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 Mon, Oct 10, 2011 at 4:06 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
> 2011/10/10 Richard Guenther <richard.guenther@gmail.com>:
>> On Mon, Oct 10, 2011 at 2:29 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>>> Recent patch had a thinko on rhs of inner lhs check for TRUTH-IF. ?It
>>> has to be checked that the LHS code is same as outer CODE, as
>>> otherwise we wouldn't apply different TRUTH-IF only on inner RHS of
>>> LHS, which is of course wrong.
>>>
>>> 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)));
>>
>> And that's still wrong.
>>
>> Stopped reading here.
>>
>> Richard.
>
> Oh, there is a not missing. ?I didn't spot that, sorry.
>
> To the point why we need to handle comparisons within simple_operand_p.
>
> If we reject comparisons and logical not here, we won't have any
> branching optimization anymore, as this the patch moves into
> fold_truthandor.
>
> The result with rejecting in simple_operand_p compares and logic-not
> provides for the following example:

But you change what simple_operand_p accepts and thus change what
fold_truthop accepts as operands to its simplifications.

Richard.

> extern int doo (void);
> extern int doo2 (void);
>
> int foo (int a, int b, int c, int d, int e, int f)
> {
> ?if (a && b && c && d && e && f)
> ? ?return doo ();
> ?return doo2 ();
> }
>
> we get the following gimple dump (-fdump-tree-gimple):
>
> foo (int a, int b, int c, int d, int e, int f)
> {
> ?int D.2752;
>
> ?if (a != 0) goto <D.2740>; else goto <D.2741>;
> ?<D.2740>:
> ?if (b != 0) goto <D.2742>; else goto <D.2743>;
> ?<D.2742>:
> ?if (c != 0) goto <D.2744>; else goto <D.2745>;
> ?<D.2744>:
> ?if (d != 0) goto <D.2746>; else goto <D.2747>;
> ?<D.2746>:
> ?if (e != 0) goto <D.2748>; else goto <D.2749>;
> ?<D.2748>:
> ?if (f != 0) goto <D.2750>; else goto <D.2751>;
> ?<D.2750>:
> ?D.2752 = doo ();
> ?return D.2752;
> ?<D.2751>:
> ?<D.2749>:
> ?<D.2747>:
> ?<D.2745>:
> ?<D.2743>:
> ?<D.2741>:
> ?D.2752 = doo2 ();
> ?return D.2752;
> }
>
> So this result is caused by the fact that a logical-and/or is always a
> comparison. ?As we are rejecting comparisons/logical-not, ?we end up
> with a sequence of TRUTH_(AND|OR)_IFs. ?So the hole loop in
> fold_truth_andor for trying to pack simple and side-effect-less
> operands in tupels of two is useless.
>
> For the new patch (attached with proper ! fix for compares) we get for
> the same sample gimple output:
>
> foo (int a, int b, int c, int d, int e, int f)
> {
> ?_Bool D.2740;
> ?_Bool D.2741;
> ?_Bool D.2742;
> ?_Bool D.2745;
> ?_Bool D.2746;
> ?_Bool D.2747;
> ?_Bool D.2750;
> ?_Bool D.2751;
> ?_Bool D.2752;
> ?int D.2755;
>
> ?D.2740 = a != 0;
> ?D.2741 = b != 0;
> ?D.2742 = D.2740 & D.2741;
> ?if (D.2742 != 0) goto <D.2743>; else goto <D.2744>;
> ?<D.2743>:
> ?D.2745 = c != 0;
> ?D.2746 = d != 0;
> ?D.2747 = D.2745 & D.2746;
> ?if (D.2747 != 0) goto <D.2748>; else goto <D.2749>;
> ?<D.2748>:
> ?D.2750 = e != 0;
> ?D.2751 = f != 0;
> ?D.2752 = D.2750 & D.2751;
> ?if (D.2752 != 0) goto <D.2753>; else goto <D.2754>;
> ?<D.2753>:
> ?D.2755 = doo ();
> ?return D.2755;
> ?<D.2754>:
> ?<D.2749>:
> ?<D.2744>:
> ?D.2755 = doo2 ();
> ?return D.2755;
> }
>
> which is the proper output for high branching cost, as it tries to
> avoid branches and not to tend them.
>
> --
> 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)));
> +
> + ?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;
> + ? ?}
> +
> ? 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
> ? ?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 +8395,46 @@ 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 leafs to an non-IF
> + ? ? ? ? If tree-code of left-hand operand isn't an AND/OR-IF code and not
> + ? ? ? ? equal to CODE, then we don't want to add right-hand operand.
> + ? ? ? ? If the inner right-hand side of left-hand operand has side-effects,
> + ? ? ? ? or isn't simple, then we can't add to it, as otherwise we might
> + ? ? ? ? destroy if-sequence. ?*/
> + ? ? ?if (TREE_CODE (arg0) == code
> + ? ? ? ? /* Needed for sequence points to handle trappings, and
> + ? ? ? ? ? ?side-effects. ?*/
> + ? ? ? ? && !TREE_SIDE_EFFECTS (TREE_OPERAND (arg0, 1))
> + ? ? ? ? && simple_operand_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);
> + ? ? ? }
> + ? ? /* Needed for sequence points to handle trappings, and side-effects. ?*/
> + ? ? else if (!TREE_SIDE_EFFECTS (arg0)
> + ? ? ? ? ? ? && simple_operand_p (arg0))
> + ? ? ? return fold_build2_loc (loc, ncode, type, arg0, arg1);
> + ? ?}
> +
> ? return NULL_TREE;
> ?}
>