1 files changed, 106 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/looprotate.go b/src/cmd/compile/internal/ssa/looprotate.go
new file mode 100644
index 0000000..2e5e421
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/looprotate.go
@@ -0,0 +1,106 @@
+// 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")
+	}
+}