diff --git a/src/cmd/compile/internal/ssa/TODO b/src/cmd/compile/internal/ssa/TODO
index 4e39d1e9c3..a457e67101 100644
--- a/src/cmd/compile/internal/ssa/TODO
+++ b/src/cmd/compile/internal/ssa/TODO
@@ -24,7 +24,7 @@ Optimizations (better compiled code)
 - Figure out how to make PARAMOUT variables ssa-able.
   They need to get spilled automatically at end-of-function somehow.
 - If strings are being passed around without being interpreted (ptr
-  and len feilds being accessed) pass them in xmm registers?
+  and len fields being accessed) pass them in xmm registers?
   Same for interfaces?
 - OpArrayIndex should take its index in AuxInt, not a full value.
 - remove FLAGS from REP instruction clobbers
@@ -32,7 +32,6 @@ Optimizations (better compiled code)
   Note that this is challenging for ops that generate flags
   because flagalloc wants to move those instructions around for
   flag regeneration.
-- In forms like if ... { call } else { no call }, mark the call branch as unlikely.
 - Non-constant rotate detection.
 - Do 0 <= x && x < n with one unsigned compare
 - nil-check removal in indexed load/store case:
diff --git a/src/cmd/compile/internal/ssa/compile.go b/src/cmd/compile/internal/ssa/compile.go
index 2780e5bcfc..f68819c3c2 100644
--- a/src/cmd/compile/internal/ssa/compile.go
+++ b/src/cmd/compile/internal/ssa/compile.go
@@ -178,7 +178,8 @@ var passes = [...]pass{
 	{name: "late phielim", fn: phielim},
 	{name: "late copyelim", fn: copyelim},
 	{name: "late deadcode", fn: deadcode},
-	{name: "critical", fn: critical, required: true},   // remove critical edges
+	{name: "critical", fn: critical, required: true}, // remove critical edges
+	{name: "likelyadjust", fn: likelyadjust},
 	{name: "layout", fn: layout, required: true},       // schedule blocks
 	{name: "schedule", fn: schedule, required: true},   // schedule values
 	{name: "flagalloc", fn: flagalloc, required: true}, // allocate flags register
diff --git a/src/cmd/compile/internal/ssa/likelyadjust.go b/src/cmd/compile/internal/ssa/likelyadjust.go
new file mode 100755
index 0000000000..6ce8705272
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/likelyadjust.go
@@ -0,0 +1,300 @@
+// 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
+
+import (
+	"fmt"
+)
+
+type loop struct {
+	header *Block // The header node of this (reducible) loop
+	outer  *loop  // loop containing this loop
+	// Next two fields not currently used, but cheap to maintain,
+	// and aid in computation of inner-ness and list of blocks.
+	nBlocks int32 // Number of blocks in this loop but not within inner loops
+	isInner bool  // True if never discovered to contain a loop
+}
+
+// outerinner records that outer contains inner
+func (sdom sparseTree) outerinner(outer, inner *loop) {
+	oldouter := inner.outer
+	if oldouter == nil || sdom.isAncestorEq(oldouter.header, outer.header) {
+		inner.outer = outer
+		outer.isInner = false
+	}
+}
+
+type loopnest struct {
+	f     *Func
+	b2l   []*loop
+	po    []*Block
+	sdom  sparseTree
+	loops []*loop
+}
+
+func min8(a, b int8) int8 {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func max8(a, b int8) int8 {
+	if a > b {
+		return a
+	}
+	return b
+}
+
+const (
+	blDEFAULT = 0
+	blMin     = blDEFAULT
+	blCALL    = 1
+	blRET     = 2
+	blEXIT    = 3
+)
+
+var bllikelies [4]string = [4]string{"default", "call", "ret", "exit"}
+
+func describePredictionAgrees(b *Block, prediction BranchPrediction) string {
+	s := ""
+	if prediction == b.Likely {
+		s = " (agrees with previous)"
+	} else if b.Likely != BranchUnknown {
+		s = " (disagrees with previous, ignored)"
+	}
+	return s
+}
+
+func describeBranchPrediction(f *Func, b *Block, likely, not int8, prediction BranchPrediction) {
+	f.Config.Warnl(int(b.Line), "Branch prediction rule %s < %s%s",
+		bllikelies[likely-blMin], bllikelies[not-blMin], describePredictionAgrees(b, prediction))
+}
+
+func likelyadjust(f *Func) {
+	// The values assigned to certain and local only matter
+	// in their rank order.  0 is default, more positive
+	// is less likely.  It's possible to assign a negative
+	// unlikeliness (though not currently the case).
+	certain := make([]int8, f.NumBlocks()) // In the long run, all outcomes are at least this bad. Mainly for Exit
+	local := make([]int8, f.NumBlocks())   // for our immediate predecessors.
+
+	nest := loopnestfor(f)
+	po := nest.po
+	b2l := nest.b2l
+
+	for _, b := range po {
+		switch b.Kind {
+		case BlockExit:
+			// Very unlikely.
