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diff --git a/src/cmd/compile/internal/ssa/lca.go b/src/cmd/compile/internal/ssa/lca.go
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+++ b/src/cmd/compile/internal/ssa/lca.go
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+// Copyright 2016 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
+
+// Code to compute lowest common ancestors in the dominator tree.
+// https://en.wikipedia.org/wiki/Lowest_common_ancestor
+// https://en.wikipedia.org/wiki/Range_minimum_query#Solution_using_constant_time_and_linearithmic_space
+
+// lcaRange is a data structure that can compute lowest common ancestor queries
+// in O(n lg n) precomputed space and O(1) time per query.
+type lcaRange struct {
+	// Additional information about each block (indexed by block ID).
+	blocks []lcaRangeBlock
+
+	// Data structure for range minimum queries.
+	// rangeMin[k][i] contains the ID of the minimum depth block
+	// in the Euler tour from positions i to i+1<<k-1, inclusive.
+	rangeMin [][]ID
+}
+
+type lcaRangeBlock struct {
+	b          *Block
+	parent     ID    // parent in dominator tree.  0 = no parent (entry or unreachable)
+	firstChild ID    // first child in dominator tree
+	sibling    ID    // next child of parent
+	pos        int32 // an index in the Euler tour where this block appears (any one of its occurrences)
+	depth      int32 // depth in dominator tree (root=0, its children=1, etc.)
+}
+
+func makeLCArange(f *Func) *lcaRange {
+	dom := f.Idom()
+
+	// Build tree
+	blocks := make([]lcaRangeBlock, f.NumBlocks())
+	for _, b := range f.Blocks {
+		blocks[b.ID].b = b
+		if dom[b.ID] == nil {
+			continue // entry or unreachable
+		}
+		parent := dom[b.ID].ID
+		blocks[b.ID].parent = parent
+		blocks[b.ID].sibling = blocks[parent].firstChild
+		blocks[parent].firstChild = b.ID
+	}
+
+	// Compute euler tour ordering.
+	// Each reachable block will appear #children+1 times in the tour.
+	tour := make([]ID, 0, f.NumBlocks()*2-1)
+	type queueEntry struct {
+		bid ID // block to work on
+		cid ID // child we're already working on (0 = haven't started yet)
+	}
+	q := []queueEntry{{f.Entry.ID, 0}}
+	for len(q) > 0 {
+		n := len(q) - 1
+		bid := q[n].bid
+		cid := q[n].cid
+		q = q[:n]
+
+		// Add block to tour.
+		blocks[bid].pos = int32(len(tour))
+		tour = append(tour, bid)
+
+		// Proceed down next child edge (if any).
+		if cid == 0 {
+			// This is our first visit to b. Set its depth.
+			blocks[bid].depth = blocks[blocks[bid].parent].depth + 1
+			// Then explore its first child.
+			cid = blocks[bid].firstChild
+		} else {
+			// We've seen b before. Explore the next child.
+			cid = blocks[cid].sibling
+		}
+		if cid != 0 {
+			q = append(q, queueEntry{bid, cid}, queueEntry{cid, 0})
+		}
+	}
+
+	// Compute fast range-minimum query data structure
+	var rangeMin [][]ID
+	rangeMin = append(rangeMin, tour) // 1-size windows are just the tour itself.
+	for logS, s := 1, 2; s < len(tour); logS, s = logS+1, s*2 {
+		r := make([]ID, len(tour)-s+1)
+		for i := 0; i < len(tour)-s+1; i++ {
+			bid := rangeMin[logS-1][i]
+			bid2 := rangeMin[logS-1][i+s/2]
+			if blocks[bid2].depth < blocks[bid].depth {
+				bid = bid2
+			}
+			r[i] = bid
+		}
+		rangeMin = append(rangeMin, r)
+	}
+
+	return &lcaRange{blocks: blocks, rangeMin: rangeMin}
+}
+
+// find returns the lowest common ancestor of a and b.
+func (lca *lcaRange) find(a, b *Block) *Block {
+	if a == b {
+		return a
+	}
+	// Find the positions of a and bin the Euler tour.
+	p1 := lca.blocks[a.ID].pos
+	p2 := lca.blocks[b.ID].pos
+	if p1 > p2 {
+		p1, p2 = p2, p1
+	}
+
+	// The lowest common ancestor is the minimum depth block
+	// on the tour from p1 to p2.  We've precomputed minimum
+	// depth blocks for powers-of-two subsequences of the tour.
+	// Combine the right two precomputed values to get the answer.
+	logS := uint(log64(int64(p2 - p1)))
+	bid1 := lca.rangeMin[logS][p1]
+	bid2 := lca.rangeMin[logS][p2-1<<logS+1]
+	if lca.blocks[bid1].depth < lca.blocks[bid2].depth {
+		return lca.blocks[bid1].b
+	}
+	return lca.blocks[bid2].b
+}