General loop multi-versioning (pass) can be a very powerful enabler,
especially when combined with FDO. The loop MV can be done based on
many different conditions:
1) loop trip count (e.g, when we do value profiling on trip count, not
based on average count);
2) memory aliasing
3) target options -- though we do not yet support region specific
target options, the MVed loop can be outlined if believed to be
profitable.
David
On Wed, Oct 2, 2013 at 3:51 PM, Cong Hou <co...@google.com> wrote:
> On Wed, Oct 2, 2013 at 2:18 PM, Xinliang David Li <davi...@google.com> wrote:
>> On Wed, Oct 2, 2013 at 4:24 AM, Richard Biener <rguent...@suse.de> wrote:
>>> On Tue, 1 Oct 2013, Cong Hou wrote:
>>>
>>>> When alias exists between data refs in a loop, to vectorize it GCC
>>>> does loop versioning and adds runtime alias checks. Basically for each
>>>> pair of data refs with possible data dependence, there will be two
>>>> comparisons generated to make sure there is no aliasing between them
>>>> in each iteration of the vectorized loop. If there are many such data
>>>> refs pairs, the number of comparisons can be very large, which is a
>>>> big overhead.
>>>>
>>>> However, in some cases it is possible to reduce the number of those
>>>> comparisons. For example, for the following loop, we can detect that
>>>> b[0] and b[1] are two consecutive member accesses so that we can
>>>> combine the alias check between a[0:100]&b[0] and a[0:100]&b[1] into
>>>> checking a[0:100]&b[0:2]:
>>>>
>>>> void foo(int*a, int* b)
>>>> {
>>>> for (int i = 0; i < 100; ++i)
>>>> a[i] = b[0] + b[1];
>>>> }
>>>>
>>>> Actually, the requirement of consecutive memory accesses is too
>>>> strict. For the following loop, we can still combine the alias checks
>>>> between a[0:100]&b[0] and a[0:100]&b[100]:
>>>>
>>>> void foo(int*a, int* b)
>>>> {
>>>> for (int i = 0; i < 100; ++i)
>>>> a[i] = b[0] + b[100];
>>>> }
>>>>
>>>> This is because if b[0] is not in a[0:100] and b[100] is not in
>>>> a[0:100] then a[0:100] cannot be between b[0] and b[100]. We only need
>>>> to check a[0:100] and b[0:101] don't overlap.
>>>>
>>>> More generally, consider two pairs of data refs (a, b1) and (a, b2).
>>>> Suppose addr_b1 and addr_b2 are basic addresses of data ref b1 and b2;
>>>> offset_b1 and offset_b2 (offset_b1 < offset_b2) are offsets of b1 and
>>>> b2, and segment_length_a, segment_length_b1, and segment_length_b2 are
>>>> segment length of a, b1, and b2. Then we can combine the two
>>>> comparisons into one if the following condition is satisfied:
>>>>
>>>> offset_b2- offset_b1 - segment_length_b1 < segment_length_a
>>>>
>>>>
>>>> This patch detects those combination opportunities to reduce the
>>>> number of alias checks. It is tested on an x86-64 machine.
>>>
>>> Apart from the other comments you got (to which I agree) the patch
>>> seems to do two things, namely also:
>>>
>>> + /* Extract load and store statements on pointers with zero-stride
>>> + accesses. */
>>> + if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
>>> + {
>>>
>>> which I'd rather see in a separate patch (and done also when
>>> the loop doesn't require versioning for alias).
>>
>> yes.
>>
>>>
>>> Also combining the alias checks in vect_create_cond_for_alias_checks
>>> is nice but doesn't properly fix the use of the
>>> vect-max-version-for-alias-checks param
>>
>> Yes. The handling of this should be moved to
>> 'vect_prune_runtime_alias_test_list' to avoid premature decisions.
>>
>>
>>
>>>which currently inhibits
>>> vectorization of the HIMENO benchmark by default (and make us look bad
>>> compared to LLVM).
