Add bench_inv tool

Makefile.am

@@ -50,10 +50,13 @@ libsecp256k1_la_LIBADD = libsecp256k1_common.la $(SECP_LIBS)
 
 noinst_PROGRAMS =
 if USE_BENCHMARK
-noinst_PROGRAMS += bench
+noinst_PROGRAMS += bench bench_inv
 bench_SOURCES = src/bench.c
 bench_LDADD = libsecp256k1.la $(SECP_LIBS)
 bench_LDFLAGS = -static
+bench_inv_SOURCES = src/bench_inv.c
+bench_inv_LDADD = libsecp256k1_common.la $(SECP_LIBS)
+bench_inv_LDFLAGS = -static
 endif
 
 if USE_TESTS

src/bench_inv.c

@@ -0,0 +1,42 @@
+// Copyright (c) 2014 Pieter Wuille
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <stdio.h>
+
+#include "util.h"
+#include "num_impl.h"
+#include "field_impl.h"
+#include "group_impl.h"
+#include "scalar_impl.h"
+
+int main() {
+    static const unsigned char init[32] = {
+        0x02, 0x03, 0x05, 0x07, 0x0b, 0x0d, 0x11, 0x13,
+        0x17, 0x1d, 0x1f, 0x25, 0x29, 0x2b, 0x2f, 0x35,
+        0x3b, 0x3d, 0x43, 0x47, 0x49, 0x4f, 0x53, 0x59,
+        0x61, 0x65, 0x67, 0x6b, 0x6d, 0x71, 0x7f, 0x83
+    };
+    static const unsigned char fini[32] = {
+        0xba, 0x28, 0x58, 0xd8, 0xaa, 0x11, 0xd6, 0xf2,
+        0xfa, 0xce, 0x50, 0xb1, 0x67, 0x19, 0xb1, 0xa6,
+        0xe0, 0xaa, 0x84, 0x53, 0xf6, 0x80, 0xfc, 0x23,
+        0x88, 0x3c, 0xd6, 0x74, 0x9f, 0x27, 0x09, 0x03
+    };
+    secp256k1_ge_start();
+    secp256k1_scalar_t base, x;
+    secp256k1_scalar_init(&base);
+    secp256k1_scalar_init(&x);
+    secp256k1_scalar_set_b32(&base, init, NULL);
+    secp256k1_scalar_set_b32(&x, init, NULL);
+    for (int i=0; i<1000000; i++) {
+        secp256k1_scalar_inverse(&x, &x);
+        secp256k1_scalar_add(&x, &x, &base);
+    }
+    unsigned char res[32];
+    secp256k1_scalar_get_b32(res, &x);
+    CHECK(memcmp(res, fini, 32) == 0);
+    secp256k1_scalar_free(&base);
+    secp256k1_scalar_free(&x);
+    return 0;
+}