Re: [patch tree-optimization]: Improve handling of conditional-branches on targets with high branch costs

[Richard Guenther <richard dot guenther at gmail dot com>]

On Fri, Oct 7, 2011 at 11:36 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
> Hello,
>
> this is the updated version with the suggestion
>
> 2011/10/7 Richard Guenther <richard.guenther@gmail.com>:
>> On Thu, Oct 6, 2011 at 4:25 PM, Kai Tietz <ktietz@redhat.com> wrote:
>>> + ? ? ?&& ((TREE_CODE_CLASS (TREE_CODE (arg1)) != tcc_comparison
>>> + ? ? ? ? ? && TREE_CODE (arg1) != TRUTH_NOT_EXPR
>>> + ? ? ? ? ? && simple_operand_p (arg1))
>>
>> As I said previously simple_operand_p already rejects covers
>> comparisons and TRUTH_NOT_EXPR. ?Also arg1 had better
>> TREE_SIDE_EFFECTS set if the comparison might trap, as
>> it might just be hidden in something more complicated - so
>> the simple check isn't enough anyway (and if simple_operand_p
>> would cover it, the check would be better placed there).
>
> I reworked simple_operand_p so that it does this special-casing and additionally
> also checks for trapping.
>
>>> + ? ? ?if (TREE_CODE (arg0) == code
>>> + ? ? ? ? ?&& !TREE_SIDE_EFFECTS (TREE_OPERAND (arg0, 1))
>>> + ? ? ? ? ?&& simple_operand_p (TREE_OPERAND (arg0, 1)))
>>> + ? ? ? {
>>> + ? ? ? ? tem = build2_loc (loc,
>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? (code == TRUTH_ANDIF_EXPR ? TRUTH_AND_EXPR
>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? : TRUTH_OR_EXPR),
>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? type, TREE_OPERAND (arg0, 1), arg1);
>>> + ? ? ? ? return build2_loc (loc, code, type, TREE_OPERAND (arg0, 0),
>>> + ? ? ? ? ? ? ? ? ? ? ? ? ? ?tem);
>>
>> All trees should be folded, don't use plain build without a good reason.
>
> Ok, done
>
>>> + ? ? ? }
>>> + ? ? ?/* Convert X TRUTH-ANDORIF Y to X TRUTH-ANDOR Y, if X and Y
>>> + ? ? ? ?are simple operands and have no side-effects. ?*/
>>> + ? ? ?if (simple_operand_p (arg0)
>>> + ? ? ? ? ?&& !TREE_SIDE_EFFECTS (arg0))
>>
>> Again, the checks you do for arg0 do not match those for arg1. ?OTOH
>> it doesn't matter whether arg0 is simple or not or has side-effects or
>> not for this transformation, so why check it at all?
>
> It is required. ?For left-hand operand, if it isn't a logical
> and/or/xor, we need to check for side-effects (and for trapping). ?I
> see that calling of simple_operand_p is wrong here, as it rejects too
> much. ?Nevertheless the check for side-effects is necessary for having
> valid sequence-points. ?Without that checking a simple test
>
> int getter (void);
>
> int foo (void)
> {
> ?int c, r = 0;
> ?while ((c = getter ()) != '"' && c >= 0)
> ? ?r +=c;
> ?return r;
> }
>
> would give a warning about sequence-points. ?As left-hand operand has
> side-effects, but right-hand not. ?If we would combine it as AND, the
> operands are exchange-able. ?So right-hand operand needs to be another
> ANDIF expression instead.
> Same apply on trapping.
>
>> In fold_truthop we still have the same (albeit more restricted transform),
>> but guarded with
>>
>> ?if (BRANCH_COST (optimize_function_for_speed_p (cfun),
>> ? ? ? ? ? ? ? ? ? false) >= 2
>>
>> too. ?Why not here? ?Please delete redundant code in fold_truthop.
