| 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 | // This file defines the lifting pass which tries to "lift" Alloc |
| 8 | // cells (new/local variables) into SSA registers, replacing loads |
| 9 | // with the dominating stored value, eliminating loads and stores, and |
| 10 | // inserting φ-nodes as needed. |
| 11 | |
| 12 | // Cited papers and resources: |
| 13 | // |
| 14 | // Ron Cytron et al. 1991. Efficiently computing SSA form... |
| 15 | // http://doi.acm.org/10.1145/115372.115320 |
| 16 | // |
| 17 | // Cooper, Harvey, Kennedy. 2001. A Simple, Fast Dominance Algorithm. |
| 18 | // Software Practice and Experience 2001, 4:1-10. |
| 19 | // http://www.hipersoft.rice.edu/grads/publications/dom14.pdf |
| 20 | // |
| 21 | // Daniel Berlin, llvmdev mailing list, 2012. |
| 22 | // http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/046638.html |
| 23 | // (Be sure to expand the whole thread.) |
| 24 | |
| 25 | // TODO(adonovan): opt: there are many optimizations worth evaluating, and |
| 26 | // the conventional wisdom for SSA construction is that a simple |
| 27 | // algorithm well engineered often beats those of better asymptotic |
| 28 | // complexity on all but the most egregious inputs. |
| 29 | // |
| 30 | // Danny Berlin suggests that the Cooper et al. algorithm for |
| 31 | // computing the dominance frontier is superior to Cytron et al. |
| 32 | // Furthermore he recommends that rather than computing the DF for the |
| 33 | // whole function then renaming all alloc cells, it may be cheaper to |
| 34 | // compute the DF for each alloc cell separately and throw it away. |
| 35 | // |
| 36 | // Consider exploiting liveness information to avoid creating dead |
| 37 | // φ-nodes which we then immediately remove. |
| 38 | // |
| 39 | // Also see many other "TODO: opt" suggestions in the code. |
| 40 | |
| 41 | import ( |
| 42 | "fmt" |
| 43 | "go/token" |
| 44 | "go/types" |
| 45 | "math/big" |
| 46 | "os" |
| 47 | |
| 48 | "golang.org/x/tools/internal/typeparams" |
| 49 | ) |
| 50 | |
| 51 | // If true, show diagnostic information at each step of lifting. |
| 52 | // Very verbose. |
| 53 | const debugLifting = false |
| 54 | |
| 55 | // domFrontier maps each block to the set of blocks in its dominance |
| 56 | // frontier. The outer slice is conceptually a map keyed by |
| 57 | // Block.Index. The inner slice is conceptually a set, possibly |
| 58 | // containing duplicates. |
| 59 | // |
| 60 | // TODO(adonovan): opt: measure impact of dups; consider a packed bit |
| 61 | // representation, e.g. big.Int, and bitwise parallel operations for |
| 62 | // the union step in the Children loop. |
| 63 | // |
| 64 | // domFrontier's methods mutate the slice's elements but not its |
| 65 | // length, so their receivers needn't be pointers. |
| 66 | type domFrontier [][]*BasicBlock |
| 67 | |
| 68 | func (df domFrontier) add(u, v *BasicBlock) { |
| 69 | p := &df[u.Index] |
| 70 | *p = append(*p, v) |
| 71 | } |
| 72 | |
| 73 | // build builds the dominance frontier df for the dominator (sub)tree |
| 74 | // rooted at u, using the Cytron et al. algorithm. |
| 75 | // |
| 76 | // TODO(adonovan): opt: consider Berlin approach, computing pruned SSA |
| 77 | // by pruning the entire IDF computation, rather than merely pruning |
| 78 | // the DF -> IDF step. |
