// Copyright 2016 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
import (
"cmd/compile/internal/types"
"fmt"
)
// an edgeMem records a backedge, together with the memory
// phi functions at the target of the backedge that must
// be updated when a rescheduling check replaces the backedge.
type edgeMem struct {
e Edge
m *Value // phi for memory at dest of e
}
// a rewriteTarget is a value-argindex pair indicating
// where a rewrite is applied. Note that this is for values,
// not for block controls, because block controls are not targets
// for the rewrites performed in inserting rescheduling checks.
type rewriteTarget struct {
v *Value
i int
}
type rewrite struct {
before, after *Value // before is the expected value before rewrite, after is the new value installed.
rewrites []rewriteTarget // all the targets for this rewrite.
}
func (r *rewrite) String() string {
s := "\n\tbefore=" + r.before.String() + ", after=" + r.after.String()
for _, rw := range r.rewrites {
s += ", (i=" + fmt.Sprint(rw.i) + ", v=" + rw.v.LongString() + ")"
}
s += "\n"
return s
}
// insertLoopReschedChecks inserts rescheduling checks on loop backedges.
func insertLoopReschedChecks(f *Func) {
// TODO: when split information is recorded in export data, insert checks only on backedges that can be reached on a split-call-free path.
// Loop reschedule checks compare the stack pointer with
// the per-g stack bound. If the pointer appears invalid,
// that means a reschedule check is needed.
//
// Steps:
// 1. locate backedges.
// 2. Record memory definitions at block end so that
// the SSA graph for mem can be properly modified.
// 3. Ensure that phi functions that will-be-needed for mem
// are present in the graph, initially with trivial inputs.
// 4. Record all to-be-modified uses of mem;
// apply modifications (split into two steps to simplify and
// avoided nagging order-dependencies).
// 5. Rewrite backedges to include reschedule check,
// and modify destination phi function appropriately with new
// definitions for mem.
if f.NoSplit { // nosplit functions don't reschedule.
return
}
backedges := backedges(f)
if len(backedges) == 0 { // no backedges means no rescheduling checks.
return
}
lastMems := findLastMems(f)
idom := f.Idom()
po := f.postorder()
// The ordering in the dominator tree matters; it's important that
// the walk of the dominator tree also be a preorder (i.e., a node is
// visited only after all its non-backedge predecessors have been visited).
sdom := newSparseOrderedTree(f, idom, po)
if f.pass.debug > 1 {
fmt.Printf("before %s = %s\n", f.Name, sdom.treestructure(f.Entry))
}
tofixBackedges := []edgeMem{}
for _, e := range backedges { // TODO: could filter here by calls in loops, if declared and inferred nosplit are recorded in export data.
tofixBackedges = append(tofixBackedges, edgeMem{e, nil})
}
// It's possible that there is no memory state (no global/pointer loads/stores or calls)
if lastMems[f.Entry.ID] == nil {
lastMems[f.Entry.ID] = f.Entry.NewValue0(f.Entry.Pos, OpInitMem, types.TypeMem)
}
memDefsAtBlockEnds := make([]*Value, f.NumBlocks()) // For each block, the mem def seen at its bottom. Could be from earlier block.
// Propagate last mem definitions forward through successor blocks.
for i := len(po) - 1; i >= 0; i-- {
b := po[i]
mem := lastMems[b.ID]
for j := 0; mem == nil; j++ { // if there's no def, then there's no phi, so the visible mem is identical in all predecessors.
// loop because there might be backedges that haven't been visited yet.
mem = memDefsAtBlockEnds[b.Preds[j].b.ID]
}
memDefsAtBlockEnds[b.ID] = mem
if f.pass.debug > 2 {
fmt.Printf("memDefsAtBlockEnds[%s] = %s\n", b, mem)
}
}
// Maps from block to newly-inserted phi function in block.
newmemphis := make(map[*Block]rewrite)
// Insert phi functions as necessary for future changes to flow graph.
for i, emc := range tofixBackedges {
e := emc.e
h := e.b
// find the phi function for the memory input at "h", if there is one.