> Well, in general this is the default definition of
> LOGICAL_OP_NON_SHORT_CIRCUIT, so I missed that. ?As for some targets
> the macro LOGICAL_OP_NON_SHORT_CIRCUIT might be defined differently,
> it might make sense to check for BRANCH_COST again.
>
>> It's also odd that this is only called from fold_truth_andor but has
>> a more generic name, so maybe rename it to fold_truth_andor_1 on the way.
>
> I renamed it.
>
>> Richard.
>
> ChangeLog
>
> 2011-10-07 ?Kai Tietz ?<ktietz@redhat.com>
>
> ? ? ? ?* fold-const.c (simple_operand_p): Make argument non-const
> ? ? ? ?and add floating-point trapping check, and special cases for
> ? ? ? ?comparisons, and logical-not's.
> ? ? ? ?(fold_truthop): Rename to
> ? ? ? ?(fold_truth_andor_1): function name.
> ? ? ? ?Additionally remove here TRUTH-AND|OR_EXPR generation.
> ? ? ? ?(fold_truth_andor): Handle branching at one place.
>
> Bootstrapped and regression-tested for all languages plus Ada and
> Obj-C++ on x86_64-pc-linux-gnu.
> Ok for apply?
>
> Regards,
> Kai
>
> Index: gcc/gcc/fold-const.c
> ===================================================================
> --- gcc.orig/gcc/fold-const.c
> +++ gcc/gcc/fold-const.c
> @@ -111,14 +111,13 @@ static tree decode_field_reference (loca
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?tree *, tree *);
> ?static int all_ones_mask_p (const_tree, int);
> ?static tree sign_bit_p (tree, const_tree);
> -static int simple_operand_p (const_tree);
> +static int simple_operand_p (tree);
> ?static tree range_binop (enum tree_code, tree, tree, int, tree, int);
> ?static tree range_predecessor (tree);
> ?static tree range_successor (tree);
> ?static tree fold_range_test (location_t, enum tree_code, tree, tree, tree);
> ?static tree fold_cond_expr_with_comparison (location_t, tree, tree,
> tree, tree);
> ?static tree unextend (tree, int, int, tree);
> -static tree fold_truthop (location_t, enum tree_code, tree, tree, tree);
> ?static tree optimize_minmax_comparison (location_t, enum tree_code,
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?tree, tree, tree);
> ?static tree extract_muldiv (tree, tree, enum tree_code, tree, bool *);
> @@ -3500,7 +3499,7 @@ optimize_bit_field_compare (location_t l
> ? return lhs;
> ?}
>
> -/* Subroutine for fold_truthop: decode a field reference.
> +/* Subroutine for fold_truth_andor_1: decode a field reference.
>
> ? ?If EXP is a comparison reference, we return the innermost reference.
>
> @@ -3668,17 +3667,43 @@ sign_bit_p (tree exp, const_tree val)
> ? return NULL_TREE;
> ?}
>
> -/* Subroutine for fold_truthop: determine if an operand is simple enough
> +/* Subroutine for fold_truth_andor_1: determine if an operand is simple enough
> ? ?to be evaluated unconditionally. ?*/
>
> ?static int
> -simple_operand_p (const_tree exp)
> +simple_operand_p (tree exp)
> ?{
> + ?enum tree_code code;
> ? /* Strip any conversions that don't change the machine mode. ?*/
> ? STRIP_NOPS (exp);
>
> + ?code = TREE_CODE (exp);
> +
> + ?/* Handle some trivials ?$$$$ */
> + ?if (TREE_CODE_CLASS (code) == tcc_comparison)
> + ? ?return (tree_could_trap_p (exp)
> + ? ? ? ? ? && simple_operand_p (TREE_OPERAND (exp, 0))
> + ? ? ? ? ? && simple_operand_p (TREE_OPERAND (exp, 1)));

