| 1 | // Copyright 2016 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 cfg |
| 6 | |
| 7 | // This file implements the CFG construction pass. |
| 8 | |
| 9 | import ( |
| 10 | "fmt" |
| 11 | "go/ast" |
| 12 | "go/token" |
| 13 | ) |
| 14 | |
| 15 | type builder struct { |
| 16 | cfg *CFG |
| 17 | mayReturn func(*ast.CallExpr) bool |
| 18 | current *Block |
| 19 | lblocks map[*ast.Object]*lblock // labeled blocks |
| 20 | targets *targets // linked stack of branch targets |
| 21 | } |
| 22 | |
| 23 | func (b *builder) stmt(_s ast.Stmt) { |
| 24 | // The label of the current statement. If non-nil, its _goto |
| 25 | // target is always set; its _break and _continue are set only |
| 26 | // within the body of switch/typeswitch/select/for/range. |
| 27 | // It is effectively an additional default-nil parameter of stmt(). |
| 28 | var label *lblock |
| 29 | start: |
| 30 | switch s := _s.(type) { |
| 31 | case *ast.BadStmt, |
| 32 | *ast.SendStmt, |
| 33 | *ast.IncDecStmt, |
| 34 | *ast.GoStmt, |
| 35 | *ast.DeferStmt, |
| 36 | *ast.EmptyStmt, |
| 37 | *ast.AssignStmt: |
| 38 | // No effect on control flow. |
| 39 | b.add(s) |
| 40 | |
| 41 | case *ast.ExprStmt: |
| 42 | b.add(s) |
| 43 | if call, ok := s.X.(*ast.CallExpr); ok && !b.mayReturn(call) { |
| 44 | // Calls to panic, os.Exit, etc, never return. |
| 45 | b.current = b.newBlock("unreachable.call") |
| 46 | } |
| 47 | |
| 48 | case *ast.DeclStmt: |
| 49 | // Treat each var ValueSpec as a separate statement. |
| 50 | d := s.Decl.(*ast.GenDecl) |
| 51 | if d.Tok == token.VAR { |
| 52 | for _, spec := range d.Specs { |
| 53 | if spec, ok := spec.(*ast.ValueSpec); ok { |
| 54 | b.add(spec) |
| 55 | } |
| 56 | } |
| 57 | } |
| 58 | |
| 59 | case *ast.LabeledStmt: |
| 60 | label = b.labeledBlock(s.Label) |
| 61 | b.jump(label._goto) |
| 62 | b.current = label._goto |
| 63 | _s = s.Stmt |
| 64 | goto start // effectively: tailcall stmt(g, s.Stmt, label) |
| 65 | |
| 66 | case *ast.ReturnStmt: |
| 67 | b.add(s) |
| 68 | b.current = b.newBlock("unreachable.return") |
| 69 | |
| 70 | case *ast.BranchStmt: |
| 71 | b.branchStmt(s) |
| 72 | |
| 73 | case *ast.BlockStmt: |
| 74 | b.stmtList(s.List) |
| 75 | |
| 76 | case *ast.IfStmt: |
| 77 | if s.Init != nil { |
| 78 | b.stmt(s.Init) |
| 79 | } |
| 80 | then := b.newBlock("if.then") |
| 81 | done := b.newBlock("if.done") |
| 82 | _else := done |
| 83 | if s.Else != nil { |
| 84 | _else = b.newBlock("if.else") |
| 85 | } |
| 86 | b.add(s.Cond) |
| 87 | b.ifelse(then, _else) |
| 88 | b.current = then |
| 89 | b.stmt(s.Body) |
| 90 | b.jump(done) |
| 91 | |
| 92 | if s.Else != nil { |
| 93 | b.current = _else |
| 94 | b.stmt(s.Else) |
| 95 | b.jump(done) |
| 96 | } |
| 97 | |
| 98 | b.current = done |
| 99 | |
| 100 | case *ast.SwitchStmt: |
| 101 | b.switchStmt(s, label) |
| 102 | |
| 103 | case *ast.TypeSwitchStmt: |
| 104 | b.typeSwitchStmt(s, label) |
| 105 | |
| 106 | case *ast.SelectStmt: |
| 107 | b.selectStmt(s, label) |
| 108 | |
| 109 | case *ast.ForStmt: |
