[PATCH] Extended if-conversion for loops marked with pragma omp simd.

Yuri Rumyantsev ysrumyan@gmail.com
Mon Sep 8 11:03:00 GMT 2014


Richard,

Did you have a chance to look at this?

Thanks.

2014-08-15 16:02 GMT+04:00 Yuri Rumyantsev <ysrumyan@gmail.com>:
> Richard!
> Here is updated patch with the following changes:
>
> 1. Any restrictions on phi-function were eliminated for extended conversion.
> 2.  Put predicate for critical edges to 'aux' field of edge, i.e.
> negate_predicate was deleted.
> 3. Deleted splitting of critical edges, i.e. both outgoing edges can
> be critical.
> 4. Use notion of cd-equivalence to set-up predicate for join basic
> blocks to simplify it.
> 5. I decided to not design pre-pass since it will lead generating
> chain of cond expressions for phi-node if conversion, whereas for phi
> of kind
>   x = PHI <1(2), 1(3), 2(4)>
> only one cond expression is required and this is considered as simple
> optimization for arbitrary phi-function. More precise,
> if phi-function have only two different arguments and one of them has
> single occurrence, if- conversion is performed as if phi have only 2
> arguments.
> For arbitrary phi function a chain of cond expressions is produced.
>
> Updated patch is attached.
>
> Any comments will be appreciated.
>
> 2014-08-15  Yuri Rumyantsev  <ysrumyan@gmail.com>
>
> * tree-if-conv.c (cgraph.h): Add include file to detect function clone.
> (flag_force_vectorize): New variable.
> (edge_predicate): New function.
> (set_edge_predicate): New function.
> (add_stmt_to_bb_predicate_gimplified_stmts): New function.
> (init_bb_predicate): Add initialization of negate_predicate field.
> (reset_bb_predicate): Reset negate_predicate to NULL_TREE.
> (convert_name_to_cmp): New function.
> (get_type_for_cond): New function.
> (convert_bool_predicate): New function.
> (predicate_disjunction): New function.
> (predicate_conjunction): New function.
> (add_to_predicate_list): Add convert_bool argument.
> Use predicate of cd-equivalent block if convert_bool is true and
> such bb exists; save it in static variable for further possible use.
> Add call of predicate_disjunction if convert_bool argument is true.
> (add_to_dst_predicate_list): Add convert_bool argument.
> Add early function exit if edge target block is always executed.
> Add call of predicate_conjunction if convert_bool argument is true.
> Pass convert_bool argument for add_to_predicate_list.
> Set-up predicate for crritical edge if convert_bool is true.
> (equal_phi_args): New function.
> (phi_has_two_different_args): New function.
> (if_convertible_phi_p): Accept phi nodes with more than two args
> if flag_force_vectorize wa set-up.
> (ifcvt_can_use_mask_load_store): Add test on flag_force_vectorize.
> (if_convertible_stmt_p): Allow calls of function clones if
> flag_force_vectorize was set-up.
> (all_edges_are_critical): New function.
> (if_convertible_bb_p): Allow bb has more than two predecessors if
> flag_force_vectorize was set-up. Use call of all_edges_are_critical
> to reject block if-conversion with imcoming critical edges only if
> flag_force_vectorize was not set-up.
> (walk_cond_tree): New function.
> (vect_bool_pattern_is_applicable): New function.
> (predicate_bbs): Add convert_bool argument which is used to transform
> comparison expressions of boolean type into conditional expressions
> with integral operands. If convert_bool argument was set-up and
> vect bool pattern can be appied perform the following transformation:
> (bool) x != 0  --> y = (int) x; x != 0;
> Add check that if fold_build2 produces bool conversion if convert_bool
> was set-up, recompute predicate using build2_loc. Additional argument
> 'convert_bool" is passed to add_to_dst_predicate_list and
> add_to_predicate_list.
> (if_convertible_loop_p_1): Recompute POST_DOMINATOR tree if
> flag_force_vectorize was set-up to calculate cd equivalent bb's.
> Call predicate_bbs with additional argument equal to false.
> (find_phi_replacement_condition): Extend function interface:
> it returns NULL if given phi node must be handled by means of
> extended phi node predication. If number of predecessors of phi-block
> is equal 2 and atleast one incoming edge is not critical original
> algorithm is used.
> (is_cond_scalar_reduction): Add 'extended' argument which signals that
> phi arguments must be evaluated through phi_has_two_different_args.
> (predicate_scalar_phi): Add invoсation of convert_name_to_cmp if cond
> is SSA_NAME. Add 'false' argument to call of is_cond_scalar_reduction.
> (get_predicate_for_edge): New function.
> (find_insertion_point): New function.