var headerMemPhi *Value // look for header mem phi
for _, v := range h.Values {
if v.Op == OpPhi && v.Type.IsMemory() {
headerMemPhi = v
}
}
if headerMemPhi == nil {
// if the header is nil, make a trivial phi from the dominator
mem0 := memDefsAtBlockEnds[idom[h.ID].ID]
headerMemPhi = newPhiFor(h, mem0)
newmemphis[h] = rewrite{before: mem0, after: headerMemPhi}
addDFphis(mem0, h, h, f, memDefsAtBlockEnds, newmemphis, sdom)
}
tofixBackedges[i].m = headerMemPhi
}
if f.pass.debug > 0 {
for b, r := range newmemphis {
fmt.Printf("before b=%s, rewrite=%s\n", b, r.String())
}
}
// dfPhiTargets notes inputs to phis in dominance frontiers that should not
// be rewritten as part of the dominated children of some outer rewrite.
dfPhiTargets := make(map[rewriteTarget]bool)
rewriteNewPhis(f.Entry, f.Entry, f, memDefsAtBlockEnds, newmemphis, dfPhiTargets, sdom)
if f.pass.debug > 0 {
for b, r := range newmemphis {
fmt.Printf("after b=%s, rewrite=%s\n", b, r.String())
}
}
// Apply collected rewrites.
for _, r := range newmemphis {
for _, rw := range r.rewrites {
rw.v.SetArg(rw.i, r.after)
}
}
// Rewrite backedges to include reschedule checks.
for _, emc := range tofixBackedges {
e := emc.e
headerMemPhi := emc.m
h := e.b
i := e.i
p := h.Preds[i]
bb := p.b
mem0 := headerMemPhi.Args[i]
// bb e->p h,
// Because we're going to insert a rare-call, make sure the
// looping edge still looks likely.
likely := BranchLikely
if p.i != 0 {
likely = BranchUnlikely
}
if bb.Kind != BlockPlain { // backedges can be unconditional. e.g., if x { something; continue }
bb.Likely = likely
}
// rewrite edge to include reschedule check
// existing edges:
//
// bb.Succs[p.i] == Edge{h, i}
// h.Preds[i] == p == Edge{bb,p.i}
//
// new block(s):
// test:
// if sp < g.limit { goto sched }
// goto join
// sched:
// mem1 := call resched (mem0)
// goto join
// join:
// mem2 := phi(mem0, mem1)
// goto h
//
// and correct arg i of headerMemPhi and headerCtrPhi
//
// EXCEPT: join block containing only phi functions is bad
// for the register allocator. Therefore, there is no
// join, and branches targeting join must instead target
// the header, and the other phi functions within header are
// adjusted for the additional input.
test := f.NewBlock(BlockIf)
sched := f.NewBlock(BlockPlain)
test.Pos = bb.Pos
sched.Pos = bb.Pos
// if sp < g.limit { goto sched }
// goto header
cfgtypes := &f.Config.Types
pt := cfgtypes.Uintptr
g := test.NewValue1(bb.Pos, OpGetG, pt, mem0)
sp := test.NewValue0(bb.Pos, OpSP, pt)
cmpOp := OpLess64U
if pt.Size() == 4 {
cmpOp = OpLess32U
}
limaddr := test.NewValue1I(bb.Pos, OpOffPtr, pt, 2*pt.Size(), g)
lim := test.NewValue2(bb.Pos, OpLoad, pt, limaddr, mem0)
cmp := test.NewValue2(bb.Pos, cmpOp, cfgtypes.Bool, sp, lim)
test.SetControl(cmp)
// if true, goto sched
test.AddEdgeTo(sched)
// if false, rewrite edge to header.