>>
>> Here is a small reproducible:
>>
>> struct A {
>> int *base;
>> int offset;
>> int offset2;
>> int offset3;
>> int offset4;
>> int offset5;
>> int offset6;
>> int offset7;
>> int offset8;
>> };
>>
>> void foo (struct A * ar1, struct A* ar2)
>> {
>> int i;
>> for (i = 0; i < 10000; i++)
>> {
>> ar1->base[i] = 2*ar2->base[i] + ar2->offset + ar2->offset2
>> + ar2->offset3 + ar2->offset4 + ar2->offset5 + ar2->offset6; /* +
>> ar2->offset7 + ar2->offset8;*/
>> }
>> }
>>
>> GCC trunk won't vectorize it at O2 due to the limit.
>>
>>
>> There is another problem we should be tracking: GCC no longer
>> vectorize the loop (with large
>> --param=vect-max-version-for-alias-checks=40) when -fno-strict-alias
>> is specified. However with additional runtime alias check, the loop
>> should be vectorizable.
>
>
> The problem can be reproduced by the following loop:
>
>
> void foo (int* a, int** b)
> {
> int i;
> for (i = 0; i < 1000; ++i)
> a[i] = (*b)[i];
> }
>
>
> When -fno-strict-aliasing is specified, the basic address of (*b)[i]
> which is *b could be modified by a[i] if alias exists between them.
> This forbids GCC from making the basic address of (*b)[i] a loop
> invariant, and hence could not do vectorization. Although we can still
> check the aliasing between a[i] and *b from a and b (b < a or b >=
> a+1000), it must be done before vectorization and after this we have
> to do one more nested aliasing check (a and *b now):
>
>
> void foo (int* a, int** b)
> {
> int i;
> if ((int*)b < a || (int*)b >= a+1000)
> {
> int* c = *b;
> for (i = 0; i < 1000; ++i)
> a[i] = c[i];
> }
> else
> {
> for (i = 0; i < 1000; ++i)
> a[i] = (*b)[i];
> }
> }
>
> I think this is too complicated to be able to get any benefit for us.
>
>
> thanks,
> Cong
>
>
>
>>
>> David
>>
>>
>>>
>>> So I believe this merging should be done incrementally when
>>> we collect the DDRs we need to test in vect_mark_for_runtime_alias_test.
>>>
>>> Thanks for working on this,
>>> Richard.
>>>
>>>>
>>>> thanks,
>>>> Cong
>>>>
>>>>
>>>>
>>>> Index: gcc/tree-vect-loop-manip.c
>>>> ===================================================================
>>>> --- gcc/tree-vect-loop-manip.c (revision 202662)
>>>> +++ gcc/tree-vect-loop-manip.c (working copy)
>>>> @@ -19,6 +19,10 @@ You should have received a copy of the G
>>>> along with GCC; see the file COPYING3. If not see
>>>> <http://www.gnu.org/licenses/>. */
>>>>
>>>> +#include <vector>
>>>> +#include <utility>
>>>> +#include <algorithm>
>>>> +
>>>> #include "config.h"
>>>> #include "system.h"
>>>> #include "coretypes.h"
>>>> @@ -2248,6 +2252,74 @@ vect_vfa_segment_size (struct data_refer
>>>> return segment_length;
>>>> }
>>>>
>>>> +namespace
>>>> +{
>>>> +
>>>> +/* struct dr_addr_with_seg_len
>>>> +
>>>> + A struct storing information of a data reference, including the data
>>>> + ref itself, its basic address, the access offset and the segment length
>>>> + for aliasing checks. */
>>>> +
>>>> +struct dr_addr_with_seg_len
>>>> +{
>>>> + dr_addr_with_seg_len (data_reference* d, tree addr, tree off, tree len)
>>>> + : dr (d), basic_addr (addr), offset (off), seg_len (len) {}
>>>> +
>>>> + data_reference* dr;
>>>> + tree basic_addr;
>>>> + tree offset;
>>>> + tree seg_len;
>>>> +};
>>>> +
>>>> +/* Operator == between two dr_addr_with_seg_len objects.
