cmd/compile: use slice extension idiom in LSym.Grow

name        old alloc/op      new alloc/op      delta
Template         35.0MB ± 0%       35.0MB ± 0%  -0.05%  (p=0.008 n=5+5)
Unicode          29.3MB ± 0%       29.3MB ± 0%    ~     (p=0.310 n=5+5)
GoTypes           115MB ± 0%        115MB ± 0%  -0.08%  (p=0.008 n=5+5)
Compiler          519MB ± 0%        519MB ± 0%  -0.08%  (p=0.008 n=5+5)
SSA              1.59GB ± 0%       1.59GB ± 0%  -0.05%  (p=0.008 n=5+5)
Flate            24.2MB ± 0%       24.2MB ± 0%  -0.06%  (p=0.008 n=5+5)
GoParser         28.2MB ± 0%       28.1MB ± 0%  -0.04%  (p=0.016 n=5+5)
Reflect          78.8MB ± 0%       78.7MB ± 0%  -0.10%  (p=0.008 n=5+5)
Tar              34.5MB ± 0%       34.4MB ± 0%  -0.07%  (p=0.008 n=5+5)
XML              43.3MB ± 0%       43.2MB ± 0%  -0.09%  (p=0.008 n=5+5)
[Geo mean]       77.5MB            77.4MB       -0.06%

name        old allocs/op     new allocs/op     delta
Template           330k ± 0%         329k ± 0%  -0.32%  (p=0.008 n=5+5)
Unicode            337k ± 0%         336k ± 0%  -0.10%  (p=0.008 n=5+5)
GoTypes           1.15M ± 0%        1.14M ± 0%  -0.34%  (p=0.008 n=5+5)
Compiler          4.78M ± 0%        4.77M ± 0%  -0.25%  (p=0.008 n=5+5)
SSA               12.9M ± 0%        12.9M ± 0%  -0.12%  (p=0.008 n=5+5)
Flate              221k ± 0%         220k ± 0%  -0.32%  (p=0.008 n=5+5)
GoParser           275k ± 0%         274k ± 0%  -0.34%  (p=0.008 n=5+5)
Reflect            944k ± 0%         940k ± 0%  -0.42%  (p=0.008 n=5+5)
Tar                323k ± 0%         322k ± 0%  -0.31%  (p=0.008 n=5+5)
XML                384k ± 0%         383k ± 0%  -0.26%  (p=0.008 n=5+5)
[Geo mean]         749k              747k       -0.28%


Updates #21266

Change-Id: I926ee3ba009c068239db70cdee8fdf85b5ee6bb4
Reviewed-on: https://go-review.googlesource.com/109816
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/data.go

@@ -46,12 +46,7 @@ func (s *LSym) Grow(lsiz int64) {
 	if len(s.P) >= siz {
 		return
 	}
-	// TODO(dfc) append cap-len at once, rather than
-	// one byte at a time.
-	for cap(s.P) < siz {
-		s.P = append(s.P[:cap(s.P)], 0)
-	}
-	s.P = s.P[:siz]
+	s.P = append(s.P, make([]byte, siz-len(s.P))...)
 }
 
 // GrowCap increases the capacity of s.P to c.