| 110 | b.forStmt(s, label) |
| 111 | |
| 112 | case *ast.RangeStmt: |
| 113 | b.rangeStmt(s, label) |
| 114 | |
| 115 | default: |
| 116 | panic(fmt.Sprintf("unexpected statement kind: %T", s)) |
| 117 | } |
| 118 | } |
| 119 | |
| 120 | func (b *builder) stmtList(list []ast.Stmt) { |
| 121 | for _, s := range list { |
| 122 | b.stmt(s) |
| 123 | } |
| 124 | } |
| 125 | |
| 126 | func (b *builder) branchStmt(s *ast.BranchStmt) { |
| 127 | var block *Block |
| 128 | switch s.Tok { |
| 129 | case token.BREAK: |
| 130 | if s.Label != nil { |
| 131 | if lb := b.labeledBlock(s.Label); lb != nil { |
| 132 | block = lb._break |
| 133 | } |
| 134 | } else { |
| 135 | for t := b.targets; t != nil && block == nil; t = t.tail { |
| 136 | block = t._break |
| 137 | } |
| 138 | } |
| 139 | |
| 140 | case token.CONTINUE: |
| 141 | if s.Label != nil { |
| 142 | if lb := b.labeledBlock(s.Label); lb != nil { |
| 143 | block = lb._continue |
| 144 | } |
| 145 | } else { |
| 146 | for t := b.targets; t != nil && block == nil; t = t.tail { |
| 147 | block = t._continue |
| 148 | } |
| 149 | } |
| 150 | |
| 151 | case token.FALLTHROUGH: |
| 152 | for t := b.targets; t != nil && block == nil; t = t.tail { |
| 153 | block = t._fallthrough |
| 154 | } |
| 155 | |
| 156 | case token.GOTO: |
| 157 | if s.Label != nil { |
| 158 | block = b.labeledBlock(s.Label)._goto |
| 159 | } |
| 160 | } |
| 161 | if block == nil { |
| 162 | block = b.newBlock("undefined.branch") |
| 163 | } |
| 164 | b.jump(block) |
| 165 | b.current = b.newBlock("unreachable.branch") |
| 166 | } |
| 167 | |
| 168 | func (b *builder) switchStmt(s *ast.SwitchStmt, label *lblock) { |
| 169 | if s.Init != nil { |
| 170 | b.stmt(s.Init) |
| 171 | } |
| 172 | if s.Tag != nil { |
| 173 | b.add(s.Tag) |
| 174 | } |
| 175 | done := b.newBlock("switch.done") |
| 176 | if label != nil { |
| 177 | label._break = done |
| 178 | } |
| 179 | // We pull the default case (if present) down to the end. |
| 180 | // But each fallthrough label must point to the next |
| 181 | // body block in source order, so we preallocate a |
| 182 | // body block (fallthru) for the next case. |
| 183 | // Unfortunately this makes for a confusing block order. |
| 184 | var defaultBody *[]ast.Stmt |
| 185 | var defaultFallthrough *Block |
| 186 | var fallthru, defaultBlock *Block |
| 187 | ncases := len(s.Body.List) |
| 188 | for i, clause := range s.Body.List { |
| 189 | body := fallthru |
| 190 | if body == nil { |
| 191 | body = b.newBlock("switch.body") // first case only |
| 192 | } |
| 193 | |
| 194 | // Preallocate body block for the next case. |
| 195 | fallthru = done |
| 196 | if i+1 < ncases { |
| 197 | fallthru = b.newBlock("switch.body") |
| 198 | } |
| 199 | |
| 200 | cc := clause.(*ast.CaseClause) |
| 201 | if cc.List == nil { |
| 202 | // Default case. |
| 203 | defaultBody = &cc.Body |
| 204 | defaultFallthrough = fallthru |
| 205 | defaultBlock = body |
| 206 | continue |
| 207 | } |
| 208 | |
| 209 | var nextCond *Block |
| 210 | for _, cond := range cc.List { |
