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>

src/os/exec/exec.go

@@ -404,12 +404,15 @@ func (c *Cmd) Start() error {
 
 	c.closeDescriptors(c.closeAfterStart)
 
+	// 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 {
 		c.waitDone = make(chan struct{})

src/os/exec_plan9.go

@@ -27,6 +27,7 @@ func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err e
 		Sys: attr.Sys,
 	}
 
+	sysattr.Files = make([]uintptr, 0, len(attr.Files))
 	for _, f := range attr.Files {
 		sysattr.Files = append(sysattr.Files, f.Fd())
 	}

src/os/exec_posix.go

@@ -40,6 +40,7 @@ func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err e
 	if sysattr.Env == nil {
 		sysattr.Env = Environ()
 	}
+	sysattr.Files = make([]uintptr, 0, len(attr.Files))
 	for _, f := range attr.Files {
 		sysattr.Files = append(sysattr.Files, f.Fd())
 	}