block.go

Documentation: golang.org/x/tools/go/ssa

     1  // Copyright 2022 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 "fmt"
     8  
     9  // This file implements the BasicBlock type.
    10  
    11  // addEdge adds a control-flow graph edge from from to to.
    12  func addEdge(from, to *BasicBlock) {
    13  	from.Succs = append(from.Succs, to)
    14  	to.Preds = append(to.Preds, from)
    15  }
    16  
    17  // Parent returns the function that contains block b.
    18  func (b *BasicBlock) Parent() *Function { return b.parent }
    19  
    20  // String returns a human-readable label of this block.
    21  // It is not guaranteed unique within the function.
    22  func (b *BasicBlock) String() string {
    23  	return fmt.Sprintf("%d", b.Index)
    24  }
    25  
    26  // emit appends an instruction to the current basic block.
    27  // If the instruction defines a Value, it is returned.
    28  func (b *BasicBlock) emit(i Instruction) Value {
    29  	i.setBlock(b)
    30  	b.Instrs = append(b.Instrs, i)
    31  	v, _ := i.(Value)
    32  	return v
    33  }
    34  
    35  // predIndex returns the i such that b.Preds[i] == c or panics if
    36  // there is none.
    37  func (b *BasicBlock) predIndex(c *BasicBlock) int {
    38  	for i, pred := range b.Preds {
    39  		if pred == c {
    40  			return i
    41  		}
    42  	}
    43  	panic(fmt.Sprintf("no edge %s -> %s", c, b))
    44  }
    45  
    46  // hasPhi returns true if b.Instrs contains φ-nodes.
    47  func (b *BasicBlock) hasPhi() bool {
    48  	_, ok := b.Instrs[0].(*Phi)
    49  	return ok
    50  }
    51  
    52  // phis returns the prefix of b.Instrs containing all the block's φ-nodes.
    53  func (b *BasicBlock) phis() []Instruction {
    54  	for i, instr := range b.Instrs {
    55  		if _, ok := instr.(*Phi); !ok {
    56  			return b.Instrs[:i]
    57  		}
    58  	}
    59  	return nil // unreachable in well-formed blocks
    60  }
    61  
    62  // replacePred replaces all occurrences of p in b's predecessor list with q.
    63  // Ordinarily there should be at most one.
    64  func (b *BasicBlock) replacePred(p, q *BasicBlock) {
    65  	for i, pred := range b.Preds {
    66  		if pred == p {
    67  			b.Preds[i] = q
    68  		}
    69  	}
    70  }
    71  
    72  // replaceSucc replaces all occurrences of p in b's successor list with q.
    73  // Ordinarily there should be at most one.
    74  func (b *BasicBlock) replaceSucc(p, q *BasicBlock) {
    75  	for i, succ := range b.Succs {
    76  		if succ == p {
    77  			b.Succs[i] = q
    78  		}
    79  	}
    80  }
    81  
    82  // removePred removes all occurrences of p in b's
    83  // predecessor list and φ-nodes.
    84  // Ordinarily there should be at most one.
    85  func (b *BasicBlock) removePred(p *BasicBlock) {
    86  	phis := b.phis()
    87  
    88  	// We must preserve edge order for φ-nodes.
    89  	j := 0
    90  	for i, pred := range b.Preds {
    91  		if pred != p {
    92  			b.Preds[j] = b.Preds[i]
    93  			// Strike out φ-edge too.
    94  			for _, instr := range phis {
    95  				phi := instr.(*Phi)
    96  				phi.Edges[j] = phi.Edges[i]
    97  			}
    98  			j++
    99  		}
   100  	}
   101  	// Nil out b.Preds[j:] and φ-edges[j:] to aid GC.
   102  	for i := j; i < len(b.Preds); i++ {
   103  		b.Preds[i] = nil
   104  		for _, instr := range phis {
   105  			instr.(*Phi).Edges[i] = nil
   106  		}
   107  	}
   108  	b.Preds = b.Preds[:j]
   109  	for _, instr := range phis {
   110  		phi := instr.(*Phi)
   111  		phi.Edges = phi.Edges[:j]
   112  	}
   113  }
   114
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