| 211 | nextCond = b.newBlock("switch.next") |
| 212 | b.add(cond) // one half of the tag==cond condition |
| 213 | b.ifelse(body, nextCond) |
| 214 | b.current = nextCond |
| 215 | } |
| 216 | b.current = body |
| 217 | b.targets = &targets{ |
| 218 | tail: b.targets, |
| 219 | _break: done, |
| 220 | _fallthrough: fallthru, |
| 221 | } |
| 222 | b.stmtList(cc.Body) |
| 223 | b.targets = b.targets.tail |
| 224 | b.jump(done) |
| 225 | b.current = nextCond |
| 226 | } |
| 227 | if defaultBlock != nil { |
| 228 | b.jump(defaultBlock) |
| 229 | b.current = defaultBlock |
| 230 | b.targets = &targets{ |
| 231 | tail: b.targets, |
| 232 | _break: done, |
| 233 | _fallthrough: defaultFallthrough, |
| 234 | } |
| 235 | b.stmtList(*defaultBody) |
| 236 | b.targets = b.targets.tail |
| 237 | } |
| 238 | b.jump(done) |
| 239 | b.current = done |
| 240 | } |
| 241 | |
| 242 | func (b *builder) typeSwitchStmt(s *ast.TypeSwitchStmt, label *lblock) { |
| 243 | if s.Init != nil { |
| 244 | b.stmt(s.Init) |
| 245 | } |
| 246 | if s.Assign != nil { |
| 247 | b.add(s.Assign) |
| 248 | } |
| 249 | |
| 250 | done := b.newBlock("typeswitch.done") |
| 251 | if label != nil { |
| 252 | label._break = done |
| 253 | } |
| 254 | var default_ *ast.CaseClause |
| 255 | for _, clause := range s.Body.List { |
| 256 | cc := clause.(*ast.CaseClause) |
| 257 | if cc.List == nil { |
| 258 | default_ = cc |
| 259 | continue |
| 260 | } |
| 261 | body := b.newBlock("typeswitch.body") |
| 262 | var next *Block |
| 263 | for _, casetype := range cc.List { |
| 264 | next = b.newBlock("typeswitch.next") |
| 265 | // casetype is a type, so don't call b.add(casetype). |
| 266 | // This block logically contains a type assertion, |
| 267 | // x.(casetype), but it's unclear how to represent x. |
| 268 | _ = casetype |
| 269 | b.ifelse(body, next) |
| 270 | b.current = next |
| 271 | } |
| 272 | b.current = body |
| 273 | b.typeCaseBody(cc, done) |
| 274 | b.current = next |
| 275 | } |
| 276 | if default_ != nil { |
| 277 | b.typeCaseBody(default_, done) |
| 278 | } else { |
| 279 | b.jump(done) |
| 280 | } |
| 281 | b.current = done |
| 282 | } |
| 283 | |
| 284 | func (b *builder) typeCaseBody(cc *ast.CaseClause, done *Block) { |
| 285 | b.targets = &targets{ |
| 286 | tail: b.targets, |
| 287 | _break: done, |
| 288 | } |
| 289 | b.stmtList(cc.Body) |
| 290 | b.targets = b.targets.tail |
| 291 | b.jump(done) |
| 292 | } |
| 293 | |
| 294 | func (b *builder) selectStmt(s *ast.SelectStmt, label *lblock) { |
| 295 | // First evaluate channel expressions. |
| 296 | // TODO(adonovan): fix: evaluate only channel exprs here. |
| 297 | for _, clause := range s.Body.List { |
| 298 | if comm := clause.(*ast.CommClause).Comm; comm != nil { |
| 299 | b.stmt(comm) |
| 300 | } |
| 301 | } |
| 302 | |
| 303 | done := b.newBlock("select.done") |
| 304 | if label != nil { |
| 305 | label._break = done |
| 306 | } |
| 307 | |
| 308 | var defaultBody *[]ast.Stmt |
| 309 | for _, cc := range s.Body.List { |
| 310 | clause := cc.(*ast.CommClause) |