+			local[b.ID] = blEXIT
+			certain[b.ID] = blEXIT
+
+			// Ret, it depends.
+		case BlockRet, BlockRetJmp:
+			local[b.ID] = blRET
+			certain[b.ID] = blRET
+
+			// Calls. TODO not all calls are equal, names give useful clues.
+			// Any name-based heuristics are only relative to other calls,
+			// and less influential than inferences from loop structure.
+		case BlockCall:
+			local[b.ID] = blCALL
+			certain[b.ID] = max8(blCALL, certain[b.Succs[0].ID])
+
+		default:
+			if len(b.Succs) == 1 {
+				certain[b.ID] = certain[b.Succs[0].ID]
+			} else if len(b.Succs) == 2 {
+				// If successor is an unvisited backedge, it's in loop and we don't care.
+				// Its default unlikely is also zero which is consistent with favoring loop edges.
+				// Notice that this can act like a "reset" on unlikeliness at loops; the
+				// default "everything returns" unlikeliness is erased by min with the
+				// backedge likeliness; however a loop with calls on every path will be
+				// tagged with call cost.  Net effect is that loop entry is favored.
+				b0 := b.Succs[0].ID
+				b1 := b.Succs[1].ID
+				certain[b.ID] = min8(certain[b0], certain[b1])
+
+				l := b2l[b.ID]
+				l0 := b2l[b0]
+				l1 := b2l[b1]
+
+				prediction := b.Likely
+				// Weak loop heuristic -- both source and at least one dest are in loops,
+				// and there is a difference in the destinations.
+				// TODO what is best arrangement for nested loops?
+				if l != nil && l0 != l1 {
+					noprediction := false
+					switch {
+					// prefer not to exit loops
+					case l1 == nil:
+						prediction = BranchLikely
+					case l0 == nil:
+						prediction = BranchUnlikely
+
+						// prefer to stay in loop, not exit to outer.
+					case l == l0:
+						prediction = BranchLikely
+					case l == l1:
+						prediction = BranchUnlikely
+					default:
+						noprediction = true
+					}
+					if f.pass.debug > 0 && !noprediction {
+						f.Config.Warnl(int(b.Line), "Branch prediction rule stay in loop%s",
+							describePredictionAgrees(b, prediction))
+					}
+
+				} else {
+					// Lacking loop structure, fall back on heuristics.
+					if certain[b1] > certain[b0] {
+						prediction = BranchLikely
+						if f.pass.debug > 0 {
+							describeBranchPrediction(f, b, certain[b0], certain[b1], prediction)
+						}
+					} else if certain[b0] > certain[b1] {
+						prediction = BranchUnlikely
+						if f.pass.debug > 0 {
+							describeBranchPrediction(f, b, certain[b1], certain[b0], prediction)
+						}
+					} else if local[b1] > local[b0] {
+						prediction = BranchLikely
+						if f.pass.debug > 0 {
+							describeBranchPrediction(f, b, local[b0], local[b1], prediction)
+						}
+					} else if local[b0] > local[b1] {
+						prediction = BranchUnlikely
+						if f.pass.debug > 0 {
+							describeBranchPrediction(f, b, local[b1], local[b0], prediction)
+						}
+					}
+				}
+				if b.Likely != prediction {
+					if b.Likely == BranchUnknown {
+						b.Likely = prediction
+					}
+				}
+			}
+		}
+		if f.pass.debug > 2 {
+			f.Config.Warnl(int(b.Line), "BP: Block %s, local=%s, certain=%s", b, bllikelies[local[b.ID]-blMin], bllikelies[certain[b.ID]-blMin])
+		}
+
+	}
+}
+
+func (l *loop) String() string {
+	return fmt.Sprintf("hdr:%s", l.header)
+}
+
+func (l *loop) LongString() string {
+	i := ""
+	o := ""
+	if l.isInner {
+		i = ", INNER"
+	}
+	if l.outer != nil {
+		o = ", o=" + l.outer.header.String()
+	}
+	return fmt.Sprintf("hdr:%s%s%s", l.header, i, o)
+}
+
+// nearestOuterLoop returns the outer loop of loop most nearly
+// containing block b; the header must dominate b.  loop itself
+// is assumed to not be that loop.  For acceptable performance,
+// we're relying on loop nests to not be terribly deep.
+func (l *loop) nearestOuterLoop(sdom sparseTree, b *Block) *loop {
+	var o *loop
+	for o = l.outer; o != nil && !sdom.isAncestorEq(o.header, b); o = o.outer {
+	}
+	return o
+}
+
+func loopnestfor(f *Func) *loopnest {
+	po := postorder(f)
+	dom := dominators(f)
+	sdom := newSparseTree(f, dom)
+	b2l := make([]*loop, f.NumBlocks())
+	loops := make([]*loop, 0)
+
+	// Reducible-loop-nest-finding.
