[PATCH v1 02/23] benchtests: Add random benchmark in bench-strchr.c
H.J. Lu
hjl.tools@gmail.com
Thu Mar 24 18:44:15 GMT 2022
On Wed, Mar 23, 2022 at 2:58 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> Add benchmark that randomizes whether return should be NULL or pointer
> to CHAR. The rationale is on many architectures there is a choice
> between a predicate execution option (i.e cmovcc on x86) or a branch.
>
> On x86 the results for cmovcc vs branch are something along the lines
> of the following:
>
> perc-zero, Br On Result, Time Br / Time cmov
> 0.10, 1, ,0.983
> 0.10, 0, ,1.246
> 0.25, 1, ,1.035
> 0.25, 0, ,1.49
> 0.33, 1, ,1.016
> 0.33, 0, ,1.579
> 0.50, 1, ,1.228
> 0.50, 0, ,1.739
> 0.66, 1, ,1.039
> 0.66, 0, ,1.764
> 0.75, 1, ,0.996
> 0.75, 0, ,1.642
> 0.90, 1, ,1.071
> 0.90, 0, ,1.409
> 1.00, 1, ,0.937
> 1.00, 0, ,0.999
> ---
> benchtests/bench-strchr.c | 143 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 143 insertions(+)
>
> diff --git a/benchtests/bench-strchr.c b/benchtests/bench-strchr.c
> index 203900d4ad..54640bde7e 100644
> --- a/benchtests/bench-strchr.c
> +++ b/benchtests/bench-strchr.c
> @@ -53,6 +53,11 @@
> # define SMALL_CHAR 851
> #endif /* WIDE */
>
> +#ifdef USE_FOR_STRCHRNUL
> +# define DO_RAND_TEST(...)
> +#else
> +# define DO_RAND_TEST(...) do_rand_test(__VA_ARGS__)
> +#endif
> #ifdef USE_FOR_STRCHRNUL
> # define NULLRET(endptr) endptr
> #else
> @@ -74,6 +79,133 @@ simple_STRCHR (const CHAR *s, int c)
> IMPL (simple_STRCHR, 0)
> IMPL (STRCHR, 1)
>
> +#ifndef USE_FOR_STRCHRNUL
> +/* Random benchmarks for strchr (if return is CHAR or NULL). The
> + rational for the benchmark is returning null/char can be done with
> + predicate execution (i.e cmovcc on x86) or a branch. */
> +
> +
> +/* Large enough that full history can't be stored in BHT. */
> +#define NUM_SEARCH_CHARS 2048
> +
> +/* Expectation is usecases of strchr check the return. Otherwise
> + strchrnul would almost always be better. Since there is another
> + branch coming we want to test the case where a potential branch in
> + strchr can be used to skip a later mispredict because of the
> + relationship between the two branches. */
> +static void __attribute__ ((noinline, noclone))
> +do_one_rand_plus_branch_test (json_ctx_t *json_ctx, impl_t *impl,
> + const CHAR *s, const CHAR *c)
> +{
> + size_t i, iters = INNER_LOOP_ITERS_LARGE;
> + int must_execute = 0;
> + timing_t start, stop, cur;
> + TIMING_NOW (start);
> + for (i = 0; i < iters; ++i)
> + {
> + if (CALL (impl, s, c[i % NUM_SEARCH_CHARS]))
> + {
> + /* We just need something that will force compiler to emit
> + a branch instead of conditional execution. */
> + ++must_execute;
> + asm volatile("" : : :);
> + }
> + }
> + TIMING_NOW (stop);
> +
> + TIMING_DIFF (cur, start, stop);
> +
> + json_element_double (json_ctx, (double)cur / (double)iters);
> +}
> +
> +static void __attribute__ ((noinline, noclone))
> +do_one_rand_test (json_ctx_t *json_ctx, impl_t *impl, const CHAR *s,
> + const CHAR *c)
> +{
> + size_t i, iters = INNER_LOOP_ITERS_LARGE;
> + timing_t start, stop, cur;
> + TIMING_NOW (start);
> + for (i = 0; i < iters; ++i)
> + {
> + CALL (impl, s, c[i % NUM_SEARCH_CHARS]);
> + }
> + TIMING_NOW (stop);
> +
> + TIMING_DIFF (cur, start, stop);
> +