> (predicate_arbitrary_phi): New function.
> (predicate_extended_scalar_phi): New function.
> (predicate_all_scalar_phis): Add code to set-up gimple statement
> iterator for predication of extended scalar phi's for insertion.
> (insert_gimplified_predicates): Add test for non-predicated basic
> blocks that there are no gimplified statements to insert. Insert
> predicates at the block begining for extended if-conversion.
> (predicate_mem_writes): Invoke convert_name_to_cmp for extended
> predication to build mask.
> (combine_blocks): Pass flag_force_vectorize to predicate_bbs.
> (tree_if_conversion): Initialize flag_force_vectorize from current
> loop or outer loop (to support pragma omp declare).Do loop versioning
> for innermost loop marked with pragma omp simd.
>
> 2014-08-01 13:40 GMT+04:00 Richard Biener <richard.guenther@gmail.com>:
>> On Wed, Jun 25, 2014 at 4:06 PM, Yuri Rumyantsev <ysrumyan@gmail.com> wrote:
>>> Hi All,
>>>
>>> We implemented additional support for pragma omp simd in part of
>>> extended if-conversion loops with such pragma. These extensions
>>> include:
>>>
>>> 1. All extensions are performed only if considered loop or its outer
>>>    loop was marked with pragma omp simd (force_vectorize); For ordinary
>>>    loops behavior was not changed.
>>> 2. Took off cfg restriction on basic block which can have more than 2
>>>    predecessors.
>>> 3. Put additional restriction on phi nodes which was missed in current design:
>>>    all phi nodes must be in non-predicated basic block to conform
>>>    semantic of COND_EXPR which is used for transformation.
>>
>> How is that so?  If the PHI is predicated then its result will be used
>> in a PHI node again and thus we'd create a sequence of COND_EXPRs.
>>
>> No?
>>
>>> 4. Extend predication of phi nodes: phi may have more than 2 arguments
>>> with some limitations:
>>>    - for phi nodes which have more than 2 arguments, but only two
>>>    arguments are different and one of them has the only occurence,
>>> transformation to  single COND_EXPR can be done.
>>>    - if phi node has more different arguments and all edge predicates
>>>    correspondent to phi-arguments are disjoint, a chain of COND_EXPR
>>>    will be generated for it. In current design very simple check is used:
>>>    check starting from end that two edges correspondent to neighbor
>>> arguments have common predecessor which is used for further check
>>> with next edge.
>>>  These guarantee that phi predication will produce the correct result.
>>
>> Btw, you can think of these extensions as unfactoring a PHI node by
>> inserting forwarder blocks.  Thus
>>
>>    x = PHI <1(2), 1(3), 2(4)>
>>
>> becomes
>>
>>   bb 5: <forwarder-from(2)-and(3)>
>>
>>   x = PHI <1(5), 2(4)>
>>
>> and
>>
>>   x = PHI <1(2), 2(3), 3(4)>
>>
>> becomes
>>
>>   bb 5:
>>   x' = PHI <1(2), 2(3)>
>>
>>   b = PHI<x'(5), 3(4)>
>>
>> which means that 3) has to work.  Note that we want this kind of
>> PHI transforms for out-of-SSA as well to reduce the number of
>> copies we need to insert on edges.
>>
>> Thus it would be nice if you implemented 4) in terms of a pre-pass
>> over the force_vect loops PHI nodes, applying that CFG transform.
>> And make 3) work properly if it doesn't already.
>>
>> It looks like you introduce a "negate predicate" to work around the
>> critical edge limitation?  Please instead change if-conversion to
>> work with edge predicates (as opposed to BB predicates).
>>
>> Thanks,
>> Richard.
>>
>>>
>>> Here is example of such extended predication (compile with -march=core-avx2):
>>> #pragma omp simd safelen(8)
>>>   for (i=0; i<512; i++)
>>>   {
>>>     float t = a[i];
>>>     if (t > 0 & t < 1.0e+17f)
>>>       if (c[i] != 0)
>>> res += 1;
>>>   }
>>>   <bb 4>:
>>>   # res_15 = PHI <res_1(5), 0(3)>
>>>   # i_16 = PHI <i_11(5), 0(3)>
>>>   # ivtmp_17 = PHI <ivtmp_14(5), 512(3)>
>>>   t_5 = a[i_16];
>>>   _6 = t_5 > 0.0;
>>>   _7 = t_5 < 9.9999998430674944e+16;
>>>   _8 = _7 & _6;
>>>   _ifc__28 = (unsigned int) _8;
>>>   _10 = &c[i_16];
>>>   _ifc__36 = _ifc__28 != 0 ? 4294967295 : 0;
>>>   _9 = MASK_LOAD (_10, 0B, _ifc__36);
>>>   _ifc__29 = _ifc__28 != 0 ? 1 : 0;
>>>   _ifc__30 = (int) _ifc__29;
>>>   _ifc__31 = _9 != 0 ? _ifc__30 : 0;
>>>   _ifc__32 = _ifc__28 != 0 ? 1 : 0;
>>>   _ifc__33 = (int) _ifc__32;
>>>   _ifc__34 = _9 == 0 ? _ifc__33 : 0;
>>>   _ifc__35 = _ifc__31 != 0 ? 1 : 0;
>>>   res_1 = res_15 + _ifc__35;
>>>   i_11 = i_16 + 1;
>>>   ivtmp_14 = ivtmp_17 - 1;
>>>   if (ivtmp_14 != 0)
>>>     goto <bb 4>;
>>>
>>> Bootstrap and regression testing did not show any new failures.
>>>
