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block.go (2848B)

      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 }