| 311 | if clause.Comm == nil { |
| 312 | defaultBody = &clause.Body |
| 313 | continue |
| 314 | } |
| 315 | body := b.newBlock("select.body") |
| 316 | next := b.newBlock("select.next") |
| 317 | b.ifelse(body, next) |
| 318 | b.current = body |
| 319 | b.targets = &targets{ |
| 320 | tail: b.targets, |
| 321 | _break: done, |
| 322 | } |
| 323 | switch comm := clause.Comm.(type) { |
| 324 | case *ast.ExprStmt: // <-ch |
| 325 | // nop |
| 326 | case *ast.AssignStmt: // x := <-states[state].Chan |
| 327 | b.add(comm.Lhs[0]) |
| 328 | } |
| 329 | b.stmtList(clause.Body) |
| 330 | b.targets = b.targets.tail |
| 331 | b.jump(done) |
| 332 | b.current = next |
| 333 | } |
| 334 | if defaultBody != nil { |
| 335 | b.targets = &targets{ |
| 336 | tail: b.targets, |
| 337 | _break: done, |
| 338 | } |
| 339 | b.stmtList(*defaultBody) |
| 340 | b.targets = b.targets.tail |
| 341 | b.jump(done) |
| 342 | } |
| 343 | b.current = done |
| 344 | } |
| 345 | |
| 346 | func (b *builder) forStmt(s *ast.ForStmt, label *lblock) { |
| 347 | // ...init... |
| 348 | // jump loop |
| 349 | // loop: |
| 350 | // if cond goto body else done |
| 351 | // body: |
| 352 | // ...body... |
| 353 | // jump post |
| 354 | // post: (target of continue) |
| 355 | // ...post... |
| 356 | // jump loop |
| 357 | // done: (target of break) |
| 358 | if s.Init != nil { |
| 359 | b.stmt(s.Init) |
| 360 | } |
| 361 | body := b.newBlock("for.body") |
| 362 | done := b.newBlock("for.done") // target of 'break' |
| 363 | loop := body // target of back-edge |
| 364 | if s.Cond != nil { |
| 365 | loop = b.newBlock("for.loop") |
| 366 | } |
| 367 | cont := loop // target of 'continue' |
| 368 | if s.Post != nil { |
| 369 | cont = b.newBlock("for.post") |
| 370 | } |
| 371 | if label != nil { |
| 372 | label._break = done |
| 373 | label._continue = cont |
| 374 | } |
| 375 | b.jump(loop) |
| 376 | b.current = loop |
| 377 | if loop != body { |
| 378 | b.add(s.Cond) |
| 379 | b.ifelse(body, done) |
| 380 | b.current = body |
| 381 | } |
| 382 | b.targets = &targets{ |
| 383 | tail: b.targets, |
| 384 | _break: done, |
| 385 | _continue: cont, |
| 386 | } |
| 387 | b.stmt(s.Body) |
| 388 | b.targets = b.targets.tail |
| 389 | b.jump(cont) |
| 390 | |
| 391 | if s.Post != nil { |
| 392 | b.current = cont |
| 393 | b.stmt(s.Post) |
| 394 | b.jump(loop) // back-edge |
| 395 | } |
| 396 | b.current = done |
| 397 | } |
| 398 | |
| 399 | func (b *builder) rangeStmt(s *ast.RangeStmt, label *lblock) { |
| 400 | b.add(s.X) |
| 401 | |
| 402 | if s.Key != nil { |
| 403 | b.add(s.Key) |
| 404 | } |
| 405 | if s.Value != nil { |
| 406 | b.add(s.Value) |
| 407 | } |
| 408 | |
| 409 | // ... |
| 410 | // loop: (target of continue) |
| 411 | // if ... goto body else done |
| 412 | // body: |
| 413 | // ... |
| 414 | // jump loop |
| 415 | // done: (target of break) |
| 416 | |
| 417 | loop := b.newBlock("range.loop") |
| 418 | b.jump(loop) |
| 419 | b.current = loop |
| 420 | |
| 421 | body := b.newBlock("range.body") |