| 79 | func (df domFrontier) build(u *BasicBlock) { |
| 80 | // Encounter each node u in postorder of dom tree. |
| 81 | for _, child := range u.dom.children { |
| 82 | df.build(child) |
| 83 | } |
| 84 | for _, vb := range u.Succs { |
| 85 | if v := vb.dom; v.idom != u { |
| 86 | df.add(u, vb) |
| 87 | } |
| 88 | } |
| 89 | for _, w := range u.dom.children { |
| 90 | for _, vb := range df[w.Index] { |
| 91 | // TODO(adonovan): opt: use word-parallel bitwise union. |
| 92 | if v := vb.dom; v.idom != u { |
| 93 | df.add(u, vb) |
| 94 | } |
| 95 | } |
| 96 | } |
| 97 | } |
| 98 | |
| 99 | func buildDomFrontier(fn *Function) domFrontier { |
| 100 | df := make(domFrontier, len(fn.Blocks)) |
| 101 | df.build(fn.Blocks[0]) |
| 102 | if fn.Recover != nil { |
| 103 | df.build(fn.Recover) |
| 104 | } |
| 105 | return df |
| 106 | } |
| 107 | |
| 108 | func removeInstr(refs []Instruction, instr Instruction) []Instruction { |
| 109 | i := 0 |
| 110 | for _, ref := range refs { |
| 111 | if ref == instr { |
| 112 | continue |
| 113 | } |
| 114 | refs[i] = ref |
| 115 | i++ |
| 116 | } |
| 117 | for j := i; j != len(refs); j++ { |
| 118 | refs[j] = nil // aid GC |
| 119 | } |
| 120 | return refs[:i] |
| 121 | } |
| 122 | |
| 123 | // lift replaces local and new Allocs accessed only with |
| 124 | // load/store by SSA registers, inserting φ-nodes where necessary. |
| 125 | // The result is a program in classical pruned SSA form. |
| 126 | // |
| 127 | // Preconditions: |
| 128 | // - fn has no dead blocks (blockopt has run). |
| 129 | // - Def/use info (Operands and Referrers) is up-to-date. |
| 130 | // - The dominator tree is up-to-date. |
| 131 | func lift(fn *Function) { |
| 132 | // TODO(adonovan): opt: lots of little optimizations may be |
| 133 | // worthwhile here, especially if they cause us to avoid |
| 134 | // buildDomFrontier. For example: |
| 135 | // |
| 136 | // - Alloc never loaded? Eliminate. |
| 137 | // - Alloc never stored? Replace all loads with a zero constant. |
| 138 | // - Alloc stored once? Replace loads with dominating store; |
| 139 | // don't forget that an Alloc is itself an effective store |
| 140 | // of zero. |
| 141 | // - Alloc used only within a single block? |
| 142 | // Use degenerate algorithm avoiding φ-nodes. |
| 143 | // - Consider synergy with scalar replacement of aggregates (SRA). |
| 144 | // e.g. *(&x.f) where x is an Alloc. |
| 145 | // Perhaps we'd get better results if we generated this as x.f |
| 146 | // i.e. Field(x, .f) instead of Load(FieldIndex(x, .f)). |
| 147 | // Unclear. |
| 148 | // |
| 149 | // But we will start with the simplest correct code. |
| 150 | df := buildDomFrontier(fn) |
| 151 | |
| 152 | if debugLifting { |
| 153 | title := false |
| 154 | for i, blocks := range df { |
| 155 | if blocks != nil { |
| 156 | if !title { |
| 157 | fmt.Fprintf(os.Stderr, "Dominance frontier of %s:\n", fn) |
| 158 | title = true |
| 159 | } |
| 160 | fmt.Fprintf(os.Stderr, "\t%s: %s\n", fn.Blocks[i], blocks) |
| 161 | } |
| 162 | } |
| 163 | } |
| 164 | |
| 165 | newPhis := make(newPhiMap) |
| 166 | |
| 167 | // During this pass we will replace some BasicBlock.Instrs |
| 168 | // (allocs, loads and stores) with nil, keeping a count in |
| 169 | // BasicBlock.gaps. At the end we will reset Instrs to the |
