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[dev.ssa] cmd/compile/internal/ssa: add arg-dominating check, fix phielim
Add a check to make sure value arguments dominate the value. Phi elim output used to fail this test. When eliminating redundant phis, phi elim was using one of the args and not the ultimate source. For example: b1: x = ... -> b2 b3 b2: y = Copy x b3: z = Copy x -> b4 -> b4 b4: w = phi y z Phi elim eliminates w, but it used to replace w with (Copy y). That's bad as b2 does not dominate b4. Instead we should replace w with (Copy x). Fixes #12347 Change-Id: I9f340cdabcda8e2e90359fb4f9250877b1fffe98 Reviewed-on: https://go-review.googlesource.com/13986 Reviewed-by: David Chase <drchase@google.com>
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@ -181,4 +181,45 @@ func checkFunc(f *Func) {
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f.Fatalf("used value v%d in free list", id)
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}
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}
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// Check to make sure all args dominate uses.
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if f.RegAlloc == nil {
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// Note: regalloc introduces non-dominating args.
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// See TODO in regalloc.go.
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idom := dominators(f)
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for _, b := range f.Blocks {
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for _, v := range b.Values {
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for i, arg := range v.Args {
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x := arg.Block
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y := b
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if v.Op == OpPhi {
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y = b.Preds[i]
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}
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if !domCheck(f, idom, x, y) {
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f.Fatalf("arg %d of value %s does not dominate", i, v.LongString())
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}
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}
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}
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if b.Control != nil && !domCheck(f, idom, b.Control.Block, b) {
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f.Fatalf("control value %s for %s doesn't dominate", b.Control, b)
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}
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}
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}
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}
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// domCheck reports whether x dominates y (including x==y).
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func domCheck(f *Func, idom []*Block, x, y *Block) bool {
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if y != f.Entry && idom[y.ID] == nil {
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// unreachable - ignore
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return true
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}
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for {
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if x == y {
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return true
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}
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y = idom[y.ID]
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if y == nil {
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return false
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}
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}
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}
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@ -11,33 +11,27 @@ package ssa
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// v = phi(x,x,x)
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// v = phi(x,v,x,v)
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func phielim(f *Func) {
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args := newSparseSet(f.NumValues())
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argSet := newSparseSet(f.NumValues())
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var args []*Value
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for _, b := range f.Blocks {
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for _, v := range b.Values {
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if v.Op != OpPhi {
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continue
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}
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args.clear()
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argSet.clear()
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args = args[:0]
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for _, x := range v.Args {
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for x.Op == OpCopy {
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x = x.Args[0]
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}
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args.add(x.ID)
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}
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switch {
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case args.size() == 1:
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v.Op = OpCopy
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v.SetArgs1(v.Args[0])
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case args.size() == 2 && args.contains(v.ID):
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var w *Value
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for _, x := range v.Args {
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if x.ID != v.ID {
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w = x
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break
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}
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if x != v && !argSet.contains(x.ID) {
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argSet.add(x.ID)
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args = append(args, x)
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}
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}
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if len(args) == 1 {
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v.Op = OpCopy
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v.SetArgs1(w)
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v.SetArgs1(args[0])
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}
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}
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}
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