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[dev.ssa] cmd/compile/internal/ssa: enforce load-store ordering in scheduler
We must make sure that all loads that use a store are scheduled before the next store. Add additional dependency edges to the value graph to enforce this constraint. Change-Id: Iab83644f68bc4c30637085b82ca7467b9d5513a5 Reviewed-on: https://go-review.googlesource.com/13470 Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
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src/cmd/compile/internal/gc/testdata/loadstore_ssa.go
vendored
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39
src/cmd/compile/internal/gc/testdata/loadstore_ssa.go
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@ -0,0 +1,39 @@
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// run
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Tests load/store ordering
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package main
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// testLoadStoreOrder tests for reordering of stores/loads.
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func testLoadStoreOrder() {
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z := uint32(1000)
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if testLoadStoreOrder_ssa(&z, 100) == 0 {
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println("testLoadStoreOrder failed")
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failed = true
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}
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}
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func testLoadStoreOrder_ssa(z *uint32, prec uint) int {
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switch {
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}
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old := *z // load
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*z = uint32(prec) // store
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if *z < old { // load
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return 1
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}
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return 0
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}
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var failed = false
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func main() {
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testLoadStoreOrder()
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if failed {
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panic("failed")
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}
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}
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@ -21,22 +21,49 @@ func schedule(f *Func) {
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var order []*Value
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// priority queue of legally schedulable (0 unscheduled uses) values
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var priq [4][]*Value
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var priq [5][]*Value
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// maps mem values to the next live memory value
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nextMem := make([]*Value, f.NumValues())
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// additional pretend arguments for each Value. Used to enforce load/store ordering.
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additionalArgs := make([][]*Value, f.NumValues())
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for _, b := range f.Blocks {
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// Compute uses.
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// Find store chain for block.
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for _, v := range b.Values {
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if v.Op != OpPhi {
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// Note: if a value is used by a phi, it does not induce
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// a scheduling edge because that use is from the
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// previous iteration.
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if v.Op != OpPhi && v.Type.IsMemory() {
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for _, w := range v.Args {
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if w.Block == b {
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uses[w.ID]++
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if w.Type.IsMemory() {
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nextMem[w.ID] = v
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}
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}
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}
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}
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// Compute uses.
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for _, v := range b.Values {
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if v.Op == OpPhi {
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// If a value is used by a phi, it does not induce
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// a scheduling edge because that use is from the
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// previous iteration.
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continue
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}
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for _, w := range v.Args {
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if w.Block == b {
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uses[w.ID]++
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}
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// Any load must come before the following store.
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if v.Type.IsMemory() || !w.Type.IsMemory() {
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continue // not a load
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}
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s := nextMem[w.ID]
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if s == nil || s.Block != b {
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continue
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}
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additionalArgs[s.ID] = append(additionalArgs[s.ID], v)
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uses[v.ID]++
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}
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}
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// Compute score. Larger numbers are scheduled closer to the end of the block.
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for _, v := range b.Values {
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switch {
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@ -44,23 +71,22 @@ func schedule(f *Func) {
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// We want all the phis first.
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score[v.ID] = 0
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case v.Type.IsMemory():
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// Schedule stores as late as possible.
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// This makes sure that loads do not get scheduled
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// after a following store (1-live-memory requirement).
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score[v.ID] = 2
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// Schedule stores as early as possible. This tends to
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// reduce register pressure.
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score[v.ID] = 1
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case v.Type.IsFlags():
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// Schedule flag register generation as late as possible.
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// This makes sure that we only have one live flags
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// value at a time.
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score[v.ID] = 2
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score[v.ID] = 3
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default:
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score[v.ID] = 1
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score[v.ID] = 2
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}
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}
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if b.Control != nil && b.Control.Op != OpPhi {
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// Force the control value to be scheduled at the end,
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// unless it is a phi value (which must be first).
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score[b.Control.ID] = 3
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score[b.Control.ID] = 4
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// TODO: some times control values are used by other values
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// in the block. So the control value will not appear at
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// the very end. Decide if this is a problem or not.
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@ -110,6 +136,14 @@ func schedule(f *Func) {
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priq[s] = append(priq[s], w)
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}
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}
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for _, w := range additionalArgs[v.ID] {
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uses[w.ID]--
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if uses[w.ID] == 0 {
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// All uses scheduled, w is now schedulable.
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s := score[w.ID]
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priq[s] = append(priq[s], w)
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}
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}
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}
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if len(order) != len(b.Values) {
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f.Fatalf("schedule does not include all values")
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