| 170 | // concatenation of all non-dead newPhis and non-nil Instrs |
| 171 | // for the block, reusing the original array if space permits. |
| 172 | |
| 173 | // While we're here, we also eliminate 'rundefers' |
| 174 | // instructions in functions that contain no 'defer' |
| 175 | // instructions. |
| 176 | usesDefer := false |
| 177 | |
| 178 | // A counter used to generate ~unique ids for Phi nodes, as an |
| 179 | // aid to debugging. We use large numbers to make them highly |
| 180 | // visible. All nodes are renumbered later. |
| 181 | fresh := 1000 |
| 182 | |
| 183 | // Determine which allocs we can lift and number them densely. |
| 184 | // The renaming phase uses this numbering for compact maps. |
| 185 | numAllocs := 0 |
| 186 | for _, b := range fn.Blocks { |
| 187 | b.gaps = 0 |
| 188 | b.rundefers = 0 |
| 189 | for _, instr := range b.Instrs { |
| 190 | switch instr := instr.(type) { |
| 191 | case *Alloc: |
| 192 | index := -1 |
| 193 | if liftAlloc(df, instr, newPhis, &fresh) { |
| 194 | index = numAllocs |
| 195 | numAllocs++ |
| 196 | } |
| 197 | instr.index = index |
| 198 | case *Defer: |
| 199 | usesDefer = true |
| 200 | case *RunDefers: |
| 201 | b.rundefers++ |
| 202 | } |
| 203 | } |
| 204 | } |
| 205 | |
| 206 | // renaming maps an alloc (keyed by index) to its replacement |
| 207 | // value. Initially the renaming contains nil, signifying the |
| 208 | // zero constant of the appropriate type; we construct the |
| 209 | // Const lazily at most once on each path through the domtree. |
| 210 | // TODO(adonovan): opt: cache per-function not per subtree. |
| 211 | renaming := make([]Value, numAllocs) |
| 212 | |
| 213 | // Renaming. |
| 214 | rename(fn.Blocks[0], renaming, newPhis) |
| 215 | |
| 216 | // Eliminate dead φ-nodes. |
| 217 | removeDeadPhis(fn.Blocks, newPhis) |
| 218 | |
| 219 | // Prepend remaining live φ-nodes to each block. |
| 220 | for _, b := range fn.Blocks { |
| 221 | nps := newPhis[b] |
| 222 | j := len(nps) |
| 223 | |
| 224 | rundefersToKill := b.rundefers |
| 225 | if usesDefer { |
| 226 | rundefersToKill = 0 |
| 227 | } |
| 228 | |
| 229 | if j+b.gaps+rundefersToKill == 0 { |
| 230 | continue // fast path: no new phis or gaps |
| 231 | } |
| 232 | |
| 233 | // Compact nps + non-nil Instrs into a new slice. |
| 234 | // TODO(adonovan): opt: compact in situ (rightwards) |
| 235 | // if Instrs has sufficient space or slack. |
| 236 | dst := make([]Instruction, len(b.Instrs)+j-b.gaps-rundefersToKill) |
| 237 | for i, np := range nps { |
| 238 | dst[i] = np.phi |
| 239 | } |
| 240 | for _, instr := range b.Instrs { |
| 241 | if instr == nil { |
| 242 | continue |
| 243 | } |
| 244 | if !usesDefer { |
| 245 | if _, ok := instr.(*RunDefers); ok { |
| 246 | continue |
| 247 | } |
| 248 | } |
| 249 | dst[j] = instr |
| 250 | j++ |
| 251 | } |
| 252 | b.Instrs = dst |
| 253 | } |
| 254 | |
| 255 | // Remove any fn.Locals that were lifted. |
| 256 | j := 0 |
| 257 | for _, l := range fn.Locals { |
| 258 | if l.index < 0 { |
| 259 | fn.Locals[j] = l |
| 260 | j++ |
| 261 | } |
| 262 | } |
| 263 | // Nil out fn.Locals[j:] to aid GC. |
| 264 | for i := j; i < len(fn.Locals); i++ { |
| 265 | fn.Locals[i] = nil |
| 266 | } |
| 267 | fn.Locals = fn.Locals[:j] |
| 268 | } |
| 269 | |