>>>> +
>>>> + This equality operator is used to make sure two data refs
>>>> + are the same one so that we will consider to combine the
>>>> + aliasing checks of those two pairs of data dependent data
>>>> + refs. */
>>>> +
>>>> +bool operator == (const dr_addr_with_seg_len& d1,
>>>> + const dr_addr_with_seg_len& d2)
>>>> +{
>>>> + return operand_equal_p (d1.basic_addr, d2.basic_addr, 0)
>>>> + && operand_equal_p (d1.offset, d2.offset, 0)
>>>> + && operand_equal_p (d1.seg_len, d2.seg_len, 0);
>>>> +}
>>>> +
>>>> +typedef std::pair <dr_addr_with_seg_len, dr_addr_with_seg_len>
>>>> + dr_addr_with_seg_len_pair_t;
>>>> +
>>>> +
>>>> +/* Operator < between two dr_addr_with_seg_len_pair_t objects.
>>>> +
>>>> + This operator is used to sort objects of dr_addr_with_seg_len_pair_t
>>>> + so that we can combine aliasing checks during one scan. */
>>>> +
>>>> +bool operator < (const dr_addr_with_seg_len_pair_t& p1,
>>>> + const dr_addr_with_seg_len_pair_t& p2)
>>>> +{
>>>> + const dr_addr_with_seg_len& p11 = p1.first;
>>>> + const dr_addr_with_seg_len& p12 = p1.second;
>>>> + const dr_addr_with_seg_len& p21 = p2.first;
>>>> + const dr_addr_with_seg_len& p22 = p2.second;
>>>> +
>>>> + if (p11.basic_addr != p21.basic_addr)
>>>> + return p11.basic_addr < p21.basic_addr;
>>>> + if (p12.basic_addr != p22.basic_addr)
>>>> + return p12.basic_addr < p22.basic_addr;
>>>> + if (TREE_CODE (p11.offset) != INTEGER_CST
>>>> + || TREE_CODE (p21.offset) != INTEGER_CST)
>>>> + return p11.offset < p21.offset;
>>>> + if (int_cst_value (p11.offset) != int_cst_value (p21.offset))
>>>> + return int_cst_value (p11.offset) < int_cst_value (p21.offset);
>>>> + if (TREE_CODE (p12.offset) != INTEGER_CST
>>>> + || TREE_CODE (p22.offset) != INTEGER_CST)
>>>> + return p12.offset < p22.offset;
>>>> + return int_cst_value (p12.offset) < int_cst_value (p22.offset);
>>>> +}
>>>> +
>>>> +}
>>>>
>>>> /* Function vect_create_cond_for_alias_checks.
>>>>
>>>> @@ -2292,20 +2364,51 @@ vect_create_cond_for_alias_checks (loop_
>>>> if (may_alias_ddrs.is_empty ())
>>>> return;
>>>>
>>>> +
>>>> + /* Basically, for each pair of dependent data refs store_ptr_0
>>>> + and load_ptr_0, we create an expression:
>>>> +
>>>> + ((store_ptr_0 + store_segment_length_0) <= load_ptr_0)
>>>> + || (load_ptr_0 + load_segment_length_0) <= store_ptr_0))
>>>> +
>>>> + for aliasing checks. However, in some cases we can decrease
>>>> + the number of checks by combining two checks into one. For
>>>> + example, suppose we have another pair of data refs store_ptr_0
>>>> + and load_ptr_1, and if the following condition is satisfied:
>>>> +
>>>> + load_ptr_0 < load_ptr_1 &&
>>>> + load_ptr_1 - load_ptr_0 - load_segment_length_0 <
>>>> store_segment_length_0
>>>> +
>>>> + (this condition means, in each iteration of vectorized loop,
>>>> + the accessed memory of store_ptr_0 cannot be between the memory
>>>> + of load_ptr_0 and load_ptr_1.)
