go/ssa/block.go

// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ssa

import "fmt"

// This file implements the BasicBlock type.

// addEdge adds a control-flow graph edge from from to to.
func addEdge(from, to *BasicBlock) {
	from.Succs = append(from.Succs, to)
	to.Preds = append(to.Preds, from)
}

// Parent returns the function that contains block b.
func (b *BasicBlock) Parent() *Function { return b.parent }

// String returns a human-readable label of this block.
// It is not guaranteed unique within the function.
func (b *BasicBlock) String() string {
	return fmt.Sprintf("%d", b.Index)
}

// emit appends an instruction to the current basic block.
// If the instruction defines a Value, it is returned.
func (b *BasicBlock) emit(i Instruction) Value {
	i.setBlock(b)
	b.Instrs = append(b.Instrs, i)
	v, _ := i.(Value)
	return v
}

// predIndex returns the i such that b.Preds[i] == c or panics if
// there is none.
func (b *BasicBlock) predIndex(c *BasicBlock) int {
	for i, pred := range b.Preds {
		if pred == c {
			return i
		}
	}
	panic(fmt.Sprintf("no edge %s -> %s", c, b))
}

// hasPhi returns true if b.Instrs contains φ-nodes.
func (b *BasicBlock) hasPhi() bool {
	_, ok := b.Instrs[0].(*Phi)
	return ok
}

// phis returns the prefix of b.Instrs containing all the block's φ-nodes.
func (b *BasicBlock) phis() []Instruction {
	for i, instr := range b.Instrs {
		if _, ok := instr.(*Phi); !ok {
			return b.Instrs[:i]
		}
	}
	return nil // unreachable in well-formed blocks
}

// replacePred replaces all occurrences of p in b's predecessor list with q.
// Ordinarily there should be at most one.
func (b *BasicBlock) replacePred(p, q *BasicBlock) {
	for i, pred := range b.Preds {
		if pred == p {
			b.Preds[i] = q
		}
	}
}

// replaceSucc replaces all occurrences of p in b's successor list with q.
// Ordinarily there should be at most one.
func (b *BasicBlock) replaceSucc(p, q *BasicBlock) {
	for i, succ := range b.Succs {
		if succ == p {
			b.Succs[i] = q
		}
	}
}

// removePred removes all occurrences of p in b's
// predecessor list and φ-nodes.
// Ordinarily there should be at most one.
func (b *BasicBlock) removePred(p *BasicBlock) {
	phis := b.phis()

	// We must preserve edge order for φ-nodes.
	j := 0
	for i, pred := range b.Preds {
		if pred != p {
			b.Preds[j] = b.Preds[i]
			// Strike out φ-edge too.
			for _, instr := range phis {
				phi := instr.(*Phi)
				phi.Edges[j] = phi.Edges[i]
			}
			j++
		}
	}
	// Nil out b.Preds[j:] and φ-edges[j:] to aid GC.
	for i := j; i < len(b.Preds); i++ {
		b.Preds[i] = nil
		for _, instr := range phis {
			instr.(*Phi).Edges[i] = nil
		}
	}
	b.Preds = b.Preds[:j]
	for _, instr := range phis {
		phi := instr.(*Phi)
		phi.Edges = phi.Edges[:j]
	}
}