[dev.ssa] cmd/internal/ssa: add deadstore pass

Eliminate dead stores.  Dead stores are those which are
unconditionally followed by another store to the same location, with
no intervening load.

Just a simple intra-block implementation for now.

Change-Id: I2bf54e3a342608fc4e01edbe1b429e83f24764ab
Reviewed-on: https://go-review.googlesource.com/10386
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

1 files changed, 103 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/ssa/deadstore.go b/src/cmd/compile/internal/ssa/deadstore.go
new file mode 100644
index 0000000000..b02b35460a
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/deadstore.go
@@ -0,0 +1,103 @@
+// 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"
+
+// dse does dead-store elimination on the Function.
+// Dead stores are those which are unconditionally followed by
+// another store to the same location, with no intervening load.
+// This implementation only works within a basic block.  TODO: use something more global.
+func dse(f *Func) {
+	var stores []*Value
+	loadUse := newSparseSet(f.NumValues())
+	storeUse := newSparseSet(f.NumValues())
+	shadowed := newSparseSet(f.NumValues())
+	for _, b := range f.Blocks {
+		// Find all the stores in this block.  Categorize their uses:
+		//  loadUse contains stores which are used by a subsequent load.
+		//  storeUse contains stores which are used by a subsequent store.
+		loadUse.clear()
+		storeUse.clear()
+		stores = stores[:0]
+		for _, v := range b.Values {
+			if v.Op == OpPhi {
+				// Ignore phis - they will always be first and can't be eliminated
+				continue
+			}
+			if v.Type.IsMemory() {
+				stores = append(stores, v)
+				for _, a := range v.Args {
+					if a.Block == b && a.Type.IsMemory() {
+						storeUse.add(a.ID)
+						if v.Op != OpStore {
+							// CALL, DUFFCOPY, etc. are both
+							// reads and writes.
+							loadUse.add(a.ID)
+						}
+					}
+				}
+			} else {
+				for _, a := range v.Args {
+					if a.Block == b && a.Type.IsMemory() {
+						loadUse.add(a.ID)
+					}
+				}
+			}
+		}
+		if len(stores) == 0 {
+			continue
+		}
+
+		// find last store in the block
+		var last *Value
+		for _, v := range stores {
+			if storeUse.contains(v.ID) {
+				continue
+			}
+			if last != nil {
+				log.Fatalf("two final stores - simultaneous live stores", last, v)
+			}
+			last = v
+		}
+		if last == nil {
+			log.Fatalf("no last store found - cycle?")
+		}
+
+		// Walk backwards looking for dead stores.  Keep track of shadowed addresses.
+		// An "address" is an SSA Value which encodes both the address and size of
+		// the write.  This code will not remove dead stores to the same address
+		// of different types.
+		shadowed.clear()
+		v := last
+
+	walkloop:
+		if loadUse.contains(v.ID) {
+			// Someone might be reading this memory state.
+			// Clear all shadowed addresses.
+			shadowed.clear()
+		}
+		if v.Op == OpStore {
+			if shadowed.contains(v.Args[0].ID) {
+				// Modify store into a copy
+				v.Op = OpCopy
+				v.Aux = nil
+				v.SetArgs1(v.Args[2])
+			} else {
+				shadowed.add(v.Args[0].ID)
+			}
+		}
+		// walk to previous store
+		if v.Op == OpPhi {
+			continue // At start of block.  Move on to next block.
+		}
+		for _, a := range v.Args {
+			if a.Block == b && a.Type.IsMemory() {
+				v = a
+				goto walkloop
+			}
+		}
+	}
+}