[dev.ssa] cmd/compile: add backing store buffers for block.{Preds,Succs,Values}

Speeds up compilation by 6%.

Change-Id: Ibaad95710323ddbe13c1b0351843fe43a48d776e
Reviewed-on: https://go-review.googlesource.com/19080Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/block.go

@@ -53,6 +53,11 @@ type Block struct {
 
 	// After flagalloc, records whether flags are live at the end of the block.
 	FlagsLiveAtEnd bool
+
+	// Storage for Succs, Preds, and Values
+	succstorage [2]*Block
+	predstorage [4]*Block
+	valstorage  [8]*Value
 }
 
 //     kind           control    successors

src/cmd/compile/internal/ssa/check.go

@@ -219,7 +219,7 @@ func checkFunc(f *Func) {
 			f.Fatalf("control value for %s is missing: %v", b, b.Control)
 		}
 	}
-	for b := f.freeBlocks; b != nil; b = b.Aux.(*Block) {
+	for b := f.freeBlocks; b != nil; b = b.succstorage[0] {
 		if blockMark[b.ID] {
 			f.Fatalf("used block b%d in free list", b.ID)
 		}

src/cmd/compile/internal/ssa/deadcode.go

@@ -183,7 +183,7 @@ func deadcode(f *Func) {
 		b.Values = b.Values[:i]
 	}
 
-	// Remove unreachable blocks.  Return dead block ids to allocator.
+	// Remove unreachable blocks.  Return dead blocks to allocator.
 	i = 0
 	for _, b := range f.Blocks {
 		if reachable[b.ID] {
@@ -193,10 +193,6 @@ func deadcode(f *Func) {
 			if len(b.Values) > 0 {
 				b.Fatalf("live values in unreachable block %v: %v", b, b.Values)
 			}
-			b.Preds = nil
-			b.Succs = nil
-			b.Control = nil
-			b.Kind = BlockDead
 			f.freeBlock(b)
 		}
 	}
@@ -206,9 +202,6 @@ func deadcode(f *Func) {
 		tail[j] = nil
 	}
 	f.Blocks = f.Blocks[:i]
-
-	// TODO: renumber Blocks and Values densely?
-	// TODO: save dead Values and Blocks for reuse?  Or should we just let GC handle it?
 }
 
 // removePred removes the predecessor p from b's predecessor list.

src/cmd/compile/internal/ssa/func.go

@@ -30,7 +30,7 @@ type Func struct {
 	Names []LocalSlot
 
 	freeValues *Value // free Values linked by argstorage[0].  All other fields except ID are 0/nil.
-	freeBlocks *Block // free Blocks linked by Aux.(*Block).  All other fields except ID are 0/nil.
+	freeBlocks *Block // free Blocks linked by succstorage[0].  All other fields except ID are 0/nil.
 }
 
 // NumBlocks returns an integer larger than the id of any Block in the Func.
@@ -68,7 +68,7 @@ func (f *Func) newValue(op Op, t Type, b *Block, line int32) *Value {
 
 // freeValue frees a value.  It must no longer be referenced.
 func (f *Func) freeValue(v *Value) {
-	if v.Type == nil {
+	if v.Block == nil {
 		f.Fatalf("trying to free an already freed value")
 	}
 	// Clear everything but ID (which we reuse).
@@ -84,8 +84,8 @@ func (f *Func) NewBlock(kind BlockKind) *Block {
 	var b *Block
 	if f.freeBlocks != nil {
 		b = f.freeBlocks
-		f.freeBlocks = b.Aux.(*Block)
-		b.Aux = nil
+		f.freeBlocks = b.succstorage[0]
+		b.succstorage[0] = nil
 	} else {
 		ID := f.bid.get()
 		if int(ID) < len(f.Config.blocks) {
@@ -96,16 +96,22 @@ func (f *Func) NewBlock(kind BlockKind) *Block {
 	}
 	b.Kind = kind
 	b.Func = f
+	b.Preds = b.predstorage[:0]
+	b.Succs = b.succstorage[:0]
+	b.Values = b.valstorage[:0]
 	f.Blocks = append(f.Blocks, b)
 	return b
 }
 
 func (f *Func) freeBlock(b *Block) {
+	if b.Func == nil {
+		f.Fatalf("trying to free an already freed block")
+	}
 	// Clear everything but ID (which we reuse).
 	id := b.ID
 	*b = Block{}
 	b.ID = id
-	b.Aux = f.freeBlocks
+	b.succstorage[0] = f.freeBlocks
 	f.freeBlocks = b
 }
 

src/cmd/compile/internal/ssa/fuse.go

@@ -22,7 +22,12 @@ func fuse(f *Func) {
 		}
 
 		// replace b->c edge with preds(b) -> c
+		c.predstorage[0] = nil
+		if len(b.Preds) > len(b.predstorage) {
 			c.Preds = b.Preds
+		} else {
+			c.Preds = append(c.predstorage[:0], b.Preds...)
+		}
 		for _, p := range c.Preds {
 			for i, q := range p.Succs {
 				if q == b {