// do NOT remove+add, because that will perturb all the other phi functions
// as well as messing up other edges to the header.
test.Succs = append(test.Succs, Edge{h, i})
h.Preds[i] = Edge{test, 1}
headerMemPhi.SetArg(i, mem0)
test.Likely = BranchUnlikely
// sched:
// mem1 := call resched (mem0)
// goto header
resched := f.fe.Syslook("goschedguarded")
mem1 := sched.NewValue1A(bb.Pos, OpStaticCall, types.TypeMem, resched, mem0)
sched.AddEdgeTo(h)
headerMemPhi.AddArg(mem1)
bb.Succs[p.i] = Edge{test, 0}
test.Preds = append(test.Preds, Edge{bb, p.i})
// Must correct all the other phi functions in the header for new incoming edge.
// Except for mem phis, it will be the same value seen on the original
// backedge at index i.
for _, v := range h.Values {
if v.Op == OpPhi && v != headerMemPhi {
v.AddArg(v.Args[i])
}
}
}
f.invalidateCFG()
if f.pass.debug > 1 {
sdom = newSparseTree(f, f.Idom())
fmt.Printf("after %s = %s\n", f.Name, sdom.treestructure(f.Entry))
}
}
// newPhiFor inserts a new Phi function into b,
// with all inputs set to v.
func newPhiFor(b *Block, v *Value) *Value {
phiV := b.NewValue0(b.Pos, OpPhi, v.Type)
for range b.Preds {
phiV.AddArg(v)
}
return phiV
}
// rewriteNewPhis updates newphis[h] to record all places where the new phi function inserted
// in block h will replace a previous definition. Block b is the block currently being processed;
// if b has its own phi definition then it takes the place of h.
// defsForUses provides information about other definitions of the variable that are present
// (and if nil, indicates that the variable is no longer live)
// sdom must yield a preorder of the flow graph if recursively walked, root-to-children.
// The result of newSparseOrderedTree with order supplied by a dfs-postorder satisfies this
// requirement.
func rewriteNewPhis(h, b *Block, f *Func, defsForUses []*Value, newphis map[*Block]rewrite, dfPhiTargets map[rewriteTarget]bool, sdom SparseTree) {
// If b is a block with a new phi, then a new rewrite applies below it in the dominator tree.
if _, ok := newphis[b]; ok {
h = b
}
change := newphis[h]
x := change.before
y := change.after
// Apply rewrites to this block
if x != nil { // don't waste time on the common case of no definition.
p := &change.rewrites
for _, v := range b.Values {
if v == y { // don't rewrite self -- phi inputs are handled below.
continue
}
for i, w := range v.Args {
if w != x {
continue
}
tgt := rewriteTarget{v, i}
// It's possible dominated control flow will rewrite this instead.
// Visiting in preorder (a property of how sdom was constructed)
// ensures that these are seen in the proper order.
if dfPhiTargets[tgt] {
continue
}
*p = append(*p, tgt)
if f.pass.debug > 1 {
fmt.Printf("added block target for h=%v, b=%v, x=%v, y=%v, tgt.v=%s, tgt.i=%d\n",
h, b, x, y, v, i)
}
}
}
// Rewrite appropriate inputs of phis reached in successors
// in dominance frontier, self, and dominated.
// If the variable def reaching uses in b is itself defined in b, then the new phi function
// does not reach the successors of b. (This assumes a bit about the structure of the
// phi use-def graph, but it's true for memory.)
if dfu := defsForUses[b.ID]; dfu != nil && dfu.Block != b {
for _, e := range b.Succs {
s := e.b
for _, v := range s.Values {
if v.Op == OpPhi && v.Args[e.i] == x {
tgt := rewriteTarget{v, e.i}
*p = append(*p, tgt)
dfPhiTargets[tgt] = true
if f.pass.debug > 1 {
fmt.Printf("added phi target for h=%v, b=%v, s=%v, x=%v, y=%v, tgt.v=%s, tgt.i=%d\n",
h, b, s, x, y, v.LongString(), e.i)
}
break
}
}
}
}
newphis[h] = change
}
for c := sdom[b.ID].child; c != nil; c = sdom[c.ID].sibling {
rewriteNewPhis(h, c, f, defsForUses, newphis, dfPhiTargets, sdom) // TODO: convert to explicit stack from recursion.
}
}
// addDFphis creates new trivial phis that are necessary to correctly reflect (within SSA)
// a new definition for variable "x" inserted at h (usually but not necessarily a phi).