>>>> +
>>>> + we then can use only the following expression to finish the
>>>> + alising checks between store_ptr_0 & load_ptr_0 and
>>>> + store_ptr_0 & load_ptr_1:
>>>> +
>>>> + ((store_ptr_0 + store_segment_length_0) <= load_ptr_0)
>>>> + || (load_ptr_1 + load_segment_length_1 <= store_ptr_0))
>>>> +
>>>> + Note that we only consider that load_ptr_0 and load_ptr_1 have the
>>>> + same basic address. */
>>>> +
>>>> + std::vector<dr_addr_with_seg_len_pair_t> ddrs_with_seg_len;
>>>> +
>>>> + /* First, we collect all data ref pairs for aliasing checks. */
>>>> +
>>>> FOR_EACH_VEC_ELT (may_alias_ddrs, i, ddr)
>>>> {
>>>> struct data_reference *dr_a, *dr_b;
>>>> gimple dr_group_first_a, dr_group_first_b;
>>>> - tree addr_base_a, addr_base_b;
>>>> tree segment_length_a, segment_length_b;
>>>> gimple stmt_a, stmt_b;
>>>> - tree seg_a_min, seg_a_max, seg_b_min, seg_b_max;
>>>>
>>>> dr_a = DDR_A (ddr);
>>>> stmt_a = DR_STMT (DDR_A (ddr));
>>>> dr_group_first_a = GROUP_FIRST_ELEMENT (vinfo_for_stmt (stmt_a));
>>>> if (dr_group_first_a)
>>>> - {
>>>> + {
>>>> stmt_a = dr_group_first_a;
>>>> dr_a = STMT_VINFO_DATA_REF (vinfo_for_stmt (stmt_a));
>>>> }
>>>> @@ -2314,20 +2417,11 @@ vect_create_cond_for_alias_checks (loop_
>>>> stmt_b = DR_STMT (DDR_B (ddr));
>>>> dr_group_first_b = GROUP_FIRST_ELEMENT (vinfo_for_stmt (stmt_b));
>>>> if (dr_group_first_b)
>>>> - {
>>>> + {
>>>> stmt_b = dr_group_first_b;
>>>> dr_b = STMT_VINFO_DATA_REF (vinfo_for_stmt (stmt_b));
>>>> }
>>>>
>>>> - addr_base_a
>>>> - = fold_build_pointer_plus (DR_BASE_ADDRESS (dr_a),
>>>> - size_binop (PLUS_EXPR, DR_OFFSET (dr_a),
>>>> - DR_INIT (dr_a)));
>>>> - addr_base_b
>>>> - = fold_build_pointer_plus (DR_BASE_ADDRESS (dr_b),
>>>> - size_binop (PLUS_EXPR, DR_OFFSET (dr_b),
>>>> - DR_INIT (dr_b)));
>>>> -
>>>> if (!operand_equal_p (DR_STEP (dr_a), DR_STEP (dr_b), 0))
>>>> length_factor = scalar_loop_iters;
>>>> else
>>>> @@ -2335,24 +2429,149 @@ vect_create_cond_for_alias_checks (loop_
>>>> segment_length_a = vect_vfa_segment_size (dr_a, length_factor);
>>>> segment_length_b = vect_vfa_segment_size (dr_b, length_factor);
>>>>
>>>> + dr_addr_with_seg_len_pair_t dr_with_seg_len_pair
>>>> + (dr_addr_with_seg_len
>>>> + (dr_a, DR_BASE_ADDRESS (dr_a),
>>>> + size_binop (PLUS_EXPR, DR_OFFSET (dr_a), DR_INIT (dr_a)),
>>>> + segment_length_a),
>>>> + dr_addr_with_seg_len
>>>> + (dr_b, DR_BASE_ADDRESS (dr_b),
>>>> + size_binop (PLUS_EXPR, DR_OFFSET (dr_b), DR_INIT (dr_b)),
>>>> + segment_length_b));
>>>> +
>>>> + if (dr_with_seg_len_pair.first.basic_addr >
>>>> + dr_with_seg_len_pair.second.basic_addr)
>>>> + std::swap (dr_with_seg_len_pair.first, dr_with_seg_len_pair.second);
>>>> +
>>>> + ddrs_with_seg_len.push_back (dr_with_seg_len_pair);
>>>> + }
>>>> +
>>>> + /* Second, we sort the collected data ref pairs so that we can scan
>>>> + them once to combine all possible aliasing checks. */
>>>> +
>>>> + std::sort (ddrs_with_seg_len.begin(), ddrs_with_seg_len.end());
>>>> +
>>>> + /* Remove duplicate data ref pairs. */