| 270 | // removeDeadPhis removes φ-nodes not transitively needed by a |
| 271 | // non-Phi, non-DebugRef instruction. |
| 272 | func removeDeadPhis(blocks []*BasicBlock, newPhis newPhiMap) { |
| 273 | // First pass: find the set of "live" φ-nodes: those reachable |
| 274 | // from some non-Phi instruction. |
| 275 | // |
| 276 | // We compute reachability in reverse, starting from each φ, |
| 277 | // rather than forwards, starting from each live non-Phi |
| 278 | // instruction, because this way visits much less of the |
| 279 | // Value graph. |
| 280 | livePhis := make(map[*Phi]bool) |
| 281 | for _, npList := range newPhis { |
| 282 | for _, np := range npList { |
| 283 | phi := np.phi |
| 284 | if !livePhis[phi] && phiHasDirectReferrer(phi) { |
| 285 | markLivePhi(livePhis, phi) |
| 286 | } |
| 287 | } |
| 288 | } |
| 289 | |
| 290 | // Existing φ-nodes due to && and || operators |
| 291 | // are all considered live (see Go issue 19622). |
| 292 | for _, b := range blocks { |
| 293 | for _, phi := range b.phis() { |
| 294 | markLivePhi(livePhis, phi.(*Phi)) |
| 295 | } |
| 296 | } |
| 297 | |
| 298 | // Second pass: eliminate unused phis from newPhis. |
| 299 | for block, npList := range newPhis { |
| 300 | j := 0 |
| 301 | for _, np := range npList { |
| 302 | if livePhis[np.phi] { |
| 303 | npList[j] = np |
| 304 | j++ |
| 305 | } else { |
| 306 | // discard it, first removing it from referrers |
| 307 | for _, val := range np.phi.Edges { |
| 308 | if refs := val.Referrers(); refs != nil { |
| 309 | *refs = removeInstr(*refs, np.phi) |
| 310 | } |
| 311 | } |
| 312 | np.phi.block = nil |
| 313 | } |
| 314 | } |
| 315 | newPhis[block] = npList[:j] |
| 316 | } |
| 317 | } |
| 318 | |
| 319 | // markLivePhi marks phi, and all φ-nodes transitively reachable via |
| 320 | // its Operands, live. |
| 321 | func markLivePhi(livePhis map[*Phi]bool, phi *Phi) { |
| 322 | livePhis[phi] = true |
| 323 | for _, rand := range phi.Operands(nil) { |
| 324 | if q, ok := (*rand).(*Phi); ok { |
| 325 | if !livePhis[q] { |
| 326 | markLivePhi(livePhis, q) |
| 327 | } |
| 328 | } |
| 329 | } |
| 330 | } |
| 331 | |
| 332 | // phiHasDirectReferrer reports whether phi is directly referred to by |
| 333 | // a non-Phi instruction. Such instructions are the |
| 334 | // roots of the liveness traversal. |
| 335 | func phiHasDirectReferrer(phi *Phi) bool { |
| 336 | for _, instr := range *phi.Referrers() { |
| 337 | if _, ok := instr.(*Phi); !ok { |
| 338 | return true |
| 339 | } |
| 340 | } |
| 341 | return false |
| 342 | } |
| 343 | |
| 344 | type blockSet struct{ big.Int } // (inherit methods from Int) |
| 345 | |
| 346 | // add adds b to the set and returns true if the set changed. |
| 347 | func (s *blockSet) add(b *BasicBlock) bool { |
| 348 | i := b.Index |
| 349 | if s.Bit(i) != 0 { |
| 350 | return false |
| 351 | } |
| 352 | s.SetBit(&s.Int, i, 1) |
| 353 | return true |
| 354 | } |
| 355 | |
| 356 | // take removes an arbitrary element from a set s and |
| 357 | // returns its index, or returns -1 if empty. |
| 358 | func (s *blockSet) take() int { |
| 359 | l := s.BitLen() |
| 360 | for i := 0; i < l; i++ { |
| 361 | if s.Bit(i) == 1 { |
| 362 | s.SetBit(&s.Int, i, 0) |
| 363 | return i |
| 364 | } |
| 365 | } |
| 366 | return -1 |
| 367 | } |
| 368 | |
