stupidgcm: add BenchmarkCCall

gocryptfs/internal/stupidgcm$ go test -bench .
goos: linux
goarch: amd64
pkg: github.com/rfjakob/gocryptfs/v2/internal/stupidgcm
cpu: Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz
BenchmarkCCall-4   	15864030	        78.60 ns/op
PASS
ok  	github.com/rfjakob/gocryptfs/v2/internal/stupidgcm	1.898s

internal/stupidgcm/common_test.go

@@ -208,3 +208,24 @@ func randBytes(n int) []byte {
 	}
 	return b
 }
+
+/*
+BenchmarkCCall benchmarks the overhead of calling from Go into C.
+Looks like things improved a bit compared to
+https://www.cockroachlabs.com/blog/the-cost-and-complexity-of-cgo/
+where they measured 171ns/op:
+
+$ go test -bench .
+goos: linux
+goarch: amd64
+pkg: github.com/rfjakob/gocryptfs/v2/internal/stupidgcm
+cpu: Intel(R) Core(TM) i5-3470 CPU @ 3.20GHz
+BenchmarkCCall-4   	13989364	        76.72 ns/op
+PASS
+ok  	github.com/rfjakob/gocryptfs/v2/internal/stupidgcm	1.735s
+*/
+func BenchmarkCCall(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		noopCFunction()
+	}
+}

internal/stupidgcm/openssl.go

@@ -108,3 +108,9 @@ func openSSLOpen(a *stupidAEADCommon, dst, iv, in, authData []byte) ([]byte, err
 	}
 	return append(dst, buf...), nil
 }
+
+// This functions exists to benchmark the C call overhead from Go.
+// See BenchmarkCCall for resuts.
+func noopCFunction() {
+	C.noop_c_function()
+}

internal/stupidgcm/openssl_aead.c

@@ -176,3 +176,8 @@ int openssl_aead_open(
 
     return plaintextLen;
 }
+
+// This functions exists to benchmark the C call overhead from Go.
+void noop_c_function(void) {
+    return;
+}

internal/stupidgcm/openssl_aead.h

@@ -27,3 +27,5 @@ int openssl_aead_open(
     const int ivLen,
     unsigned char* const plaintext,
     const int plaintextBufLen);
+
+void noop_c_function(void);