On Mon, Jul 31, 2023 at 10:17 PM Andrew Pinski via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> In some cases (usually dealing with bools only), there could be some
> statements
> left behind which are considered trivial dead.
> An example is:
> ```
> bool f(bool a, bool b)
> {
> if (!a && !b)
> return 0;
> if (!a && b)
> return 0;
> if (a && !b)
> return 0;
> return 1;
> }
> ```
> Where during phiopt2, the IR had:
> ```
> _3 = ~b_7(D);
> _4 = _3 & a_6(D);
> _4 != 0 ? 0 : 1
> ```
> match-and-simplify would transform that into:
> ```
> _11 = ~a_6(D);
> _12 = b_7(D) | _11;
> ```
> But phiopt would leave around the statements defining _4 and _3.
> This helps by marking the conditional's lhs and rhs to see if they are
> trivial dead.
>
> OK? Bootstrapped and tested on x86_64-linux-gnu.
OK.
> gcc/ChangeLog:
>
> * tree-ssa-phiopt.cc (match_simplify_replacement): Mark's cond
> statement's lhs and rhs to check if trivial dead.
> Rename inserted_exprs to exprs_maybe_dce; also move it so
> bitmap is not allocated if not needed.
> ---
> gcc/tree-ssa-phiopt.cc | 21 ++++++++++++++++-----
> 1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/gcc/tree-ssa-phiopt.cc b/gcc/tree-ssa-phiopt.cc
> index cb4e2da023d..ff36bb0119b 100644
> --- a/gcc/tree-ssa-phiopt.cc
> +++ b/gcc/tree-ssa-phiopt.cc
> @@ -767,7 +767,6 @@ match_simplify_replacement (basic_block cond_bb,
> basic_block middle_bb,
> tree result;
> gimple *stmt_to_move = NULL;
> gimple *stmt_to_move_alt = NULL;
> - auto_bitmap inserted_exprs;
> tree arg_true, arg_false;
>
> /* Special case A ? B : B as this will always simplify to B. */
> @@ -844,6 +843,18 @@ match_simplify_replacement (basic_block cond_bb,
> basic_block middle_bb,
> if (!result)
> return false;
>
> + auto_bitmap exprs_maybe_dce;
> +
> + /* Mark the cond statements' lhs/rhs as maybe dce. */
> + if (TREE_CODE (gimple_cond_lhs (stmt)) == SSA_NAME
> + && !SSA_NAME_IS_DEFAULT_DEF (gimple_cond_lhs (stmt)))
> + bitmap_set_bit (exprs_maybe_dce,
> + SSA_NAME_VERSION (gimple_cond_lhs (stmt)));
> + if (TREE_CODE (gimple_cond_rhs (stmt)) == SSA_NAME
> + && !SSA_NAME_IS_DEFAULT_DEF (gimple_cond_rhs (stmt)))
> + bitmap_set_bit (exprs_maybe_dce,
> + SSA_NAME_VERSION (gimple_cond_rhs (stmt)));
> +
> gsi = gsi_last_bb (cond_bb);
> /* Insert the sequence generated from gimple_simplify_phiopt. */
> if (seq)
> @@ -855,7 +866,7 @@ match_simplify_replacement (basic_block cond_bb,
> basic_block middle_bb,
> gimple *stmt = gsi_stmt (gsi1);
> tree name = gimple_get_lhs (stmt);
> if (name && TREE_CODE (name) == SSA_NAME)
> - bitmap_set_bit (inserted_exprs, SSA_NAME_VERSION (name));
> + bitmap_set_bit (exprs_maybe_dce, SSA_NAME_VERSION (name));
> }
> if (dump_file && (dump_flags & TDF_FOLDING))
> {
> @@ -867,10 +878,10 @@ match_simplify_replacement (basic_block cond_bb,
> basic_block middle_bb,
>
> /* If there was a statement to move, move it to right before
> the original conditional. */
> - move_stmt (stmt_to_move, &gsi, inserted_exprs);
> - move_stmt (stmt_to_move_alt, &gsi, inserted_exprs);
> + move_stmt (stmt_to_move, &gsi, exprs_maybe_dce);
> + move_stmt (stmt_to_move_alt, &gsi, exprs_maybe_dce);
>
> - replace_phi_edge_with_variable (cond_bb, e1, phi, result, inserted_exprs);
> + replace_phi_edge_with_variable (cond_bb, e1, phi, result, exprs_maybe_dce);
>
> /* Add Statistic here even though replace_phi_edge_with_variable already
> does it as we want to be able to count when match-simplify happens vs
> --
> 2.31.1
>
