| 1 | // Copyright 2013 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 | // Simple block optimizations to simplify the control flow graph. |
| 8 | |
| 9 | // TODO(adonovan): opt: instead of creating several "unreachable" blocks |
| 10 | // per function in the Builder, reuse a single one (e.g. at Blocks[1]) |
| 11 | // to reduce garbage. |
| 12 | |
| 13 | import ( |
| 14 | "fmt" |
| 15 | "os" |
| 16 | ) |
| 17 | |
| 18 | // If true, perform sanity checking and show progress at each |
| 19 | // successive iteration of optimizeBlocks. Very verbose. |
| 20 | const debugBlockOpt = false |
| 21 | |
| 22 | // markReachable sets Index=-1 for all blocks reachable from b. |
| 23 | func markReachable(b *BasicBlock) { |
| 24 | b.Index = -1 |
| 25 | for _, succ := range b.Succs { |
| 26 | if succ.Index == 0 { |
| 27 | markReachable(succ) |
| 28 | } |
| 29 | } |
| 30 | } |
| 31 | |
| 32 | // deleteUnreachableBlocks marks all reachable blocks of f and |
| 33 | // eliminates (nils) all others, including possibly cyclic subgraphs. |
| 34 | func deleteUnreachableBlocks(f *Function) { |
| 35 | const white, black = 0, -1 |
| 36 | // We borrow b.Index temporarily as the mark bit. |
| 37 | for _, b := range f.Blocks { |
| 38 | b.Index = white |
| 39 | } |
| 40 | markReachable(f.Blocks[0]) |
| 41 | if f.Recover != nil { |
| 42 | markReachable(f.Recover) |
| 43 | } |
| 44 | for i, b := range f.Blocks { |
| 45 | if b.Index == white { |
| 46 | for _, c := range b.Succs { |
| 47 | if c.Index == black { |
| 48 | c.removePred(b) // delete white->black edge |
| 49 | } |
| 50 | } |
| 51 | if debugBlockOpt { |
| 52 | fmt.Fprintln(os.Stderr, "unreachable", b) |
| 53 | } |
| 54 | f.Blocks[i] = nil // delete b |
| 55 | } |
| 56 | } |
| 57 | f.removeNilBlocks() |
| 58 | } |
| 59 | |
| 60 | // jumpThreading attempts to apply simple jump-threading to block b, |
| 61 | // in which a->b->c become a->c if b is just a Jump. |
| 62 | // The result is true if the optimization was applied. |
| 63 | func jumpThreading(f *Function, b *BasicBlock) bool { |
| 64 | if b.Index == 0 { |
| 65 | return false // don't apply to entry block |
| 66 | } |
| 67 | if b.Instrs == nil { |
| 68 | return false |
| 69 | } |
| 70 | if _, ok := b.Instrs[0].(*Jump); !ok { |
| 71 | return false // not just a jump |
| 72 | } |
| 73 | c := b.Succs[0] |
| 74 | if c == b { |
| 75 | return false // don't apply to degenerate jump-to-self. |
| 76 | } |
| 77 | if c.hasPhi() { |
| 78 | return false // not sound without more effort |
| 79 | } |
| 80 | for j, a := range b.Preds { |
| 81 | a.replaceSucc(b, c) |
| 82 | |
| 83 | // If a now has two edges to c, replace its degenerate If by Jump. |
| 84 | if len(a.Succs) == 2 && a.Succs[0] == c && a.Succs[1] == c { |
| 85 | jump := new(Jump) |
| 86 | jump.setBlock(a) |
| 87 | a.Instrs[len(a.Instrs)-1] = jump |
| 88 | a.Succs = a.Succs[:1] |
| 89 | c.removePred(b) |
| 90 | } else { |
| 91 | if j == 0 { |
| 92 | c.replacePred(b, a) |
| 93 | } else { |
| 94 | c.Preds = append(c.Preds, a) |
| 95 | } |
| 96 | } |
| 97 | |
| 98 | if debugBlockOpt { |
| 99 | fmt.Fprintln(os.Stderr, "jumpThreading", a, b, c) |
| 100 | } |
| 101 | } |
| 102 | f.Blocks[b.Index] = nil // delete b |
| 103 | return true |
| 104 | } |
| 105 | |
| 106 | // fuseBlocks attempts to apply the block fusion optimization to block |
| 107 | // a, in which a->b becomes ab if len(a.Succs)==len(b.Preds)==1. |
| 108 | // The result is true if the optimization was applied. |
| 109 | func fuseBlocks(f *Function, a *BasicBlock) bool { |
| 110 | if len(a.Succs) != 1 { |
| 111 | return false |
| 112 | } |
| 113 | b := a.Succs[0] |
| 114 | if len(b.Preds) != 1 { |
| 115 | return false |
| 116 | } |
| 117 | |
| 118 | // Degenerate &&/|| ops may result in a straight-line CFG |
| 119 | // containing φ-nodes. (Ideally we'd replace such them with |
| 120 | // their sole operand but that requires Referrers, built later.) |
| 121 | if b.hasPhi() { |
| 122 | return false // not sound without further effort |
| 123 | } |
| 124 | |
| 125 | // Eliminate jump at end of A, then copy all of B across. |
| 126 | a.Instrs = append(a.Instrs[:len(a.Instrs)-1], b.Instrs...) |
| 127 | for _, instr := range b.Instrs { |
| 128 | instr.setBlock(a) |
| 129 | } |
| 130 | |
| 131 | // A inherits B's successors |
| 132 | a.Succs = append(a.succs2[:0], b.Succs...) |
| 133 | |
| 134 | // Fix up Preds links of all successors of B. |
| 135 | for _, c := range b.Succs { |
| 136 | c.replacePred(b, a) |
| 137 | } |
| 138 | |
| 139 | if debugBlockOpt { |
| 140 | fmt.Fprintln(os.Stderr, "fuseBlocks", a, b) |
| 141 | } |
| 142 | |
| 143 | f.Blocks[b.Index] = nil // delete b |
| 144 | return true |
| 145 | } |
| 146 | |
| 147 | // optimizeBlocks() performs some simple block optimizations on a |
| 148 | // completed function: dead block elimination, block fusion, jump |
| 149 | // threading. |
| 150 | func optimizeBlocks(f *Function) { |
| 151 | deleteUnreachableBlocks(f) |
| 152 | |
| 153 | // Loop until no further progress. |
| 154 | changed := true |
| 155 | for changed { |
| 156 | changed = false |
| 157 | |
| 158 | if debugBlockOpt { |
| 159 | f.WriteTo(os.Stderr) |
| 160 | mustSanityCheck(f, nil) |
| 161 | } |
| 162 | |
| 163 | for _, b := range f.Blocks { |
| 164 | // f.Blocks will temporarily contain nils to indicate |
| 165 | // deleted blocks; we remove them at the end. |
| 166 | if b == nil { |
| 167 | continue |
| 168 | } |
| 169 | |
| 170 | // Fuse blocks. b->c becomes bc. |
| 171 | if fuseBlocks(f, b) { |
| 172 | changed = true |
| 173 | } |
| 174 | |
| 175 | // a->b->c becomes a->c if b contains only a Jump. |
| 176 | if jumpThreading(f, b) { |
| 177 | changed = true |
| 178 | continue // (b was disconnected) |
| 179 | } |
| 180 | } |
| 181 | } |
| 182 | f.removeNilBlocks() |
| 183 | } |
| 184 |
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