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| 1 | +// Copyright 2017 The Go Authors. All rights reserved. |
| 2 | +// Use of this source code is governed by a BSD-style |
| 3 | +// license that can be found in the LICENSE file. |
| 4 | + |
| 5 | +package ssa |
| 6 | + |
| 7 | +// loopRotate converts loops with a check-loop-condition-at-beginning |
| 8 | +// to loops with a check-loop-condition-at-end. |
| 9 | +// This helps loops avoid extra unnecessary jumps. |
| 10 | +// |
| 11 | +// loop: |
| 12 | +// CMPQ ... |
| 13 | +// JGE exit |
| 14 | +// ... |
| 15 | +// JMP loop |
| 16 | +// exit: |
| 17 | +// |
| 18 | +// JMP entry |
| 19 | +// loop: |
| 20 | +// ... |
| 21 | +// entry: |
| 22 | +// CMPQ ... |
| 23 | +// JLT loop |
| 24 | +func loopRotate(f *Func) { |
| 25 | + loopnest := f.loopnest() |
| 26 | + if len(loopnest.loops) == 0 { |
| 27 | + return |
| 28 | + } |
| 29 | + |
| 30 | + // Set of blocks we're moving, by ID. |
| 31 | + move := map[ID]struct{}{} |
| 32 | + |
| 33 | + // Map from block ID to the moving block that should |
| 34 | + // come right after it. |
| 35 | + after := map[ID]*Block{} |
| 36 | + |
| 37 | + // Check each loop header and decide if we want to move it. |
| 38 | + for _, loop := range loopnest.loops { |
| 39 | + b := loop.header |
| 40 | + var p *Block // b's in-loop predecessor |
| 41 | + for _, e := range b.Preds { |
| 42 | + if e.b.Kind != BlockPlain { |
| 43 | + continue |
| 44 | + } |
| 45 | + if loopnest.b2l[e.b.ID] != loop { |
| 46 | + continue |
| 47 | + } |
| 48 | + p = e.b |
| 49 | + } |
| 50 | + if p == nil || p == b { |
| 51 | + continue |
| 52 | + } |
| 53 | + |
| 54 | + // Place b after p. |
| 55 | + move[b.ID] = struct{}{} |
| 56 | + after[p.ID] = b |
| 57 | + } |
| 58 | + |
| 59 | + // Move blocks to their destinations in a single pass. |
| 60 | + // We rely here on the fact that loop headers must come |
| 61 | + // before the rest of the loop. And that relies on the |
| 62 | + // fact that we only identify reducible loops. |
| 63 | + j := 0 |
| 64 | + for i, b := range f.Blocks { |
| 65 | + if _, ok := move[b.ID]; ok { |
| 66 | + continue |
| 67 | + } |
| 68 | + f.Blocks[j] = b |
| 69 | + j++ |
| 70 | + if a := after[b.ID]; a != nil { |
| 71 | + if j > i { |
| 72 | + f.Fatalf("head before tail in loop %s", b) |
| 73 | + } |
| 74 | + f.Blocks[j] = a |
| 75 | + j++ |
| 76 | + } |
| 77 | + } |
| 78 | + if j != len(f.Blocks) { |
| 79 | + f.Fatalf("bad reordering in looprotate") |
| 80 | + } |
| 81 | +} |
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