runtime: add a benchmark of FPCallers

This allows comparing frame pointer unwinding against the default
unwinder as shown below.

goos: linux
goarch: amd64
pkg: runtime
cpu: Intel(R) Xeon(R) Platinum 8375C CPU @ 2.90GHz
                    │ callers.txt │
                    │   sec/op    │
Callers/cached-32     1.254µ ± 0%
FPCallers/cached-32   24.99n ± 0%

For #16638

Change-Id: I4dd05f82254726152ef4a5d5beceab33641e9d2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/475795
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Felix Geisendörfer <felix.geisendoerfer@datadoghq.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/runtime/callers_test.go

@@ -430,3 +430,23 @@ func callersNoCache(b *testing.B, n int) int {
 		return 1 + callersNoCache(b, n-1)
 	}
 }
+
+func BenchmarkFPCallers(b *testing.B) {
+	b.Run("cached", func(b *testing.B) {
+		// Very pcvalueCache-friendly, no inlining.
+		fpCallersCached(b, 100)
+	})
+}
+
+func fpCallersCached(b *testing.B, n int) int {
+	if n <= 0 {
+		pcs := make([]uintptr, 32)
+		b.ResetTimer()
+		for i := 0; i < b.N; i++ {
+			runtime.FPCallers(0, pcs)
+		}
+		b.StopTimer()
+		return 0
+	}
+	return 1 + fpCallersCached(b, n-1)
+}

src/runtime/export_test.go

@@ -1727,3 +1727,9 @@ func FrameStartLine(f *Frame) int {
 func PersistentAlloc(n uintptr) unsafe.Pointer {
 	return persistentalloc(n, 0, &memstats.other_sys)
 }
+
+// FPCallers works like Callers and uses frame pointer unwinding to populate
+// pcBuf with the return addresses of the physical frames on the stack.
+func FPCallers(skip int, pcBuf []uintptr) int {
+	return fpTracebackPCs(unsafe.Pointer(getcallerfp()), skip, pcBuf)
+}