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Diffstat (limited to 'vendor/golang.org/x/tools/go/ssa/lift.go')
| -rw-r--r-- | vendor/golang.org/x/tools/go/ssa/lift.go | 653 |
1 files changed, 0 insertions, 653 deletions
diff --git a/vendor/golang.org/x/tools/go/ssa/lift.go b/vendor/golang.org/x/tools/go/ssa/lift.go deleted file mode 100644 index 048e9b0..0000000 --- a/vendor/golang.org/x/tools/go/ssa/lift.go +++ /dev/null @@ -1,653 +0,0 @@ -// Copyright 2013 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 - -// This file defines the lifting pass which tries to "lift" Alloc -// cells (new/local variables) into SSA registers, replacing loads -// with the dominating stored value, eliminating loads and stores, and -// inserting φ-nodes as needed. - -// Cited papers and resources: -// -// Ron Cytron et al. 1991. Efficiently computing SSA form... -// http://doi.acm.org/10.1145/115372.115320 -// -// Cooper, Harvey, Kennedy. 2001. A Simple, Fast Dominance Algorithm. -// Software Practice and Experience 2001, 4:1-10. -// http://www.hipersoft.rice.edu/grads/publications/dom14.pdf -// -// Daniel Berlin, llvmdev mailing list, 2012. -// http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/046638.html -// (Be sure to expand the whole thread.) - -// TODO(adonovan): opt: there are many optimizations worth evaluating, and -// the conventional wisdom for SSA construction is that a simple -// algorithm well engineered often beats those of better asymptotic -// complexity on all but the most egregious inputs. -// -// Danny Berlin suggests that the Cooper et al. algorithm for -// computing the dominance frontier is superior to Cytron et al. -// Furthermore he recommends that rather than computing the DF for the -// whole function then renaming all alloc cells, it may be cheaper to -// compute the DF for each alloc cell separately and throw it away. -// -// Consider exploiting liveness information to avoid creating dead -// φ-nodes which we then immediately remove. -// -// Also see many other "TODO: opt" suggestions in the code. - -import ( - "fmt" - "go/token" - "go/types" - "math/big" - "os" -) - -// If true, show diagnostic information at each step of lifting. -// Very verbose. -const debugLifting = false - -// domFrontier maps each block to the set of blocks in its dominance -// frontier. The outer slice is conceptually a map keyed by -// Block.Index. The inner slice is conceptually a set, possibly -// containing duplicates. -// -// TODO(adonovan): opt: measure impact of dups; consider a packed bit -// representation, e.g. big.Int, and bitwise parallel operations for -// the union step in the Children loop. -// -// domFrontier's methods mutate the slice's elements but not its -// length, so their receivers needn't be pointers. -// -type domFrontier [][]*BasicBlock - -func (df domFrontier) add(u, v *BasicBlock) { - p := &df[u.Index] - *p = append(*p, v) -} - -// build builds the dominance frontier df for the dominator (sub)tree -// rooted at u, using the Cytron et al. algorithm. -// -// TODO(adonovan): opt: consider Berlin approach, computing pruned SSA -// by pruning the entire IDF computation, rather than merely pruning -// the DF -> IDF step. -func (df domFrontier) build(u *BasicBlock) { - // Encounter each node u in postorder of dom tree. - for _, child := range u.dom.children { - df.build(child) - } - for _, vb := range u.Succs { - if v := vb.dom; v.idom != u { - df.add(u, vb) - } - } - for _, w := range u.dom.children { - for _, vb := range df[w.Index] { - // TODO(adonovan): opt: use word-parallel bitwise union. - if v := vb.dom; v.idom != u { - df.add(u, vb) - } - } - } -} - -func buildDomFrontier(fn *Function) domFrontier { - df := make(domFrontier, len(fn.Blocks)) - df.build(fn.Blocks[0]) - if fn.Recover != nil { - df.build(fn.Recover) - } - return df -} - -func removeInstr(refs []Instruction, instr Instruction) []Instruction { - i := 0 - for _, ref := range refs { - if ref == instr { - continue - } - refs[i] = ref - i++ - } - for j := i; j != len(refs); j++ { - refs[j] = nil // aid GC - } - return refs[:i] -} - -// lift replaces local and new Allocs accessed only with -// load/store by SSA registers, inserting φ-nodes where necessary. -// The result is a program in classical pruned SSA form. -// -// Preconditions: -// - fn has no dead blocks (blockopt has run). -// - Def/use info (Operands and Referrers) is up-to-date. -// - The dominator tree is up-to-date. -// -func lift(fn *Function) { - // TODO(adonovan): opt: lots of little optimizations may be - // worthwhile here, especially if they cause us to avoid - // buildDomFrontier. For example: - // - // - Alloc never loaded? Eliminate. - // - Alloc never stored? Replace all loads with a zero constant. - // - Alloc stored once? Replace loads with dominating store; - // don't forget that an Alloc is itself an effective store - // of zero. - // - Alloc used only within a single block? - // Use degenerate algorithm avoiding φ-nodes. - // - Consider synergy with scalar replacement of aggregates (SRA). - // e.g. *(&x.f) where x is an Alloc. - // Perhaps we'd get better results if we generated this as x.f - // i.e. Field(x, .f) instead of Load(FieldIndex(x, .f)). - // Unclear. - // - // But we will start with the simplest correct code. - df := buildDomFrontier(fn) - - if debugLifting { - title := false - for i, blocks := range df { - if blocks != nil { - if !title { - fmt.Fprintf(os.Stderr, "Dominance frontier of %s:\n", fn) - title = true - } - fmt.Fprintf(os.Stderr, "\t%s: %s\n", fn.Blocks[i], blocks) - } - } - } - - newPhis := make(newPhiMap) - - // During this pass we will replace some BasicBlock.Instrs - // (allocs, loads and stores) with nil, keeping a count in - // BasicBlock.gaps. At the end we will reset Instrs to the - // concatenation of all non-dead newPhis and non-nil Instrs - // for the block, reusing the original array if space permits. - - // While we're here, we also eliminate 'rundefers' - // instructions in functions that contain no 'defer' - // instructions. - usesDefer := false - - // A counter used to generate ~unique ids for Phi nodes, as an - // aid to debugging. We use large numbers to make them highly - // visible. All nodes are renumbered later. - fresh := 1000 - - // Determine which allocs we can lift and number them densely. - // The renaming phase uses this numbering for compact maps. - numAllocs := 0 - for _, b := range fn.Blocks { - b.gaps = 0 - b.rundefers = 0 - for _, instr := range b.Instrs { - switch instr := instr.(type) { - case *Alloc: - index := -1 - if liftAlloc(df, instr, newPhis, &fresh) { - index = numAllocs - numAllocs++ - } - instr.index = index - case *Defer: - usesDefer = true - case *RunDefers: - b.rundefers++ - } - } - } - - // renaming maps an alloc (keyed by index) to its replacement - // value. Initially the renaming contains nil, signifying the - // zero constant of the appropriate type; we construct the - // Const lazily at most once on each path through the domtree. - // TODO(adonovan): opt: cache per-function not per subtree. - renaming := make([]Value, numAllocs) - - // Renaming. - rename(fn.Blocks[0], renaming, newPhis) - - // Eliminate dead φ-nodes. - removeDeadPhis(fn.Blocks, newPhis) - - // Prepend remaining live φ-nodes to each block. - for _, b := range fn.Blocks { - nps := newPhis[b] - j := len(nps) - - rundefersToKill := b.rundefers - if usesDefer { - rundefersToKill = 0 - } - - if j+b.gaps+rundefersToKill == 0 { - continue // fast path: no new phis or gaps - } - - // Compact nps + non-nil Instrs into a new slice. - // TODO(adonovan): opt: compact in situ (rightwards) - // if Instrs has sufficient space or slack. - dst := make([]Instruction, len(b.Instrs)+j-b.gaps-rundefersToKill) - for i, np := range nps { - dst[i] = np.phi - } - for _, instr := range b.Instrs { - if instr == nil { - continue - } - if !usesDefer { - if _, ok := instr.