+	for _, b := range po {
+		if f.pass.debug > 3 {
+			fmt.Printf("loop finding (0) at %s\n", b)
+		}
+
+		var innermost *loop // innermost header reachable from this block
+
+		// IF any successor s of b is in a loop headed by h
+		// AND h dominates b
+		// THEN b is in the loop headed by h.
+		//
+		// Choose the first/innermost such h.
+		//
+		// IF s itself dominates b, the s is a loop header;
+		// and there may be more than one such s.
+		// Since there's at most 2 successors, the inner/outer ordering
+		// between them can be established with simple comparisons.
+		for _, bb := range b.Succs {
+			l := b2l[bb.ID]
+
+			if sdom.isAncestorEq(bb, b) { // Found a loop header
+				if l == nil {
+					l = &loop{header: bb, isInner: true}
+					loops = append(loops, l)
+					b2l[bb.ID] = l
+				}
+			} else { // Perhaps a loop header is inherited.
+				// is there any loop containing our successor whose
+				// header dominates b?
+				if l != nil && !sdom.isAncestorEq(l.header, b) {
+					l = l.nearestOuterLoop(sdom, b)
+				}
+			}
+
+			if l == nil || innermost == l {
+				continue
+			}
+
+			if innermost == nil {
+				innermost = l
+				continue
+			}
+
+			if sdom.isAncestor(innermost.header, l.header) {
+				sdom.outerinner(innermost, l)
+				innermost = l
+			} else if sdom.isAncestor(l.header, innermost.header) {
+				sdom.outerinner(l, innermost)
+			}
+		}
+
+		if innermost != nil {
+			b2l[b.ID] = innermost
+			innermost.nBlocks++
+		}
+	}
+	if f.pass.debug > 1 && len(loops) > 0 {
+		fmt.Printf("Loops in %s:\n", f.Name)
+		for _, l := range loops {
+			fmt.Printf("%s, b=", l.LongString())
+			for _, b := range f.Blocks {
+				if b2l[b.ID] == l {
+					fmt.Printf(" %s", b)
+				}
+			}
+			fmt.Print("\n")
+		}
+		fmt.Printf("Nonloop blocks in %s:", f.Name)
+		for _, b := range f.Blocks {
+			if b2l[b.ID] == nil {
+				fmt.Printf(" %s", b)
+			}
+		}
+		fmt.Print("\n")
+	}
+	return &loopnest{f, b2l, po, sdom, loops}
+}
diff --git a/test/opt_branchlikely.go b/test/opt_branchlikely.go
new file mode 100644
index 0000000000..99e914654f
--- /dev/null
+++ b/test/opt_branchlikely.go
@@ -0,0 +1,85 @@
+// +build amd64
+// errorcheck -0 -d=ssa/likelyadjust/debug=1
+
+// 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.
+
+// Test that branches have some prediction properties.
+package foo
+
+func f(x, y, z int) int {
+	a := 0
+	for i := 0; i < x; i++ { // ERROR "Branch prediction rule stay in loop"
+		for j := 0; j < y; j++ { // ERROR "Branch prediction rule stay in loop"
+			a += j
+		}
+		for k := 0; k < z; k++ { // ERROR "Branch prediction rule stay in loop"
+			a -= x + y + z
+		}
+	}
+	return a
+}
+
+func g(x, y, z int) int {
+	a := 0
+	if y == 0 { // ERROR "Branch prediction rule default < call"
+		y = g(y, z, x)
+	} else {
+		y++
+	}
+	if y == x { // ERROR "Branch prediction rule default < call"
+		y = g(y, z, x)
+	} else {
+	}
+	if y == 2 { // ERROR "Branch prediction rule default < call"
+		z++
+	} else {
+		y = g(z, x, y)
+	}
+	if y+z == 3 { // ERROR "Branch prediction rule call < exit"
+		println("ha ha")
+	} else {
+		panic("help help help")
+	}
+	if x != 0 { // ERROR "Branch prediction rule default < ret"
+		for i := 0; i < x; i++ { // ERROR "Branch prediction rule stay in loop"
+			if x == 4 { // ERROR "Branch prediction rule stay in loop"
+				return a
+			}
+			for j := 0; j < y; j++ { // ERROR "Branch prediction rule stay in loop"
+				for k := 0; k < z; k++ { // ERROR "Branch prediction rule stay in loop"
+					a -= j * i
+				}
+				a += j
+			}
+		}
+	}
+	return a
+}
+
+func h(x, y, z int) int {
+	a := 0
+	for i := 0; i < x; i++ { // ERROR "Branch prediction rule stay in loop"
+		for j := 0; j < y; j++ { // ERROR "Branch prediction rule stay in loop"
+			a += j
+			if i == j { // ERROR "Branch prediction rule stay in loop"
+				break
+			}
+			a *= j
+		}
+		for k := 0; k < z; k++ { // ERROR "Branch prediction rule stay in loop"
+			a -= k
+			if i == k {
+				continue
+			}
+			a *= k
+		}
+	}
+	if a > 0 { // ERROR "Branch prediction rule default < call"
+		a = g(x, y, z)
+	} else {
+		a = -a
+	}
+	return a
+}