os/exec: less allocs in the common case

When Stdin, Stdout, and Stderr are nil, there are no goroutines to keep
track of, so we don't need a channel.

And in startProcess, preallocate the right size for sysattr.Files,
saving a bit of space and a couple of slice growth allocs.

name            old time/op    new time/op    delta
ExecHostname-8     419µs ± 0%     417µs ± 1%    ~     (p=0.093 n=6+6)

name            old alloc/op   new alloc/op   delta
ExecHostname-8    6.40kB ± 0%    6.28kB ± 0%  -1.86%  (p=0.002 n=6+6)

name            old allocs/op  new allocs/op  delta
ExecHostname-8      34.0 ± 0%      31.0 ± 0%  -8.82%  (p=0.002 n=6+6)

Change-Id: Ic1d617f29e9c6431cdcadc7f9bb992750a6d5f48
Reviewed-on: https://go-review.googlesource.com/c/164801
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/src/os/exec/exec.go b/src/os/exec/exec.go
index 30fd64a4b1..424b49cf06 100644
--- a/src/os/exec/exec.go
+++ b/src/os/exec/exec.go
@@ -404,11 +404,14 @@ func (c *Cmd) Start() error {
 
 	c.closeDescriptors(c.closeAfterStart)
 
-	c.errch = make(chan error, len(c.goroutine))
-	for _, fn := range c.goroutine {
-		go func(fn func() error) {
-			c.errch <- fn()
-		}(fn)
+	// Don't allocate the channel unless there are goroutines to fire.
+	if len(c.goroutine) > 0 {
+		c.errch = make(chan error, len(c.goroutine))
+		for _, fn := range c.goroutine {
+			go func(fn func() error) {
+				c.errch <- fn()
+			}(fn)
+		}
 	}
 
 	if c.ctx != nil {