uint8(b.AuxInt))
   395  	case "": // no aux int type
   396  		return ""
   397  	default: // type specified but not implemented - print as int64
   398  		return fmt.Sprintf("%v", b.AuxInt)
   399  	}
   400  }
   401  
   402  // likelyBranch reports whether block b is the likely branch of all of its predecessors.
   403  func (b *Block) likelyBranch() bool {
   404  	if len(b.Preds) == 0 {
   405  		return false
   406  	}
   407  	for _, e := range b.Preds {
   408  		p := e.b
   409  		if len(p.Succs) == 1 || len(p.Succs) == 2 && (p.Likely == BranchLikely && p.Succs[0].b == b ||
   410  			p.Likely == BranchUnlikely && p.Succs[1].b == b) {
   411  			continue
   412  		}
   413  		return false
   414  	}
   415  	return true
   416  }
   417  
   418  func (b *Block) Logf(msg string, args ...interface{})   { b.Func.Logf(msg, args...) }
   419  func (b *Block) Log() bool                              { return b.Func.Log() }
   420  func (b *Block) Fatalf(msg string, args ...interface{}) { b.Func.Fatalf(msg, args...) }
   421  
   422  type BranchPrediction int8
   423  
   424  const (
   425  	BranchUnlikely = BranchPrediction(-1)
   426  	BranchUnknown  = BranchPrediction(0)
   427  	BranchLikely   = BranchPrediction(+1)
   428  )
   429
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