>>>> + ddrs_with_seg_len.erase (std::unique (ddrs_with_seg_len.begin(),
>>>> + ddrs_with_seg_len.end()),
>>>> + ddrs_with_seg_len.end());
>>>> +
>>>> + /* We then scan the sorted dr pairs and check if we can combine
>>>> + alias checks of two neighbouring dr pairs. */
>>>> +
>>>> + for (size_t i = 1; i < ddrs_with_seg_len.size (); ++i)
>>>> + {
>>>> + dr_addr_with_seg_len& dr_a1 = ddrs_with_seg_len[i-1].first;
>>>> + dr_addr_with_seg_len& dr_b1 = ddrs_with_seg_len[i-1].second;
>>>> + dr_addr_with_seg_len& dr_a2 = ddrs_with_seg_len[i].first;
>>>> + dr_addr_with_seg_len& dr_b2 = ddrs_with_seg_len[i].second;
>>>> +
>>>> + if (dr_a1 == dr_a2)
>>>> + {
>>>> + if (dr_b1.basic_addr != dr_b2.basic_addr
>>>> + || TREE_CODE (dr_b1.offset) != INTEGER_CST
>>>> + || TREE_CODE (dr_b2.offset) != INTEGER_CST)
>>>> + continue;
>>>> +
>>>> + int diff = int_cst_value (dr_b2.offset) -
>>>> + int_cst_value (dr_b1.offset);
>>>> +
>>>> + gcc_assert (diff > 0);
>>>> +
>>>> + if (diff <= vect_factor
>>>> + || (TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>>> + && diff - int_cst_value (dr_b1.seg_len) < vect_factor)
>>>> + || (TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>>> + && TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>>> + && diff - int_cst_value (dr_b1.seg_len) <
>>>> + int_cst_value (dr_a1.seg_len)))
>>>> + {
>>>> + if (dump_enabled_p ())
>>>> + {
>>>> + dump_printf_loc
>>>> + (MSG_NOTE, vect_location,
>>>> + "combining two runtime checks for data references ");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b1.dr));
>>>> + dump_printf (MSG_NOTE, " and ");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b2.dr));
>>>> + dump_printf (MSG_NOTE, "\n");
>>>> + }
>>>> +
>>>> + dr_b1.seg_len = size_binop (PLUS_EXPR,
>>>> + dr_b2.seg_len, size_int (diff));
>>>> + ddrs_with_seg_len.erase (ddrs_with_seg_len.begin () + i);
>>>> + --i;
>>>> + }
>>>> + }
>>>> + else if (dr_b1 == dr_b2)
>>>> + {
>>>> + if (dr_a1.basic_addr != dr_a2.basic_addr
>>>> + || TREE_CODE (dr_a1.offset) != INTEGER_CST
>>>> + || TREE_CODE (dr_a2.offset) != INTEGER_CST)
>>>> + continue;
>>>> +
>>>> + int diff = int_cst_value (dr_a2.offset) -
>>>> + int_cst_value (dr_a1.offset);
>>>> +
>>>> + gcc_assert (diff > 0);
>>>> +
>>>> + if (diff <= vect_factor
>>>> + || (TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>>> + && diff - int_cst_value (dr_a1.seg_len) < vect_factor)
>>>> + || (TREE_CODE (dr_a1.seg_len) == INTEGER_CST
>>>> + && TREE_CODE (dr_b1.seg_len) == INTEGER_CST
>>>> + && diff - int_cst_value (dr_a1.seg_len) <
>>>> + int_cst_value (dr_b1.seg_len)))
>>>> + {
>>>> + if (dump_enabled_p ())
>>>> + {
>>>> + dump_printf_loc
>>>> + (MSG_NOTE, vect_location,
>>>> + "combining two runtime checks for data references ");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a1.dr));
>>>> + dump_printf (MSG_NOTE, " and ");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a2.dr));
>>>> + dump_printf (MSG_NOTE, "\n");
>>>> + }
>>>> +
>>>> + dr_a1.seg_len = size_binop (PLUS_EXPR,
>>>> + dr_a2.seg_len, size_int (diff));
