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// Copyright 2017 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
// loopRotate converts loops with a check-loop-condition-at-beginning
// to loops with a check-loop-condition-at-end.
// This helps loops avoid extra unnecessary jumps.
//
// loop:
// CMPQ ...
// JGE exit
// ...
// JMP loop
// exit:
//
// JMP entry
// loop:
// ...
// entry:
// CMPQ ...
// JLT loop
func loopRotate(f *Func) {
loopnest := f.loopnest()
if loopnest.hasIrreducible {
return
}
if len(loopnest.loops) == 0 {
return
}
idToIdx := make([]int, f.NumBlocks())
for i, b := range f.Blocks {
idToIdx[b.ID] = i
}
// Set of blocks we're moving, by ID.
move := map[ID]struct{}{}
// Map from block ID to the moving blocks that should
// come right after it.
after := map[ID][]*Block{}
// Check each loop header and decide if we want to move it.
for _, loop := range loopnest.loops {
b := loop.header
var p *Block // b's in-loop predecessor
for _, e := range b.Preds {
if e.b.Kind != BlockPlain {
continue
}
if loopnest.b2l[e.b.ID] != loop {
continue
}
p = e.b
}
if p == nil || p == b {
continue
}
after[p.ID] = []*Block{b}
for {
nextIdx := idToIdx[b.ID] + 1
if nextIdx >= len(f.Blocks) { // reached end of function (maybe impossible?)
break
}
nextb := f.Blocks[nextIdx]
if nextb == p { // original loop predecessor is next
break
}
if loopnest.b2l[nextb.ID] != loop { // about to leave loop
break
}
after[p.ID] = append(after[p.ID], nextb)
b = nextb
}
// Place b after p.
for _, b := range after[p.ID] {
move[b.ID] = struct{}{}
}
}
// Move blocks to their destinations in a single pass.
// We rely here on the fact that loop headers must come
// before the rest of the loop. And that relies on the
// fact that we only identify reducible loops.
j := 0
for i, b := range f.Blocks {
if _, ok := move[b.ID]; ok {
continue
}
f.Blocks[j] = b
j++
for _, a := range after[b.ID] {
if j > i {
f.Fatalf("head before tail in loop %s", b)
}
f.Blocks[j] = a
j++
}
}
if j != len(f.Blocks) {
f.Fatalf("bad reordering in looprotate")
}
}
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