> Hi.
>
> The patch makes a loop upper bound estimation based on
> __builtin_expect_with_probability.
>
> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>
> The patch is pre-approved by Honza.
> Thanks,
> Martin
>
> gcc/ChangeLog:
>
> 2019-07-04 Martin Liska <mli...@suse.cz>
>
> * tree-ssa-loop-niter.c
> (get_upper_bound_based_on_builtin_expr_with_prob):
> New function.
> (estimate_numbers_of_iterations):
> Support __builtin_expect_with_probability for analysis
> of # of loop iterations.
perhaps we want to also document that builtin-expect can be used this way?
It owuld be also nice to have a testcase.
Honza
> ---
> gcc/tree-ssa-loop-niter.c | 66 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 66 insertions(+)
>
>
> diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
> index f51385900ed..5e75a412d93 100644
> --- a/gcc/tree-ssa-loop-niter.c
> +++ b/gcc/tree-ssa-loop-niter.c
> @@ -4183,6 +4183,55 @@ maybe_lower_iteration_bound (struct loop *loop)
> delete not_executed_last_iteration;
> }
>
> +/* Get expected upper bound for number of loop iterations for
> + BUILT_IN_EXPECT_WITH_PROBABILITY for a condition COND. */
> +
> +static tree
> +get_upper_bound_based_on_builtin_expr_with_prob (gcond *cond)
> +{
> + if (cond == NULL)
> + return NULL_TREE;
> +
> + tree lhs = gimple_cond_lhs (cond);
> + if (TREE_CODE (lhs) != SSA_NAME)
> + return NULL_TREE;
> +
> + gimple *stmt = SSA_NAME_DEF_STMT (gimple_cond_lhs (cond));
> + gcall *def = dyn_cast<gcall *> (stmt);
> + if (def == NULL)
> + return NULL_TREE;
> +
> + tree decl = gimple_call_fndecl (def);
> + if (!decl
> + || !fndecl_built_in_p (decl, BUILT_IN_EXPECT_WITH_PROBABILITY)
> + || gimple_call_num_args (stmt) != 3)
> + return NULL_TREE;
> +
> + tree c = gimple_call_arg (def, 1);
> + tree condt = TREE_TYPE (lhs);
> + tree res = fold_build2 (gimple_cond_code (cond),
> + condt, c,
> + gimple_cond_rhs (cond));
> + if (TREE_CODE (res) != INTEGER_CST)
> + return NULL_TREE;
> +
> +
> + tree prob = gimple_call_arg (def, 2);
> + tree t = TREE_TYPE (prob);
> + tree one
> + = build_real_from_int_cst (t,
> + integer_one_node);
> + if (integer_zerop (res))
> + prob = fold_build2 (MINUS_EXPR, t, one, prob);
> + tree r = fold_build2 (RDIV_EXPR, t, one, prob);
> + if (TREE_CODE (r) != REAL_CST)
> + return NULL_TREE;
> +
> + HOST_WIDE_INT probi
> + = real_to_integer (TREE_REAL_CST_PTR (r));
> + return build_int_cst (condt, probi);
> +}
> +
> /* Records estimates on numbers of iterations of LOOP. If USE_UNDEFINED_P
> is true also use estimates derived from undefined behavior. */
>
> @@ -4231,6 +4280,23 @@ estimate_numbers_of_iterations (struct loop *loop)
> likely_exit = single_likely_exit (loop);
> FOR_EACH_VEC_ELT (exits, i, ex)
> {
> + if (ex == likely_exit)
> + {
> + gimple *stmt = last_stmt (ex->src);
> + if (stmt != NULL)
> + {
> + gcond *cond = dyn_cast<gcond *> (stmt);
> + tree niter_bound
> + = get_upper_bound_based_on_builtin_expr_with_prob (cond);
> + if (niter_bound != NULL_TREE)
> + {
> + widest_int max = derive_constant_upper_bound (niter_bound);
> + record_estimate (loop, niter_bound, max, cond,
> + true, true, false);
> + }
> + }
> + }
> +
> if (!number_of_iterations_exit (loop, ex, &niter_desc, false, false))
> continue;
>
>
