On Thu, 10 Nov 2022, Philipp Tomsich wrote:
> Our ifcombine pass combines 2 single-bit tests into a single test of
> the form "(a & T) == T", requiring the same polarity (i.e., tests for
> bit set/cleared) for both bit-tests. However some applications test
> against two bits expecting one set and the other cleared.
>
> This adds support for the case "(a & T) == C" where T is a constant
> with 2 bits set and C has only one of those bits set.
>
> gcc/ChangeLog:
>
> * tree-ssa-ifcombine.cc (ifcombine_ifandif): Add support for
> combining two bit-tests that test for bits of different
> polarity.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/ssa-ifcombine-15.c: New test.
>
> Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu>
> ---
>
> .../gcc.dg/tree-ssa/ssa-ifcombine-15.c | 14 +++++
> gcc/tree-ssa-ifcombine.cc | 56 +++++++++++++++++++
> 2 files changed, 70 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ssa-ifcombine-15.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-ifcombine-15.c
> b/gcc/testsuite/gcc.dg/tree-ssa/ssa-ifcombine-15.c
> new file mode 100644
> index 00000000000..081faa39628
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-ifcombine-15.c
> @@ -0,0 +1,14 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O -fdump-tree-ifcombine-details-blocks" } */
> +
> +void sink();
> +
> +void reversed(unsigned char *a)
> +{
> + if (*a & 0x60)
> + if (!(*a & 0x02))
> + g();
> +}
> +
> +/* { dg-final { scan-tree-dump "optimizing double bit test" } } */
> +
> diff --git a/gcc/tree-ssa-ifcombine.cc b/gcc/tree-ssa-ifcombine.cc
> index cd6331f84db..ea49cc2bff1 100644
> --- a/gcc/tree-ssa-ifcombine.cc
> +++ b/gcc/tree-ssa-ifcombine.cc
> @@ -498,6 +498,62 @@ ifcombine_ifandif (basic_block inner_cond_bb, bool
> inner_inv,
> return true;
> }
>
> + /* See if we test polarity-reversed single bits of the same name in
> + both tests. In that case remove the outer test, merging both
> + else edges, and change the inner one to test for
> + name & (bit1 | bit2) == (bit2). */
> + else if ((recognize_single_bit_test (inner_cond, &name1, &bit1, !inner_inv)
> + && recognize_single_bit_test (outer_cond, &name2, &bit2, outer_inv)
> + && name1 == name2)
> + || (recognize_single_bit_test (inner_cond, &name2, &bit2, inner_inv)
> + && recognize_single_bit_test (outer_cond, &name1, &bit1,
> !outer_inv)
> + && name1 == name2))
Instead of explicitely testing for the combinations of !inv and inv can
you make recognize_single_bit_test output whether it recognizes a bit
set or bit clear pattern and then appropriately combine that with
inner_inv/outer_inv to the correct test? It seems we can handle all cases
just fine?
Thanks,
Richard.
> + {
> + tree t, t2, t3;
> +
> + /* Do it. */
> + gsi = gsi_for_stmt (inner_cond);
> + t = fold_build2 (LSHIFT_EXPR, TREE_TYPE (name1),
> + build_int_cst (TREE_TYPE (name1), 1), bit1);
> + t2 = fold_build2 (LSHIFT_EXPR, TREE_TYPE (name1),
> + build_int_cst (TREE_TYPE (name1), 1), bit2);
> + t = fold_build2 (BIT_IOR_EXPR, TREE_TYPE (name1), t, t2);
> + t = force_gimple_operand_gsi (&gsi, t, true, NULL_TREE,
> + true, GSI_SAME_STMT);
> + t3 = fold_build2 (BIT_AND_EXPR, TREE_TYPE (name1), name1, t);
> + t3 = force_gimple_operand_gsi (&gsi, t3, true, NULL_TREE,
> + true, GSI_SAME_STMT);
> + t = fold_build2 (result_inv ? NE_EXPR : EQ_EXPR,
> + boolean_type_node, t2, t3);
> + t = canonicalize_cond_expr_cond (t);
> + if (!t)
> + return false;
> + gimple_cond_set_condition_from_tree (inner_cond, t);
> + update_stmt (inner_cond);
> +
> + /* Leave CFG optimization to cfg_cleanup. */
> + gimple_cond_set_condition_from_tree (outer_cond,
> + outer_inv ? boolean_false_node : boolean_true_node);
> + update_stmt (outer_cond);
> +
> + update_profile_after_ifcombine (inner_cond_bb, outer_cond_bb);
> +
> + if (dump_file)
> + {
> + fprintf (dump_file, "optimizing double bit test to ");
> + print_generic_expr (dump_file, name1);
> + fprintf (dump_file, " & T == C\nwith temporary T = (1 << ");
> + print_generic_expr (dump_file, bit1);
> + fprintf (dump_file, ") | (1 << ");
> + print_generic_expr (dump_file, bit2);
> + fprintf (dump_file, ")\nand temporary C = (1 << ");
> + print_generic_expr (dump_file, bit2);
> + fprintf (dump_file, ")\n");
> + }
> +
> + return true;
> + }
> +
> /* See if we have two bit tests of the same name in both tests.
> In that case remove the outer test and change the inner one to
> test for name & (bits1 | bits2) != 0. */
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