(*RunDefers); ok { - continue - } - } - dst[j] = instr - j++ - } - b.Instrs = dst - } - - // Remove any fn.Locals that were lifted. - j := 0 - for _, l := range fn.Locals { - if l.index < 0 { - fn.Locals[j] = l - j++ - } - } - // Nil out fn.Locals[j:] to aid GC. - for i := j; i < len(fn.Locals); i++ { - fn.Locals[i] = nil - } - fn.Locals = fn.Locals[:j] -} - -// removeDeadPhis removes φ-nodes not transitively needed by a -// non-Phi, non-DebugRef instruction. -func removeDeadPhis(blocks []*BasicBlock, newPhis newPhiMap) { - // First pass: find the set of "live" φ-nodes: those reachable - // from some non-Phi instruction. - // - // We compute reachability in reverse, starting from each φ, - // rather than forwards, starting from each live non-Phi - // instruction, because this way visits much less of the - // Value graph. - livePhis := make(map[*Phi]bool) - for _, npList := range newPhis { - for _, np := range npList { - phi := np.phi - if !livePhis[phi] && phiHasDirectReferrer(phi) { - markLivePhi(livePhis, phi) - } - } - } - - // Existing φ-nodes due to && and || operators - // are all considered live (see Go issue 19622). - for _, b := range blocks { - for _, phi := range b.phis() { - markLivePhi(livePhis, phi.(*Phi)) - } - } - - // Second pass: eliminate unused phis from newPhis. - for block, npList := range newPhis { - j := 0 - for _, np := range npList { - if livePhis[np.phi] { - npList[j] = np - j++ - } else { - // discard it, first removing it from referrers - for _, val := range np.phi.Edges { - if refs := val.Referrers(); refs != nil { - *refs = removeInstr(*refs, np.phi) - } - } - np.phi.block = nil - } - } - newPhis[block] = npList[:j] - } -} - -// markLivePhi marks phi, and all φ-nodes transitively reachable via -// its Operands, live. -func markLivePhi(livePhis map[*Phi]bool, phi *Phi) { - livePhis[phi] = true - for _, rand := range phi.Operands(nil) { - if q, ok := (*rand).(*Phi); ok { - if !livePhis[q] { - markLivePhi(livePhis, q) - } - } - } -} - -// phiHasDirectReferrer reports whether phi is directly referred to by -// a non-Phi instruction. Such instructions are the -// roots of the liveness traversal. -func phiHasDirectReferrer(phi *Phi) bool { - for _, instr := range *phi.Referrers() { - if _, ok := instr.(*Phi); !ok { - return true - } - } - return false -} - -type blockSet struct{ big.Int } // (inherit methods from Int) - -// add adds b to the set and returns true if the set changed. -func (s *blockSet) add(b *BasicBlock) bool { - i := b.Index - if s.Bit(i) != 0 { - return false - } - s.SetBit(&s.Int, i, 1) - return true -} - -// take removes an arbitrary element from a set s and -// returns its index, or returns -1 if empty. -func (s *blockSet) take() int { - l := s.BitLen() - for i := 0; i < l; i++ { - if s.Bit(i) == 1 { - s.SetBit(&s.Int, i, 0) - return i - } - } - return -1 -} - -// newPhi is a pair of a newly introduced φ-node and the lifted Alloc -// it replaces. -type newPhi struct { - phi *Phi - alloc *Alloc -} - -// newPhiMap records for each basic block, the set of newPhis that -// must be prepended to the block. -type newPhiMap map[*BasicBlock][]newPhi - -// liftAlloc determines whether alloc can be lifted into registers, -// and if so, it populates newPhis with all the φ-nodes it may require -// and returns true. -// -// fresh is a source of fresh ids for phi nodes. -// -func liftAlloc(df domFrontier, alloc *Alloc, newPhis newPhiMap, fresh *int) bool { - // Don't lift aggregates into registers, because we don't have - // a way to express their zero-constants. - switch deref(alloc.Type()).Underlying().