[dev.ssa] cmd/internal/ssa: SSA backend compiler skeleton

First pass adding code for SSA backend.  It is standalone for now.
I've included just a few passes to make the review size manageable -
I have more passes coming.

cmd/internal/ssa is the library containing the ssa compiler proper.

cmd/internal/ssa/ssac is a driver that loads an sexpr-based IR,
converts it to SSA form, and calls the above library.  It is essentially
throwaway code - it will disappear once the Go compiler calls
cmd/internal/ssa itself.  The .goir files in ssac/ are dumps of fibonacci
programs I made from a hacked-up compiler.  They are just for testing.

Change-Id: I5ee89356ec12c87cd916681097cd3c2cd591040c
Reviewed-on: https://go-review.googlesource.com/6681
Reviewed-by: Alan Donovan <adonovan@google.com>

src/cmd/internal/ssa/deadcode.go

@@ -0,0 +1,153 @@
+// Copyright 2015 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 "log"
+
+// deadcode removes dead code from f.
+func deadcode(f *Func) {
+
+	// Find all reachable basic blocks.
+	reachable := make([]bool, f.NumBlocks())
+	reachable[f.Entry.ID] = true
+	p := []*Block{f.Entry} // stack-like worklist
+	for len(p) > 0 {
+		// pop a reachable block
+		b := p[len(p)-1]
+		p = p[:len(p)-1]
+
+		// constant-fold conditionals
+		// TODO: rewrite rules instead?
+		if b.Kind == BlockIf && b.Control.Op == OpConstBool {
+			cond := b.Control.Aux.(bool)
+			var c *Block
+			if cond {
+				// then branch is always taken
+				c = b.Succs[1]
+			} else {
+				// else branch is always taken
+				c = b.Succs[0]
+				b.Succs[0] = b.Succs[1]
+			}
+			b.Succs[1] = nil // aid GC
+			b.Succs = b.Succs[:1]
+			removePredecessor(b, c)
+			b.Kind = BlockPlain
+			b.Control = nil
+		}
+
+		for _, c := range b.Succs {
+			if !reachable[c.ID] {
+				reachable[c.ID] = true
+				p = append(p, c) // push
+			}
+		}
+	}
+
+	// Find all live values
+	live := make([]bool, f.NumValues()) // flag to set for each live value
+	var q []*Value                      // stack-like worklist of unscanned values
+
+	// Starting set: all control values of reachable blocks are live.
+	for _, b := range f.Blocks {
+		if !reachable[b.ID] {
+			continue
+		}
+		if v := b.Control; v != nil && !live[v.ID] {
+			live[v.ID] = true
+			q = append(q, v)
+		}
+	}
+
+	// Compute transitive closure of live values.
+	for len(q) > 0 {
+		// pop a reachable value
+		v := q[len(q)-1]
+		q = q[:len(q)-1]
+		for _, x := range v.Args {
+			if !live[x.ID] {
+				live[x.ID] = true
+				q = append(q, x) // push
+			}
+		}
+	}
+
+	// Remove dead values from blocks' value list.  Return dead
+	// value ids to the allocator.
+	for _, b := range f.Blocks {
+		i := 0
+		for _, v := range b.Values {
+			if live[v.ID] {
+				b.Values[i] = v
+				i++
+			} else {
+				f.vid.put(v.ID)
+			}
+		}
+		for j := i; j < len(b.Values); j++ {
+			b.Values[j] = nil // aid GC
+		}
+		b.Values = b.Values[:i]
+	}
+
+	// Remove unreachable blocks.  Return dead block ids to allocator.
+	i := 0
+	for _, b := range f.Blocks {
+		if reachable[b.ID] {
+			f.Blocks[i] = b
+			i++
+		} else {
+			if len(b.Values) > 0 {
+				panic("live value in unreachable block")
+			}
+			f.bid.put(b.ID)
+		}
+	}
+	// zero remainder to help gc
+	for j := i; j < len(f.Blocks); j++ {
+		f.Blocks[j] = nil
+	}
+	f.Blocks = f.Blocks[:i]
+
+	// TODO: renumber Blocks and Values densely?
+}
+
+// There was an edge b->c.  It has been removed from b's successors.
+// Fix up c to handle that fact.
+func removePredecessor(b, c *Block) {
+	n := len(c.Preds) - 1
+	if n == 0 {
+		// c is now dead - don't bother working on it
+		if c.Preds[0] != b {
+			log.Panicf("%s.Preds[0]==%s, want %s", c, c.Preds[0], b)
+		}
+		return
+	}
+
+	// find index of b in c's predecessor list
+	var i int
+	for j, p := range c.Preds {
+		if p == b {
+			i = j
+			break
+		}
+	}
+
+	c.Preds[i] = c.Preds[n]
+	c.Preds[n] = nil // aid GC
+	c.Preds = c.Preds[:n]
+	// rewrite phi ops to match the new predecessor list
+	for _, v := range c.Values {
+		if v.Op != OpPhi {
+			continue
+		}
+		v.Args[i] = v.Args[n]
+		v.Args[n] = nil // aid GC
+		v.Args = v.Args[:n]
+		if n == 1 {
+			v.Op = OpCopy
+		}
+	}
+}