cmd/compile: eliminate more allocs in newblock

name       old allocs/op    new allocs/op    delta
Template         389k ± 0%        386k ± 0%  -0.84%        (p=0.000 n=10+10)
Unicode          323k ± 0%        323k ± 0%  -0.25%        (p=0.000 n=10+10)
GoTypes         1.17M ± 0%       1.16M ± 0%  -0.93%        (p=0.000 n=10+10)
Compiler        4.13M ± 0%       4.09M ± 0%  -1.05%        (p=0.000 n=10+10)

Change-Id: I6c00850d07511c2e65761c7373fc3df738499105
Reviewed-on: https://go-review.googlesource.com/32235
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/plive.go

@@ -109,16 +109,22 @@ func newblock(prog *obj.Prog) *BasicBlock {
 	if prog == nil {
 		Fatalf("newblock: prog cannot be nil")
 	}
-	result := new(BasicBlock)
+	// type block allows us to allocate a BasicBlock
+	// and its pred/succ slice together.
+	type block struct {
+		result BasicBlock
+		pred   [2]*BasicBlock
+		succ   [2]*BasicBlock
+	}
+	b := new(block)
+
+	result := &b.result
 	result.rpo = -1
 	result.mark = UNVISITED
 	result.first = prog
 	result.last = prog
-	// We want two 0-len slices with capacity 2.
-	// Carve them out of a single allocation.
-	blocks := make([]*BasicBlock, 4)
-	result.pred = blocks[0:][:0:2]
-	result.succ = blocks[2:][:0:2]
+	result.pred = b.pred[:0]
+	result.succ = b.succ[:0]
 	return result
 }