| 369 | // newPhi is a pair of a newly introduced φ-node and the lifted Alloc |
| 370 | // it replaces. |
| 371 | type newPhi struct { |
| 372 | phi *Phi |
| 373 | alloc *Alloc |
| 374 | } |
| 375 | |
| 376 | // newPhiMap records for each basic block, the set of newPhis that |
| 377 | // must be prepended to the block. |
| 378 | type newPhiMap map[*BasicBlock][]newPhi |
| 379 | |
| 380 | // liftAlloc determines whether alloc can be lifted into registers, |
| 381 | // and if so, it populates newPhis with all the φ-nodes it may require |
| 382 | // and returns true. |
| 383 | // |
| 384 | // fresh is a source of fresh ids for phi nodes. |
| 385 | func liftAlloc(df domFrontier, alloc *Alloc, newPhis newPhiMap, fresh *int) bool { |
| 386 | // TODO(taking): zero constants of aggregated types can now be lifted. |
| 387 | switch deref(alloc.Type()).Underlying().(type) { |
| 388 | case *types.Array, *types.Struct, *typeparams.TypeParam: |
| 389 | return false |
| 390 | } |
| 391 | |
| 392 | // Don't lift named return values in functions that defer |
| 393 | // calls that may recover from panic. |
| 394 | if fn := alloc.Parent(); fn.Recover != nil { |
| 395 | for _, nr := range fn.namedResults { |
| 396 | if nr == alloc { |
| 397 | return false |
| 398 | } |
| 399 | } |
| 400 | } |
| 401 | |
| 402 | // Compute defblocks, the set of blocks containing a |
| 403 | // definition of the alloc cell. |
| 404 | var defblocks blockSet |
| 405 | for _, instr := range *alloc.Referrers() { |
| 406 | // Bail out if we discover the alloc is not liftable; |
| 407 | // the only operations permitted to use the alloc are |
| 408 | // loads/stores into the cell, and DebugRef. |
| 409 | switch instr := instr.(type) { |
| 410 | case *Store: |
| 411 | if instr.Val == alloc { |
| 412 | return false // address used as value |
| 413 | } |
| 414 | if instr.Addr != alloc { |
| 415 | panic("Alloc.Referrers is inconsistent") |
| 416 | } |
| 417 | defblocks.add(instr.Block()) |
| 418 | case *UnOp: |
| 419 | if instr.Op != token.MUL { |
| 420 | return false // not a load |
| 421 | } |
| 422 | if instr.X != alloc { |
| 423 | panic("Alloc.Referrers is inconsistent") |
| 424 | } |
| 425 | case *DebugRef: |
| 426 | // ok |
| 427 | default: |
| 428 | return false // some other instruction |
| 429 | } |
| 430 | } |
| 431 | // The Alloc itself counts as a (zero) definition of the cell. |
| 432 | defblocks.add(alloc.Block()) |
| 433 | |
| 434 | if debugLifting { |
| 435 | fmt.Fprintln(os.Stderr, "\tlifting ", alloc, alloc.Name()) |
| 436 | } |
| 437 | |
| 438 | fn := alloc.Parent() |
| 439 | |
| 440 | // Φ-insertion. |
| 441 | // |
| 442 | // What follows is the body of the main loop of the insert-φ |
| 443 | // function described by Cytron et al, but instead of using |
| 444 | // counter tricks, we just reset the 'hasAlready' and 'work' |
| 445 | // sets each iteration. These are bitmaps so it's pretty cheap. |
| 446 | // |
| 447 | // TODO(adonovan): opt: recycle slice storage for W, |
| 448 | // hasAlready, defBlocks across liftAlloc calls. |
| 449 | var hasAlready blockSet |
| 450 | |
| 451 | // Initialize W and work to defblocks. |
| 452 | var work blockSet = defblocks // blocks seen |
| 453 | var W blockSet // blocks to do |
| 454 | W.Set(&defblocks.Int) |
| 455 | |
| 456 | // Traverse iterated dominance frontier, inserting φ-nodes. |