>>> gcc/ChageLog
>>>
>>> 2014-06-25  Yuri Rumyantsev  <ysrumyan@gmail.com>
>>>
>>> * tree-if-conv.c (flag_force_vectorize): New variable.
>>> (struct bb_predicate_s): Add negate_predicate field.
>>> (bb_negate_predicate): New function.
>>> (set_bb_negate_predicate): New function.
>>> (bb_copy_predicate): New function.
>>> (add_stmt_to_bb_predicate_gimplified_stmts): New function.
>>> (init_bb_predicate): Add initialization of negate_predicate field.
>>> (reset_bb_predicate): Reset negate_predicate to NULL_TREE.
>>> (convert_name_to_cmp): New function.
>>> (get_type_for_cond): New function.
>>> (convert_bool_predicate): New function.
>>> (predicate_disjunction): New function.
>>> (predicate_conjunction): New function.
>>> (add_to_predicate_list): Add convert_bool argument.
>>> Add call of predicate_disjunction if convert_bool argument is true.
>>> (add_to_dst_predicate_list): Add convert_bool argument.
>>> Add early function exit if edge target block is always executed.
>>> Add call of predicate_conjunction if convert_bool argument is true.
>>> Pass convert_bool argument for add_to_predicate_list.
>>> (equal_phi_args): New function.
>>> (phi_has_two_different_args): New function.
>>> (phi_args_disjoint): New function.
>>> (if_convertible_phi_p): Accept phi nodes with more than two args
>>> for loops marked with pragma omp simd. Add check that phi nodes are
>>> in non-predicated basic blocks.
>>> (ifcvt_can_use_mask_load_store): Use flag_force_vectorize.
>>> (all_edges_are_critical): New function.
>>> (if_convertible_bb_p): Allow bb has more than two predecessors if
>>> flag_force_vectorize was setup. Use call of all_edges_are_critical
>>> to reject block if-conversion with imcoming critical edges only if
>>> flag_force_vectorize was not setup.
>>> (walk_cond_tree): New function.
>>> (vect_bool_pattern_is_applicable): New function.
>>> (predicate_bbs): Add convert_bool argument that is used to transform
>>> comparison expressions of boolean type into conditional expressions
>>> with integral operands. If bool_conv argument is false or both
>>> outgoing edges are not critical old algorithm of predicate assignments
>>> is used, otherwise the following code was added: check on applicable
>>> of vect-bool-pattern recognition and trnasformation of
>>> (bool) x != 0  --> y = (int) x; x != 0;
>>> compute predicates for both outgoing edges one of which is critical
>>> one using 'normal' edge, i.e. compute true and false predicates using
>>> normal outgoing edge only; evaluated predicates are stored in
>>> predicate and negate_predicate fields of struct bb_predicate_s and
>>> negate_predicate of normal edge conatins predicate of critical edge,
>>> but generated gimplified statements are stored in their destination
>>> block fields. Additional argument 'convert_bool" is passed to
>>> add_to_dst_predicate_list and add_to_predicate_list.
>>> (if_convertible_loop_p_1): Call predicate_bbs with additional argument
>>> equal to false.
>>> (find_phi_replacement_condition): Extend function interface:
>>> it returns NULL if given phi node must be handled by means of
>>> extended phi node predication. If number of predecessors of phi-block
>>> is equal 2 and atleast one incoming edge is not critical original
>>> algorithm is used.
>>> (is_cond_scalar_reduction): Add 'extended' argument which signals that
>>> both phi arguments must be evaluated through phi_has_two_different_args.
>>> (predicate_scalar_phi): Add invoсation of convert_name_to_cmp if cond
>>> is SSA_NAME. Add 'false' argument to call of is_cond_scalar_reduction.
>>> (get_predicate_for_edge): New function.
>>> (find_insertion_point): New function.
>>> (predicate_phi_disjoint_args): New function.
>>> (predicate_extended_scalar_phi): New function.
>>> (predicate_all_scalar_phis): Add code to set-up gimple statement
>>> iterator for predication of extended scalar phi's for insertion.
>>> (insert_gimplified_predicates): Add test for non-predicated basic
>>> blocks that there are no gimplified statements to insert. Insert
>>> predicates at the block begining for extended if-conversion.
>>> (predicate_mem_writes): Invoke convert_name_to_cmp for extended
>>> predication to build mask.
>>> (combine_blocks): Pass flag_force_vectorize to predicate_bbs.
>>> (split_crit_edge): New function.
>>> (tree_if_conversion): Initialize flag_force_vectorize from current
>>> loop or outer loop (to support pragma omp declare). Invoke
>>> split_crit_edge for extended predication. Do loop versioning for
>>> innermost loop marked with pragma omp simd.



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