(type) { - case *types.Array, *types.Struct: - return false - } - - // Don't lift named return values in functions that defer - // calls that may recover from panic. - if fn := alloc.Parent(); fn.Recover != nil { - for _, nr := range fn.namedResults { - if nr == alloc { - return false - } - } - } - - // Compute defblocks, the set of blocks containing a - // definition of the alloc cell. - var defblocks blockSet - for _, instr := range *alloc.Referrers() { - // Bail out if we discover the alloc is not liftable; - // the only operations permitted to use the alloc are - // loads/stores into the cell, and DebugRef. - switch instr := instr.(type) { - case *Store: - if instr.Val == alloc { - return false // address used as value - } - if instr.Addr != alloc { - panic("Alloc.Referrers is inconsistent") - } - defblocks.add(instr.Block()) - case *UnOp: - if instr.Op != token.MUL { - return false // not a load - } - if instr.X != alloc { - panic("Alloc.Referrers is inconsistent") - } - case *DebugRef: - // ok - default: - return false // some other instruction - } - } - // The Alloc itself counts as a (zero) definition of the cell. - defblocks.add(alloc.Block()) - - if debugLifting { - fmt.Fprintln(os.Stderr, "\tlifting ", alloc, alloc.Name()) - } - - fn := alloc.Parent() - - // Φ-insertion. - // - // What follows is the body of the main loop of the insert-φ - // function described by Cytron et al, but instead of using - // counter tricks, we just reset the 'hasAlready' and 'work' - // sets each iteration. These are bitmaps so it's pretty cheap. - // - // TODO(adonovan): opt: recycle slice storage for W, - // hasAlready, defBlocks across liftAlloc calls. - var hasAlready blockSet - - // Initialize W and work to defblocks. - var work blockSet = defblocks // blocks seen - var W blockSet // blocks to do - W.Set(&defblocks.Int) - - // Traverse iterated dominance frontier, inserting φ-nodes. - for i := W.take(); i != -1; i = W.take() { - u := fn.Blocks[i] - for _, v := range df[u.Index] { - if hasAlready.add(v) { - // Create φ-node. - // It will be prepended to v.Instrs later, if needed. - phi := &Phi{ - Edges: make([]Value, len(v.Preds)), - Comment: alloc.Comment, - } - // This is merely a debugging aid: - phi.setNum(*fresh) - *fresh++ - - phi.pos = alloc.Pos() - phi.setType(deref(alloc.Type())) - phi.block = v - if debugLifting { - fmt.Fprintf(os.Stderr, "\tplace %s = %s at block %s\n", phi.Name(), phi, v) - } - newPhis[v] = append(newPhis[v], newPhi{phi, alloc}) - - if work.add(v) { - W.add(v) - } - } - } - } - - return true -} - -// replaceAll replaces all intraprocedural uses of x with y, -// updating x.Referrers and y.Referrers. -// Precondition: x.Referrers() != nil, i.e. x must be local to some function. -// -func replaceAll(x, y Value) { - var rands []*Value - pxrefs := x.Referrers() - pyrefs := y.Referrers() - for _, instr := range *pxrefs { - rands = instr.Operands(rands[:0]) // recycle storage - for _, rand := range rands { - if *rand != nil { - if *rand == x { - *rand = y - } - } - } - if pyrefs != nil { - *pyrefs = append(*pyrefs, instr) // dups ok - } - } - *pxrefs = nil // x is now unreferenced -} - -// renamed returns the value to which alloc is being renamed, -// constructing it lazily if it's the implicit zero