Clearly wrong.  And what's $$$$ supposed to be?

> +
> + ?if (FLOAT_TYPE_P (TREE_TYPE (exp))
> + ? ? ?&& tree_could_trap_p (exp))
> + ? ?return false;
> +
> + ?switch (code)
> + ? ?{
> + ? ?case SSA_NAME:
> + ? ? ?return true;
> + ? ?case TRUTH_NOT_EXPR:
> + ? ? ?return simple_operand_p (TREE_OPERAND (exp, 0));
> + ? ?case BIT_NOT_EXPR:
> + ? ? ?if (TREE_CODE (TREE_TYPE (exp)) != BOOLEAN_TYPE)
> + ? ? ? ?return false;
> + ? ? ?return simple_operand_p (TREE_OPERAND (exp, 0));
> + ? ?default:
> + ? ? ?break;
> + ? ?}
> +

You add a lot of cases here without a good reason.  Why in this patch?
Simply removing the tcc_comparison checks would have been enough ...

> ? return (CONSTANT_CLASS_P (exp)
> - ? ? ? ? || TREE_CODE (exp) == SSA_NAME
> ? ? ? ? ?|| (DECL_P (exp)
> ? ? ? ? ? ? ?&& ! TREE_ADDRESSABLE (exp)
> ? ? ? ? ? ? ?&& ! TREE_THIS_VOLATILE (exp)
> @@ -4888,7 +4913,7 @@ fold_range_test (location_t loc, enum tr
> ? return 0;
> ?}
>
> -/* Subroutine for fold_truthop: C is an INTEGER_CST interpreted as a P
> +/* Subroutine for fold_truth_andor_1: C is an INTEGER_CST interpreted as a P

fold_truth_andor

> ? ?bit value. ?Arrange things so the extra bits will be set to zero if and
> ? ?only if C is signed-extended to its full width. ?If MASK is nonzero,
> ? ?it is an INTEGER_CST that should be AND'ed with the extra bits. ?*/
> @@ -5025,8 +5050,8 @@ merge_truthop_with_opposite_arm (locatio
> ? ?We return the simplified tree or 0 if no optimization is possible. ?*/
>
> ?static tree
> -fold_truthop (location_t loc, enum tree_code code, tree truth_type,
> - ? ? ? ? ? ? tree lhs, tree rhs)
> +fold_truth_andor_1 (location_t loc, enum tree_code code, tree truth_type,
> + ? ? ? ? ? ? ? ? ? tree lhs, tree rhs)
> ?{
> ? /* If this is the "or" of two comparisons, we can do something if
> ? ? ?the comparisons are NE_EXPR. ?If this is the "and", we can do something
> @@ -5054,8 +5079,6 @@ fold_truthop (location_t loc, enum tree_
> ? tree lntype, rntype, result;
> ? HOST_WIDE_INT first_bit, end_bit;
> ? int volatilep;
> - ?tree orig_lhs = lhs, orig_rhs = rhs;
> - ?enum tree_code orig_code = code;
>
> ? /* Start by getting the comparison codes. ?Fail if anything is volatile.
> ? ? ?If one operand is a BIT_AND_EXPR with the constant one, treat it as if
> @@ -5119,8 +5142,7 @@ fold_truthop (location_t loc, enum tree_
> ? /* If the RHS can be evaluated unconditionally and its operands are
> ? ? ?simple, it wins to evaluate the RHS unconditionally on machines
> ? ? ?with expensive branches. ?In this case, this isn't a comparison
> - ? ? that can be merged. ?Avoid doing this if the RHS is a floating-point
> - ? ? comparison since those can trap. ?*/
> + ? ? that can be merged. ?*/
>
> ? if (BRANCH_COST (optimize_function_for_speed_p (cfun),
> ? ? ? ? ? ? ? ? ? false) >= 2
> @@ -5149,13 +5171,6 @@ fold_truthop (location_t loc, enum tree_
> ? ? ? ? ? ? ? ? ? ? ? ? ? build2 (BIT_IOR_EXPR, TREE_TYPE (ll_arg),
> ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ll_arg, rl_arg),
> ? ? ? ? ? ? ? ? ? ? ? ? ? build_int_cst (TREE_TYPE (ll_arg), 0));
> -
> - ? ? ?if (LOGICAL_OP_NON_SHORT_CIRCUIT)
> - ? ? ? {
> - ? ? ? ? if (code != orig_code || lhs != orig_lhs || rhs != orig_rhs)
> - ? ? ? ? ? return build2_loc (loc, code, truth_type, lhs, rhs);
> - ? ? ? ? return NULL_TREE;
> - ? ? ? }
> ? ? }
>
> ? /* See if the comparisons can be merged. ?Then get all the parameters for
> @@ -8380,13 +8395,52 @@ fold_truth_andor (location_t loc, enum t
> ? ? ?lhs is another similar operation, try to merge its rhs with our
> ? ? ?rhs. ?Then try to merge our lhs and rhs. ?*/
> ? if (TREE_CODE (arg0) == code
> - ? ? ?&& 0 != (tem = fold_truthop (loc, code, type,
> - ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?TREE_OPERAND (arg0, 1), arg1)))
> + ? ? ?&& 0 != (tem = fold_truth_andor_1 (loc, code, type,
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?TREE_OPERAND (arg0, 1), arg1)))
> ? ? return fold_build2_loc (loc, code, type, TREE_OPERAND (arg0, 0), tem);
>
> - ?if ((tem = fold_truthop (loc, code, type, arg0, arg1)) != 0)
> + ?if ((tem = fold_truth_andor_1 (loc, code, type, arg0, arg1)) != 0)
> ? ? return tem;
>
> + ?if ((code == TRUTH_ANDIF_EXPR || code == TRUTH_ORIF_EXPR)
> + ? ? ?&& (BRANCH_COST (optimize_function_for_speed_p (cfun),
> + ? ? ? ? ? ? ? ? ? ? ?false) >= 2)
> + ? ? ?&& !TREE_SIDE_EFFECTS (arg1)
> + ? ? ?&& LOGICAL_OP_NON_SHORT_CIRCUIT
> + ? ? ?&& simple_operand_p (arg1))
> + ? ?{
> + ? ? ?enum tree_code ncode = (code == TRUTH_ANDIF_EXPR ? TRUTH_AND_EXPR
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?: TRUTH_OR_EXPR);
> +
> + ? ? ?/* We don't want to pack more then two non-IF branches
> + ? ? ? ? together. ?Therefore we need to check, if rhs isn't
> + ? ? ? ? already an TRUTH_(XOR|OR|AND)[IF]_EXPR. ?*/