// These new phis can only occur at the dominance frontier of h; block s is in the dominance
// frontier of h if h does not strictly dominate s and if s is a successor of a block b where
// either b = h or h strictly dominates b.
// These newly created phis are themselves new definitions that may require addition of their
// own trivial phi functions in their own dominance frontier, and this is handled recursively.
func addDFphis(x *Value, h, b *Block, f *Func, defForUses []*Value, newphis map[*Block]rewrite, sdom SparseTree) {
oldv := defForUses[b.ID]
if oldv != x { // either a new definition replacing x, or nil if it is proven that there are no uses reachable from b
return
}
idom := f.Idom()
outer:
for _, e := range b.Succs {
s := e.b
// check phi functions in the dominance frontier
if sdom.isAncestor(h, s) {
continue // h dominates s, successor of b, therefore s is not in the frontier.
}
if _, ok := newphis[s]; ok {
continue // successor s of b already has a new phi function, so there is no need to add another.
}
if x != nil {
for _, v := range s.Values {
if v.Op == OpPhi && v.Args[e.i] == x {
continue outer // successor s of b has an old phi function, so there is no need to add another.
}
}
}
old := defForUses[idom[s.ID].ID] // new phi function is correct-but-redundant, combining value "old" on all inputs.
headerPhi := newPhiFor(s, old)
// the new phi will replace "old" in block s and all blocks dominated by s.
newphis[s] = rewrite{before: old, after: headerPhi} // record new phi, to have inputs labeled "old" rewritten to "headerPhi"
addDFphis(old, s, s, f, defForUses, newphis, sdom) // the new definition may also create new phi functions.
}
for c := sdom[b.ID].child; c != nil; c = sdom[c.ID].sibling {
addDFphis(x, h, c, f, defForUses, newphis, sdom) // TODO: convert to explicit stack from recursion.
}
}
// findLastMems maps block ids to last memory-output op in a block, if any
func findLastMems(f *Func) []*Value {
var stores []*Value
lastMems := make([]*Value, f.NumBlocks())
storeUse := f.newSparseSet(f.NumValues())
defer f.retSparseSet(storeUse)
for _, b := range f.Blocks {
// Find all the stores in this block. Categorize their uses:
// storeUse contains stores which are used by a subsequent store.
storeUse.clear()
stores = stores[:0]
var memPhi *Value
for _, v := range b.Values {
if v.Op == OpPhi {
if v.Type.IsMemory() {
memPhi = v
}
continue
}
if v.Type.IsMemory() {
stores = append(stores, v)
for _, a := range v.Args {
if a.Block == b && a.Type.IsMemory() {
storeUse.add(a.ID)
}
}
}
}
if len(stores) == 0 {
lastMems[b.ID] = memPhi
continue
}
// find last store in the block
var last *Value
for _, v := range stores {
if storeUse.contains(v.ID) {
continue
}
if last != nil {
b.Fatalf("two final stores - simultaneous live stores %s %s", last, v)
}
last = v
}
if last == nil {
b.Fatalf("no last store found - cycle?")
}
lastMems[b.ID] = last
}
return lastMems
}
type backedgesState struct {
b *Block
i int
}
// backedges returns a slice of successor edges that are back
// edges. For reducible loops, edge.b is the header.
func backedges(f *Func) []Edge {
edges := []Edge{}
mark := make([]markKind, f.NumBlocks())
stack := []backedgesState{}
mark[f.Entry.ID] = notExplored
stack = append(stack, backedgesState{f.Entry, 0})
for len(stack) > 0 {
l := len(stack)
x := stack[l-1]
if x.i < len(x.b.Succs) {
e := x.b.Succs[x.i]
stack[l-1].i++
s := e.b
if mark[s.ID] == notFound {
mark[s.ID] = notExplored
stack = append(stack, backedgesState{s, 0})
} else if mark[s.ID] == notExplored {
edges = append(edges, e)
}
} else {
mark[x.b.ID] = done
stack = stack[0 : l-1]
}
}
return edges
}
|
