Merge bitcoin-core/secp256k1#1445: bench: add --help option to bench_internal

1ddd76a bench: add --help option to bench_internal (Sebastian Falbesoner)

Pull request description:

  While coming up with commands for running the benchmarks for issue bitcoin-core/secp256k1#726 (comment), I noticed that in contrast to `bench{_ecmult}`, `bench_internal` doesn't have a help option yet and figured it would be nice to have one. A comparable past PR is bitcoin-core/secp256k1#1008. Benchmark categories appear in the same order as they are executed, the concrete benchmark names in parantheses per category are listed in alphabetical order.

ACKs for top commit:
  real-or-random:
    utACK 1ddd76a
  siv2r:
    ACK 1ddd76a, tested the `--help` option locally, and it works as expected.

Tree-SHA512: d117641a5f25a7cbf83881f3acceae99624528a0cbb2405efdbe1a3a2762b4d6b251392e954aaa32f6771069d31143743770fccafe198084c12258dedb0856fc

Author: real-or-random

src/bench_internal.c

@@ -17,6 +17,25 @@
 #include "ecmult_impl.h"
 #include "bench.h"
 
+static void help(int default_iters) {
+    printf("Benchmarks various internal routines.\n");
+    printf("\n");
+    printf("The default number of iterations for each benchmark is %d. This can be\n", default_iters);
+    printf("customized using the SECP256K1_BENCH_ITERS environment variable.\n");
+    printf("\n");
+    printf("Usage: ./bench_internal [args]\n");
+    printf("By default, all benchmarks will be run.\n");
+    printf("args:\n");
+    printf("    help       : display this help and exit\n");
+    printf("    scalar     : all scalar operations (add, half, inverse, mul, negate, split)\n");
+    printf("    field      : all field operations (half, inverse, issquare, mul, normalize, sqr, sqrt)\n");
+    printf("    group      : all group operations (add, double, to_affine)\n");
+    printf("    ecmult     : all point multiplication operations (ecmult_wnaf) \n");
+    printf("    hash       : all hash algorithms (hmac, rng6979, sha256)\n");
+    printf("    context    : all context object operations (context_create)\n");
+    printf("\n");
+}
+
 typedef struct {
     secp256k1_scalar scalar[2];
     secp256k1_fe fe[4];
@@ -365,8 +384,19 @@ static void bench_context(void* arg, int iters) {
 
 int main(int argc, char **argv) {
     bench_inv data;
-    int iters = get_iters(20000);
+    int default_iters = 20000;
+    int iters = get_iters(default_iters);
     int d = argc == 1; /* default */
+
+    if (argc > 1) {
+        if (have_flag(argc, argv, "-h")
+           || have_flag(argc, argv, "--help")
+           || have_flag(argc, argv, "help")) {
+            help(default_iters);
+            return 0;
+        }
+    }
+
     print_output_table_header_row();
 
     if (d || have_flag(argc, argv, "scalar") || have_flag(argc, argv, "half")) run_benchmark("scalar_half", bench_scalar_half, bench_setup, NULL, &data, 10, iters*100);