initialization. -// -func renamed(renaming []Value, alloc *Alloc) Value { - v := renaming[alloc.index] - if v == nil { - v = zeroConst(deref(alloc.Type())) - renaming[alloc.index] = v - } - return v -} - -// rename implements the (Cytron et al) SSA renaming algorithm, a -// preorder traversal of the dominator tree replacing all loads of -// Alloc cells with the value stored to that cell by the dominating -// store instruction. For lifting, we need only consider loads, -// stores and φ-nodes. -// -// renaming is a map from *Alloc (keyed by index number) to its -// dominating stored value; newPhis[x] is the set of new φ-nodes to be -// prepended to block x. -// -func rename(u *BasicBlock, renaming []Value, newPhis newPhiMap) { - // Each φ-node becomes the new name for its associated Alloc. - for _, np := range newPhis[u] { - phi := np.phi - alloc := np.alloc - renaming[alloc.index] = phi - } - - // Rename loads and stores of allocs. - for i, instr := range u.Instrs { - switch instr := instr.(type) { - case *Alloc: - if instr.index >= 0 { // store of zero to Alloc cell - // Replace dominated loads by the zero value. - renaming[instr.index] = nil - if debugLifting { - fmt.Fprintf(os.Stderr, "\tkill alloc %s\n", instr) - } - // Delete the Alloc. - u.Instrs[i] = nil - u.gaps++ - } - - case *Store: - if alloc, ok := instr.Addr.(*Alloc); ok && alloc.index >= 0 { // store to Alloc cell - // Replace dominated loads by the stored value. - renaming[alloc.index] = instr.Val - if debugLifting { - fmt.Fprintf(os.Stderr, "\tkill store %s; new value: %s\n", - instr, instr.Val.Name()) - } - // Remove the store from the referrer list of the stored value. - if refs := instr.Val.Referrers(); refs != nil { - *refs = removeInstr(*refs, instr) - } - // Delete the Store. - u.Instrs[i] = nil - u.gaps++ - } - - case *UnOp: - if instr.Op == token.MUL { - if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // load of Alloc cell - newval := renamed(renaming, alloc) - if debugLifting { - fmt.Fprintf(os.Stderr, "\tupdate load %s = %s with %s\n", - instr.Name(), instr, newval.Name()) - } - // Replace all references to - // the loaded value by the - // dominating stored value. - replaceAll(instr, newval) - // Delete the Load. - u.Instrs[i] = nil - u.gaps++ - } - } - - case *DebugRef: - if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // ref of Alloc cell - if instr.IsAddr { - instr.X = renamed(renaming, alloc) - instr.IsAddr = false - - // Add DebugRef to instr.X's referrers. - if refs := instr.X.Referrers(); refs != nil { - *refs = append(*refs, instr) - } - } else { - // A source expression denotes the address - // of an Alloc that was optimized away. - instr.X = nil - - // Delete the DebugRef. - u.Instrs[i] = nil - u.gaps++ - } - } - } - } - - // For each φ-node in a CFG successor, rename the edge. - for _, v := range u.Succs { - phis := newPhis[v] - if len(phis) == 0 { - continue - } - i := v.predIndex(u) - for _, np := range phis { - phi := np.phi - alloc := np.alloc - newval := renamed(renaming, alloc) - if debugLifting { - fmt.Fprintf(os.Stderr, "\tsetphi %s edge %s -> %s (#%d) (alloc=%s) := %s\n", - phi.Name(), u, v, i, alloc.Name(), newval.Name()) - } - phi.Edges[i] = newval - if prefs := newval.Referrers(); prefs != nil { - *prefs = append(*prefs, phi) - } - } - } - - // Continue depth-first recursion over domtree, pushing a - // fresh copy of the renaming map for each subtree. - for i, v := range u.dom.children { - r := renaming - if i < len(u.dom.children)-1 { - // On all but the final iteration, we must make - // a copy to avoid destructive update. - r = make([]Value, len(renaming)) - copy(r, renaming) - } - rename(v, r, newPhis) - } - -} |