runtime: add a benchmark of Callers

We're about to make major changes to tracebacks. We have benchmarks of
stack copying, but not of PC buffer filling, so add some that we can
track through these changes.

For #54466.

Change-Id: I3ed61d75144ba03b61517cd9834eeb71c99d74df
Reviewed-on: https://go-review.googlesource.com/c/go/+/472956
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

Author: aclements

src/runtime/callers_test.go

@@ -339,3 +339,94 @@ func callerLine(t *testing.T, skip int) int {
 	}
 	return line
 }
+
+func BenchmarkCallers(b *testing.B) {
+	b.Run("cached", func(b *testing.B) {
+		// Very pcvalueCache-friendly, no inlining.
+		callersCached(b, 100)
+	})
+	b.Run("inlined", func(b *testing.B) {
+		// Some inlining, still pretty cache-friendly.
+		callersInlined(b, 100)
+	})
+	b.Run("no-cache", func(b *testing.B) {
+		// Cache-hostile
+		callersNoCache(b, 100)
+	})
+}
+
+func callersCached(b *testing.B, n int) int {
+	if n <= 0 {
+		pcs := make([]uintptr, 32)
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			runtime.Callers(0, pcs)
+		}
+		b.StopTimer()
+		return 0
+	}
+	return 1 + callersCached(b, n-1)
+}
+
+func callersInlined(b *testing.B, n int) int {
+	if n <= 0 {
+		pcs := make([]uintptr, 32)
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			runtime.Callers(0, pcs)
+		}
+		b.StopTimer()
+		return 0
+	}
+	return 1 + callersInlined1(b, n-1)
+}
+func callersInlined1(b *testing.B, n int) int { return callersInlined2(b, n) }
+func callersInlined2(b *testing.B, n int) int { return callersInlined3(b, n) }
+func callersInlined3(b *testing.B, n int) int { return callersInlined4(b, n) }
+func callersInlined4(b *testing.B, n int) int { return callersInlined(b, n) }
+
+func callersNoCache(b *testing.B, n int) int {
+	if n <= 0 {
+		pcs := make([]uintptr, 32)
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			runtime.Callers(0, pcs)
+		}
+		b.StopTimer()
+		return 0
+	}
+	switch n % 16 {
+	case 0:
+		return 1 + callersNoCache(b, n-1)
+	case 1:
+		return 1 + callersNoCache(b, n-1)
+	case 2:
+		return 1 + callersNoCache(b, n-1)
+	case 3:
+		return 1 + callersNoCache(b, n-1)
+	case 4:
+		return 1 + callersNoCache(b, n-1)
+	case 5:
+		return 1 + callersNoCache(b, n-1)
+	case 6:
+		return 1 + callersNoCache(b, n-1)
+	case 7:
+		return 1 + callersNoCache(b, n-1)
+	case 8:
+		return 1 + callersNoCache(b, n-1)
+	case 9:
+		return 1 + callersNoCache(b, n-1)
+	case 10:
+		return 1 + callersNoCache(b, n-1)
+	case 11:
+		return 1 + callersNoCache(b, n-1)
+	case 12:
+		return 1 + callersNoCache(b, n-1)
+	case 13:
+		return 1 + callersNoCache(b, n-1)
+	case 14:
+		return 1 + callersNoCache(b, n-1)
+	default:
+		return 1 + callersNoCache(b, n-1)
+	}
+}