| 457 | for i := W.take(); i != -1; i = W.take() { |
| 458 | u := fn.Blocks[i] |
| 459 | for _, v := range df[u.Index] { |
| 460 | if hasAlready.add(v) { |
| 461 | // Create φ-node. |
| 462 | // It will be prepended to v.Instrs later, if needed. |
| 463 | phi := &Phi{ |
| 464 | Edges: make([]Value, len(v.Preds)), |
| 465 | Comment: alloc.Comment, |
| 466 | } |
| 467 | // This is merely a debugging aid: |
| 468 | phi.setNum(*fresh) |
| 469 | *fresh++ |
| 470 | |
| 471 | phi.pos = alloc.Pos() |
| 472 | phi.setType(deref(alloc.Type())) |
| 473 | phi.block = v |
| 474 | if debugLifting { |
| 475 | fmt.Fprintf(os.Stderr, "\tplace %s = %s at block %s\n", phi.Name(), phi, v) |
| 476 | } |
| 477 | newPhis[v] = append(newPhis[v], newPhi{phi, alloc}) |
| 478 | |
| 479 | if work.add(v) { |
| 480 | W.add(v) |
| 481 | } |
| 482 | } |
| 483 | } |
| 484 | } |
| 485 | |
| 486 | return true |
| 487 | } |
| 488 | |
| 489 | // replaceAll replaces all intraprocedural uses of x with y, |
| 490 | // updating x.Referrers and y.Referrers. |
| 491 | // Precondition: x.Referrers() != nil, i.e. x must be local to some function. |
| 492 | func replaceAll(x, y Value) { |
| 493 | var rands []*Value |
| 494 | pxrefs := x.Referrers() |
| 495 | pyrefs := y.Referrers() |
| 496 | for _, instr := range *pxrefs { |
| 497 | rands = instr.Operands(rands[:0]) // recycle storage |
| 498 | for _, rand := range rands { |
| 499 | if *rand != nil { |
| 500 | if *rand == x { |
| 501 | *rand = y |
| 502 | } |
| 503 | } |
| 504 | } |
| 505 | if pyrefs != nil { |
| 506 | *pyrefs = append(*pyrefs, instr) // dups ok |
| 507 | } |
| 508 | } |
| 509 | *pxrefs = nil // x is now unreferenced |
| 510 | } |
| 511 | |
| 512 | // renamed returns the value to which alloc is being renamed, |
| 513 | // constructing it lazily if it's the implicit zero initialization. |
| 514 | func renamed(renaming []Value, alloc *Alloc) Value { |
| 515 | v := renaming[alloc.index] |
| 516 | if v == nil { |
| 517 | v = zeroConst(deref(alloc.Type())) |
| 518 | renaming[alloc.index] = v |
| 519 | } |
| 520 | return v |
| 521 | } |
| 522 | |
| 523 | // rename implements the (Cytron et al) SSA renaming algorithm, a |
| 524 | // preorder traversal of the dominator tree replacing all loads of |
| 525 | // Alloc cells with the value stored to that cell by the dominating |
| 526 | // store instruction. For lifting, we need only consider loads, |
| 527 | // stores and φ-nodes. |
| 528 | // |
| 529 | // renaming is a map from *Alloc (keyed by index number) to its |
| 530 | // dominating stored value; newPhis[x] is the set of new φ-nodes to be |
| 531 | // prepended to block x. |
| 532 | func rename(u *BasicBlock, renaming []Value, newPhis newPhiMap) { |
| 533 | // Each φ-node becomes the new name for its associated Alloc. |
| 534 | for _, np := range newPhis[u] { |
| 535 | phi := np.phi |
| 536 | alloc := np.alloc |
| 537 | renaming[alloc.index] = phi |
| 538 | } |
| 539 | |
| 540 | // Rename loads and stores of allocs. |
| 541 | for i, instr := range u.Instrs { |
| 542 | switch instr := instr.(type) { |
| 543 | case *Alloc: |
| 544 | if instr.index >= 0 { // store of zero to Alloc cell |
| 545 | // Replace dominated loads by the zero value. |
| 546 | renaming[instr.index] = nil |
| 547 | if debugLifting { |