which means, just check if (simple_operand_p (arg0)) ...

> + ? ? ?if (TREE_CODE (arg0) == code
> + ? ? ? ? && TREE_CODE (TREE_OPERAND (arg0, 1)) != TRUTH_AND_EXPR
> + ? ? ? ? && TREE_CODE (TREE_OPERAND (arg0, 1)) != TRUTH_OR_EXPR
> + ? ? ? ? && TREE_CODE (TREE_OPERAND (arg0, 1)) != TRUTH_XOR_EXPR
> + ? ? ? ? && TREE_CODE (TREE_OPERAND (arg0, 1)) != TRUTH_ANDIF_EXPR
> + ? ? ? ? && TREE_CODE (TREE_OPERAND (arg0, 1)) != TRUTH_ORIF_EXPR
> + ? ? ? ? /* Needed for sequence points and trappings, or side-effects. ?*/
> + ? ? ? ? && !TREE_SIDE_EFFECTS (TREE_OPERAND (arg0, 1))
> + ? ? ? ? && !tree_could_trap_p (TREE_OPERAND (arg0, 1)))
> + ? ? ? {
> + ? ? ? ? tem = fold_build2_loc (loc, ncode, type, TREE_OPERAND (arg0, 1),
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? arg1);
> + ? ? ? ? return fold_build2_loc (loc, code, type, TREE_OPERAND (arg0, 0),
> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?tem);

Don't do this association here.  Why should it not be applied to
(a TRUTH_ANDIF_EXPR b) TRUTH_AND_EXPR c?  Thus, if you
want to associate the above if b and c do not have side-effects then
do so generally, not only when you are converting a TRUTH_ANDIF_EXPR
to TRUTH_AND_EXPR.

Thus, please remove this association code.

> + ? ? ? }
> + ? ? else if (TREE_CODE (arg0) != TRUTH_AND_EXPR
> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_OR_EXPR
> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_ANDIF_EXPR
> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_ORIF_EXPR
> + ? ? ? ? ? ? && TREE_CODE (arg0) != TRUTH_XOR_EXPR
> + ? ? ? ? ? ? /* Needed for sequence points and trappings, or side-effects. ?*/
> + ? ? ? ? ? ? && !TREE_SIDE_EFFECTS (arg0)
> + ? ? ? ? ? ? && !tree_could_trap_p (arg0))

See above.

You made the patch more complex when I asked for a simpler one.

Richard.

> + ? ? ? return fold_build2_loc (loc, ncode, type, arg0, arg1);
> + ? ?}
> +
> ? return NULL_TREE;
> ?}
>