compilebench: handle missing MemStats more gracefully

Reporting MemStats requires the legacy profile format (see golang/go#18641).
This CL detects when it couldn't get the MemStats from the profile and
reports this, rather than reporting 0 allocs.

Change-Id: Ib621ad975290cf05835fafa81e8e47762d82a519
Reviewed-on: https://go-review.googlesource.com/c/tools/+/175802
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Cherry Zhang <[email protected]>

Author: aclements

cmd/compilebench/main.go

@@ -437,6 +437,7 @@ func runBuildCmd(name string, count int, dir, tool string, args []string) error
 	}
 	end := time.Now()
 
+	haveAllocs := false
 	var allocs, allocbytes int64
 	if *flagAlloc || *flagMemprofile != "" {
 		out, err := ioutil.ReadFile(dir + "/_compilebench_.memprof")
@@ -452,13 +453,17 @@ func runBuildCmd(name string, count int, dir, tool string, args []string) error
 			if err != nil {
 				continue
 			}
+			haveAllocs = true
 			switch f[1] {
 			case "TotalAlloc":
 				allocbytes = val
 			case "Mallocs":
 				allocs = val
 			}
 		}
+		if !haveAllocs {
+			log.Println("missing stats in memprof (golang.org/issue/18641)")
+		}
 
 		if *flagMemprofile != "" {
 			outpath := *flagMemprofile
@@ -491,7 +496,7 @@ func runBuildCmd(name string, count int, dir, tool string, args []string) error
 	userns := cmd.ProcessState.UserTime().Nanoseconds()
 
 	fmt.Printf("%s 1 %d ns/op %d user-ns/op", name, wallns, userns)
-	if *flagAlloc {
+	if haveAllocs {
 		fmt.Printf(" %d B/op %d allocs/op", allocbytes, allocs)
 	}