| 422 | done := b.newBlock("range.done") |
| 423 | b.ifelse(body, done) |
| 424 | b.current = body |
| 425 | |
| 426 | if label != nil { |
| 427 | label._break = done |
| 428 | label._continue = loop |
| 429 | } |
| 430 | b.targets = &targets{ |
| 431 | tail: b.targets, |
| 432 | _break: done, |
| 433 | _continue: loop, |
| 434 | } |
| 435 | b.stmt(s.Body) |
| 436 | b.targets = b.targets.tail |
| 437 | b.jump(loop) // back-edge |
| 438 | b.current = done |
| 439 | } |
| 440 | |
| 441 | // -------- helpers -------- |
| 442 | |
| 443 | // Destinations associated with unlabeled for/switch/select stmts. |
| 444 | // We push/pop one of these as we enter/leave each construct and for |
| 445 | // each BranchStmt we scan for the innermost target of the right type. |
| 446 | type targets struct { |
| 447 | tail *targets // rest of stack |
| 448 | _break *Block |
| 449 | _continue *Block |
| 450 | _fallthrough *Block |
| 451 | } |
| 452 | |
| 453 | // Destinations associated with a labeled block. |
| 454 | // We populate these as labels are encountered in forward gotos or |
| 455 | // labeled statements. |
| 456 | type lblock struct { |
| 457 | _goto *Block |
| 458 | _break *Block |
| 459 | _continue *Block |
| 460 | } |
| 461 | |
| 462 | // labeledBlock returns the branch target associated with the |
| 463 | // specified label, creating it if needed. |
| 464 | func (b *builder) labeledBlock(label *ast.Ident) *lblock { |
| 465 | lb := b.lblocks[label.Obj] |
| 466 | if lb == nil { |
| 467 | lb = &lblock{_goto: b.newBlock(label.Name)} |
| 468 | if b.lblocks == nil { |
| 469 | b.lblocks = make(map[*ast.Object]*lblock) |
| 470 | } |
| 471 | b.lblocks[label.Obj] = lb |
| 472 | } |
| 473 | return lb |
| 474 | } |
| 475 | |
| 476 | // newBlock appends a new unconnected basic block to b.cfg's block |
| 477 | // slice and returns it. |
| 478 | // It does not automatically become the current block. |
| 479 | // comment is an optional string for more readable debugging output. |
| 480 | func (b *builder) newBlock(comment string) *Block { |
| 481 | g := b.cfg |
| 482 | block := &Block{ |
| 483 | Index: int32(len(g.Blocks)), |
| 484 | comment: comment, |
| 485 | } |
| 486 | block.Succs = block.succs2[:0] |
| 487 | g.Blocks = append(g.Blocks, block) |
| 488 | return block |
| 489 | } |
| 490 | |
| 491 | func (b *builder) add(n ast.Node) { |
| 492 | b.current.Nodes = append(b.current.Nodes, n) |
| 493 | } |
| 494 | |
| 495 | // jump adds an edge from the current block to the target block, |
| 496 | // and sets b.current to nil. |
| 497 | func (b *builder) jump(target *Block) { |
| 498 | b.current.Succs = append(b.current.Succs, target) |
| 499 | b.current = nil |
| 500 | } |
| 501 | |
| 502 | // ifelse emits edges from the current block to the t and f blocks, |
| 503 | // and sets b.current to nil. |
| 504 | func (b *builder) ifelse(t, f *Block) { |
| 505 | b.current.Succs = append(b.current.Succs, t, f) |
| 506 | b.current = nil |
| 507 | } |
| 508 |
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