From b1247d2d0d51108c910a73891ff3116e5f032ab1 Mon Sep 17 00:00:00 2001 From: "Kali Kaneko (leap communications)" Date: Sat, 12 Jan 2019 18:39:45 +0100 Subject: [pkg] all your deps are vendored to us --- vendor/golang.org/x/tools/go/ssa/lift.go | 653 +++++++++++++++++++++++++++++++ 1 file changed, 653 insertions(+) create mode 100644 vendor/golang.org/x/tools/go/ssa/lift.go (limited to 'vendor/golang.org/x/tools/go/ssa/lift.go') diff --git a/vendor/golang.org/x/tools/go/ssa/lift.go b/vendor/golang.org/x/tools/go/ssa/lift.go new file mode 100644 index 0000000..048e9b0 --- /dev/null +++ b/vendor/golang.org/x/tools/go/ssa/lift.go @@ -0,0 +1,653 @@ +// 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) + } + +} -- cgit v1.2.3