runtime: avoid a bit of unneeded work when MemProfileRate==1

Change-Id: I1dc355bcaeb0e5fb06a7fddc4cf5db596d22e0b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/236148
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Trust: Emmanuel Odeke <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: bradfitz

src/runtime/malloc.go

@@ -1221,6 +1221,13 @@ func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
 // distribution (exp(MemProfileRate)), so the best return value is a random
 // number taken from an exponential distribution whose mean is MemProfileRate.
 func nextSample() uintptr {
+	if MemProfileRate == 1 {
+		// Callers assign our return value to
+		// mcache.next_sample, but next_sample is not used
+		// when the rate is 1. So avoid the math below and
+		// just return something.
+		return 0
+	}
 	if GOOS == "plan9" {
 		// Plan 9 doesn't support floating point in note handler.
 		if g := getg(); g == g.m.gsignal {