| 548 | fmt.Fprintf(os.Stderr, "\tkill alloc %s\n", instr) |
| 549 | } |
| 550 | // Delete the Alloc. |
| 551 | u.Instrs[i] = nil |
| 552 | u.gaps++ |
| 553 | } |
| 554 | |
| 555 | case *Store: |
| 556 | if alloc, ok := instr.Addr.(*Alloc); ok && alloc.index >= 0 { // store to Alloc cell |
| 557 | // Replace dominated loads by the stored value. |
| 558 | renaming[alloc.index] = instr.Val |
| 559 | if debugLifting { |
| 560 | fmt.Fprintf(os.Stderr, "\tkill store %s; new value: %s\n", |
| 561 | instr, instr.Val.Name()) |
| 562 | } |
| 563 | // Remove the store from the referrer list of the stored value. |
| 564 | if refs := instr.Val.Referrers(); refs != nil { |
| 565 | *refs = removeInstr(*refs, instr) |
| 566 | } |
| 567 | // Delete the Store. |
| 568 | u.Instrs[i] = nil |
| 569 | u.gaps++ |
| 570 | } |
| 571 | |
| 572 | case *UnOp: |
| 573 | if instr.Op == token.MUL { |
| 574 | if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // load of Alloc cell |
| 575 | newval := renamed(renaming, alloc) |
| 576 | if debugLifting { |
| 577 | fmt.Fprintf(os.Stderr, "\tupdate load %s = %s with %s\n", |
| 578 | instr.Name(), instr, newval.Name()) |
| 579 | } |
| 580 | // Replace all references to |
| 581 | // the loaded value by the |
| 582 | // dominating stored value. |
| 583 | replaceAll(instr, newval) |
| 584 | // Delete the Load. |
| 585 | u.Instrs[i] = nil |
| 586 | u.gaps++ |
| 587 | } |
| 588 | } |
| 589 | |
| 590 | case *DebugRef: |
| 591 | if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // ref of Alloc cell |
| 592 | if instr.IsAddr { |
| 593 | instr.X = renamed(renaming, alloc) |
| 594 | instr.IsAddr = false |
| 595 | |
| 596 | // Add DebugRef to instr.X's referrers. |
| 597 | if refs := instr.X.Referrers(); refs != nil { |
| 598 | *refs = append(*refs, instr) |
| 599 | } |
| 600 | } else { |
| 601 | // A source expression denotes the address |
| 602 | // of an Alloc that was optimized away. |
| 603 | instr.X = nil |
| 604 | |
| 605 | // Delete the DebugRef. |
| 606 | u.Instrs[i] = nil |
| 607 | u.gaps++ |
| 608 | } |
| 609 | } |
| 610 | } |
| 611 | } |
| 612 | |
| 613 | // For each φ-node in a CFG successor, rename the edge. |
| 614 | for _, v := range u.Succs { |
| 615 | phis := newPhis[v] |
| 616 | if len(phis) == 0 { |
| 617 | continue |
| 618 | } |
| 619 | i := v.predIndex(u) |
| 620 | for _, np := range phis { |
| 621 | phi := np.phi |
| 622 | alloc := np.alloc |
| 623 | newval := renamed(renaming, alloc) |
| 624 | if debugLifting { |
| 625 | fmt.Fprintf(os.Stderr, "\tsetphi %s edge %s -> %s (#%d) (alloc=%s) := %s\n", |
| 626 | phi.Name(), u, v, i, alloc.Name(), newval.Name()) |
| 627 | } |
| 628 | phi.Edges[i] = newval |
| 629 | if prefs := newval.Referrers(); prefs != nil { |
| 630 | *prefs = append(*prefs, phi) |
| 631 | } |
| 632 | } |
| 633 | } |
| 634 | |
| 635 | // Continue depth-first recursion over domtree, pushing a |
| 636 | // fresh copy of the renaming map for each subtree. |
| 637 | for i, v := range u.dom.children { |
| 638 | r := renaming |
| 639 | if i < len(u.dom.children)-1 { |
| 640 | // On all but the final iteration, we must make |
| 641 | // a copy to avoid destructive update. |
| 642 | r = make([]Value, len(renaming)) |
| 643 | copy(r, renaming) |
| 644 | } |
| 645 | rename(v, r, newPhis) |
| 646 | } |
| 647 | |
| 648 | } |
| 649 |
Members