cmd/compile: combine slice allocations in newblock

name       old allocs/op    new allocs/op    delta
Template         394k ± 0%        391k ± 0%  -0.80%        (p=0.000 n=10+10)
Unicode          350k ± 0%        349k ± 0%  -0.27%        (p=0.000 n=10+10)
GoTypes         1.18M ± 0%       1.17M ± 0%  -0.92%        (p=0.000 n=10+10)
Compiler        4.18M ± 0%       4.14M ± 0%  -1.05%        (p=0.000 n=10+10)


Change-Id: I838a4e2110afe6496c535b9a0ec5aa882d63a707
Reviewed-on: https://go-review.googlesource.com/32223
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -114,8 +114,11 @@ func newblock(prog *obj.Prog) *BasicBlock {
 	result.mark = UNVISITED
 	result.first = prog
 	result.last = prog
-	result.pred = make([]*BasicBlock, 0, 2)
-	result.succ = make([]*BasicBlock, 0, 2)
+	// 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]
 	return result
 }