> + json_element_double (json_ctx, (double)cur / (double)iters);
> +}
> +
> +static void
> +do_rand_test (json_ctx_t *json_ctx, size_t align, size_t pos, size_t len,
> + float perc_zero)
> +{
> + size_t i;
> + int perc_zero_int;
> + CHAR *buf = (CHAR *)buf1;
> + CHAR *c = (CHAR *)buf2;
> + align &= 127;
> + if ((align + len) * sizeof (CHAR) >= page_size)
> + return;
> +
> + /* Test is only interesting if we can hit both cases. */
> + if (pos >= len)
> + return;
> +
> + /* Segfault if we run the test. */
> + if (NUM_SEARCH_CHARS * sizeof (CHAR) > page_size)
> + return;
> +
> + for (i = 0; i < len; ++i)
> + {
> + buf[align + i] = 2;
> + }
> + buf[align + len] = 0;
> + buf[align + pos] = 1;
> +
> + perc_zero_int = perc_zero * RAND_MAX;
> + for (i = 0; i < NUM_SEARCH_CHARS; ++i)
> + {
> + if (rand () > perc_zero_int)
> + c[i] = 0;
> + else
> + c[i] = 1;
> + }
> + {
> + json_element_object_begin (json_ctx);
> + json_attr_uint (json_ctx, "rand", 1);
> + json_attr_uint (json_ctx, "branch", 1);
> + json_attr_double (json_ctx, "perc-zero", perc_zero);
> + json_attr_uint (json_ctx, "length", len);
> + json_attr_uint (json_ctx, "pos", pos);
> + json_attr_uint (json_ctx, "alignment", align);
> + json_array_begin (json_ctx, "timings");
> +
> + FOR_EACH_IMPL (impl, 0)
> + do_one_rand_plus_branch_test (json_ctx, impl, buf + align, c);
> +
> + json_array_end (json_ctx);
> + json_element_object_end (json_ctx);
> + }
> + {
> + json_element_object_begin (json_ctx);
> + json_attr_uint (json_ctx, "rand", 1);
> + json_attr_uint (json_ctx, "branch", 0);
> + json_attr_double (json_ctx, "perc-zero", perc_zero);
> + json_attr_uint (json_ctx, "length", len);
> + json_attr_uint (json_ctx, "pos", pos);
> + json_attr_uint (json_ctx, "alignment", align);
> + json_array_begin (json_ctx, "timings");
> +
> + FOR_EACH_IMPL (impl, 0)
> + do_one_rand_test (json_ctx, impl, buf + align, c);
> +
> + json_array_end (json_ctx);
> + json_element_object_end (json_ctx);
> + }
> +}
> +#endif
> +
> static void
> do_one_test (json_ctx_t *json_ctx, impl_t *impl, const CHAR *s, int c,
> const CHAR *exp_res)
> @@ -136,6 +268,7 @@ do_test (json_ctx_t *json_ctx, size_t align, size_t pos, size_t len,
> result = NULLRET (buf + align + len);
>
> json_element_object_begin (json_ctx);
> + json_attr_uint (json_ctx, "rand", 0);
> json_attr_uint (json_ctx, "length", len);
> json_attr_uint (json_ctx, "pos", pos);
> json_attr_uint (json_ctx, "seek_char", seek_char);
> @@ -234,6 +367,16 @@ test_main (void)
> do_test (&json_ctx, 0, i, i + 1, 0, BIG_CHAR);
> }
>
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.0);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.1);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.25);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.33);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.5);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.66);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.75);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 0.9);
> + DO_RAND_TEST(&json_ctx, 0, 15, 16, 1.0);
> +
> json_array_end (&json_ctx);
> json_attr_object_end (&json_ctx);
> json_attr_object_end (&json_ctx);
> --
> 2.25.1
>
LGTM.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Thanks.
--
H.J.
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