cmd/compile: optimize postorder

name        old alloc/op      new alloc/op      delta
Template         37.1MB ± 0%       36.8MB ± 0%  -0.57%  (p=0.008 n=5+5)
Unicode          28.1MB ± 0%       28.1MB ± 0%  -0.07%  (p=0.008 n=5+5)
GoTypes           125MB ± 0%        124MB ± 0%  -0.61%  (p=0.008 n=5+5)
Compiler          571MB ± 0%        568MB ± 0%  -0.60%  (p=0.008 n=5+5)
SSA              1.88GB ± 0%       1.86GB ± 0%  -0.82%  (p=0.008 n=5+5)
Flate            22.9MB ± 0%       22.8MB ± 0%  -0.59%  (p=0.008 n=5+5)
GoParser         27.5MB ± 0%       27.3MB ± 0%  -0.53%  (p=0.008 n=5+5)
Reflect          79.8MB ± 0%       79.5MB ± 0%  -0.40%  (p=0.008 n=5+5)
Tar              34.9MB ± 0%       34.7MB ± 0%  -0.44%  (p=0.008 n=5+5)
XML              45.7MB ± 0%       45.4MB ± 0%  -0.58%  (p=0.008 n=5+5)
[Geo mean]       80.3MB            79.9MB       -0.52%

name        old allocs/op     new allocs/op     delta
Template           380k ± 0%         378k ± 0%  -0.57%  (p=0.008 n=5+5)
Unicode            340k ± 0%         340k ± 0%  -0.08%  (p=0.008 n=5+5)
GoTypes           1.36M ± 0%        1.36M ± 0%  -0.44%  (p=0.008 n=5+5)
Compiler          5.52M ± 0%        5.49M ± 0%  -0.45%  (p=0.008 n=5+5)
SSA               17.6M ± 0%        17.5M ± 0%  -0.42%  (p=0.008 n=5+5)
Flate              235k ± 0%         234k ± 0%  -0.65%  (p=0.008 n=5+5)
GoParser           302k ± 0%         300k ± 0%  -0.70%  (p=0.008 n=5+5)
Reflect            982k ± 0%         978k ± 0%  -0.40%  (p=0.008 n=5+5)
Tar                353k ± 0%         351k ± 0%  -0.53%  (p=0.008 n=5+5)
XML                437k ± 0%         435k ± 0%  -0.48%  (p=0.008 n=5+5)
[Geo mean]         844k              840k       -0.47%

Updates #27739

Change-Id: I5d533013270cbbd7c0bad1b43da96c8499be76f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/177917
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -20,7 +20,7 @@ const (
 // postorder computes a postorder traversal ordering for the
 // basic blocks in f. Unreachable blocks will not appear.
 func postorder(f *Func) []*Block {
-	return postorderWithNumbering(f, []int32{})
+	return postorderWithNumbering(f, nil)
 }
 
 type blockAndIndex struct {
@@ -34,7 +34,7 @@ func postorderWithNumbering(f *Func, ponums []int32) []*Block {
 	mark := make([]markKind, f.NumBlocks())
 
 	// result ordering
-	var order []*Block
+	order := make([]*Block, 0, len(f.Blocks))
 
 	// stack of blocks and next child to visit
 	// A constant bound allows this to be stack-allocated. 32 is