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The scheduler assumes two special invariants that apply to tuple selectors (Select0 and Select1 ops): 1. There is only one tuple selector of each type per generator. 2. Tuple selectors and generators reside in the same block. Prior to this CL the assumption was that these invariants would only be broken by the CSE pass. The CSE pass therefore contained code to move and de-duplicate selectors to fix these invariants. However it is also possible to write relatively basic optimization rules that cause these invariants to be broken. For example: (A (Select0 (B))) -> (Select1 (B)) This rule could result in the newly added selector (Select1) being in a different block to the tuple generator (see issue #38356). It could also result in duplicate selectors if this rule matches multiple times for the same tuple generator (see issue #39472). The CSE pass will 'fix' these invariants. However it will only do so when optimizations are enabled (since disabling optimizations disables the CSE pass). This CL moves the CSE tuple selector fixup code into its own pass and makes it mandatory even when optimizations are disabled. This allows tuple selectors to be treated like normal ops for most of the compilation pipeline until after the new pass has run, at which point we need to be careful to maintain the invariant again. Fixes #39472. Change-Id: Ia3f79e09d9c65ac95f897ce37e967ee1258a080b Reviewed-on: https://go-review.googlesource.com/c/go/+/237118 Run-TryBot: Michael Munday <mike.munday@ibm.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org>
59 lines
1.7 KiB
Go
59 lines
1.7 KiB
Go
// Copyright 2020 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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package ssa
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// tightenTupleSelectors ensures that tuple selectors (Select0 and
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// Select1 ops) are in the same block as their tuple generator. The
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// function also ensures that there are no duplicate tuple selectors.
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// These properties are expected by the scheduler but may not have
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// been maintained by the optimization pipeline up to this point.
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//
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// See issues 16741 and 39472.
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func tightenTupleSelectors(f *Func) {
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selectors := make(map[struct {
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id ID
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op Op
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}]*Value)
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for _, b := range f.Blocks {
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for _, selector := range b.Values {
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if selector.Op != OpSelect0 && selector.Op != OpSelect1 {
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continue
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}
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// Get the tuple generator to use as a key for de-duplication.
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tuple := selector.Args[0]
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if !tuple.Type.IsTuple() {
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f.Fatalf("arg of tuple selector %s is not a tuple: %s", selector.String(), tuple.LongString())
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}
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// If there is a pre-existing selector in the target block then
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// use that. Do this even if the selector is already in the
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// target block to avoid duplicate tuple selectors.
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key := struct {
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id ID
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op Op
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}{tuple.ID, selector.Op}
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if t := selectors[key]; t != nil {
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if selector != t {
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selector.copyOf(t)
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}
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continue
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}
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// If the selector is in the wrong block copy it into the target
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// block.
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if selector.Block != tuple.Block {
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t := selector.copyInto(tuple.Block)
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selector.copyOf(t)
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selectors[key] = t
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continue
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}
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// The selector is in the target block. Add it to the map so it
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// cannot be duplicated.
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selectors[key] = selector
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}
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}
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}
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