add a trivial benchmark function

src/tests/sha256_bench.vtc

+# looks like -*- vcl -*-
+
+varnishtest "SHA256 bench"
+
+# hmac object
+varnish v1 -vcl {
+	import blobdigest from "${vmod_topbuild}/src/.libs/libvmod_blobdigest.so";
+	import blob;
+	backend b { .host = "${bad_ip}"; }
+
+	sub vcl_init {
+		# RFC4231 test cases
+		new k1 = blob.blob(HEX,
+		                    "0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b");
+		new rfc4231t1 = blobdigest.hmac(SHA256, k1.get());
+
+		new k6 = blob.blob(HEX,
+"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");
+	}
+
+	sub vcl_recv {
+		return(synth(200));
+	}
+
+	sub vcl_synth {
+		set resp.http.dur = rfc4231t1.hmac_bench(1000000, k6.get());
+	}
+} -start
+
+client c1 {
+	txreq
+	rxresp
+	expect resp.status == 200
+} -run

src/vmod_blobdigest.c

@@ -34,6 +34,7 @@
 #include "vcl.h"
 #include "vas.h"
 #include "vsb.h"
+#include "vtim.h"
 
 #include "vcc_if.h"
 #include "vmod_blobdigest.h"
@@ -565,6 +566,27 @@ vmod_hmac_hmac(VRT_CTX, struct vmod_blobdigest_hmac *h, VCL_BLOB msg)
 			  h->vcl_name, "hmac");
 }
 
+VCL_DURATION
+vmod_hmac_hmac_bench(VRT_CTX, struct vmod_blobdigest_hmac *h, VCL_INT n,
+		     VCL_BLOB msg)
+{
+	double t0, t1;
+	uintptr_t snap;
+
+	if (n <= 0) {
+		ERR(ctx, "number of rounds must be greater than zero");
+		return (-1);
+	}
+	snap = WS_Snapshot(ctx->ws);
+	t0 = VTIM_mono();
+	while (n--) {
+		WS_Reset(ctx->ws, snap);
+		(void) vmod_hmac_hmac(ctx, h, msg);
+	}
+	t1 = VTIM_mono();
+	return (t1 - t0);
+}
+
 VCL_VOID
 vmod_hmac__fini(struct vmod_blobdigest_hmac **hmacp)
 {

src/vmod_blobdigest.vcc

@@ -410,6 +410,10 @@ Example::
 		return(synth(401));
 	}
 
+$Method DURATION .hmac_bench(INT n, BLOB msg)
+
+Run `n` rounds of ``.hmac`` and return the net total duration.
+
 $Function BLOB hmacf(ENUM {MD5, SHA1, SHA224, SHA256, SHA384, SHA512, SHA3_224,
                            SHA3_256, SHA3_384, SHA3_512} hash, BLOB key,
 		     BLOB msg)