>>>> + ddrs_with_seg_len.erase (ddrs_with_seg_len.begin () + i);
>>>> + --i;
>>>> + }
>>>> + }
>>>> + }
>>>> +
>>>> + for (size_t i = 0, s = ddrs_with_seg_len.size (); i < s; ++i)
>>>> + {
>>>> + const dr_addr_with_seg_len& dr_a = ddrs_with_seg_len[i].first;
>>>> + const dr_addr_with_seg_len& dr_b = ddrs_with_seg_len[i].second;
>>>> + tree segment_length_a = dr_a.seg_len;
>>>> + tree segment_length_b = dr_b.seg_len;
>>>> +
>>>> + tree addr_base_a
>>>> + = fold_build_pointer_plus (dr_a.basic_addr, dr_a.offset);
>>>> + tree addr_base_b
>>>> + = fold_build_pointer_plus (dr_b.basic_addr, dr_b.offset);
>>>> +
>>>> if (dump_enabled_p ())
>>>> {
>>>> dump_printf_loc (MSG_NOTE, vect_location,
>>>> - "create runtime check for data references ");
>>>> - dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a));
>>>> + "create runtime check for data references ");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_a.dr));
>>>> dump_printf (MSG_NOTE, " and ");
>>>> - dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b));
>>>> - dump_printf (MSG_NOTE, "\n");
>>>> + dump_generic_expr (MSG_NOTE, TDF_SLIM, DR_REF (dr_b.dr));
>>>> + dump_printf (MSG_NOTE, "\n");
>>>> }
>>>>
>>>> - seg_a_min = addr_base_a;
>>>> - seg_a_max = fold_build_pointer_plus (addr_base_a, segment_length_a);
>>>> - if (tree_int_cst_compare (DR_STEP (dr_a), size_zero_node) < 0)
>>>> + tree seg_a_min = addr_base_a;
>>>> + tree seg_a_max = fold_build_pointer_plus (addr_base_a,
>>>> segment_length_a);
>>>> + if (tree_int_cst_compare (DR_STEP (dr_a.dr), size_zero_node) < 0)
>>>> seg_a_min = seg_a_max, seg_a_max = addr_base_a;
>>>>
>>>> - seg_b_min = addr_base_b;
>>>> - seg_b_max = fold_build_pointer_plus (addr_base_b, segment_length_b);
>>>> - if (tree_int_cst_compare (DR_STEP (dr_b), size_zero_node) < 0)
>>>> + tree seg_b_min = addr_base_b;
>>>> + tree seg_b_max = fold_build_pointer_plus (addr_base_b,
>>>> segment_length_b);
>>>> + if (tree_int_cst_compare (DR_STEP (dr_b.dr), size_zero_node) < 0)
>>>> seg_b_min = seg_b_max, seg_b_max = addr_base_b;
>>>>
>>>> part_cond_expr =
>>>> @@ -2477,6 +2696,81 @@ vect_loop_versioning (loop_vec_info loop
>>>> adjust_phi_and_debug_stmts (orig_phi, e, PHI_RESULT (new_phi));
>>>> }
>>>>
>>>> + /* Extract load and store statements on pointers with zero-stride
>>>> + accesses. */
>>>> + if (LOOP_REQUIRES_VERSIONING_FOR_ALIAS (loop_vinfo))
>>>> + {
>>>> +
>>>> + /* In the loop body, we iterate each statement to check if it is a
>>>> load
>>>> + or store. Then we check the DR_STEP of the data reference. If
>>>> + DR_STEP is zero, then we will hoist the load statement to the loop
>>>> + preheader, and move the store statement to the loop exit. */
>>>> +
>>>> + for (gimple_stmt_iterator si = gsi_start_bb (loop->header);
>>>> + !gsi_end_p (si); )
>>>> + {
>>>> + gimple stmt = gsi_stmt (si);
>>>> + stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
>>>> + struct data_reference *dr = STMT_VINFO_DATA_REF (stmt_info);
>>>> +
>>>> +
>>>> + if (dr && integer_zerop (DR_STEP (dr)))
>>>> + {
>>>> + if (DR_IS_READ (dr))
>>>> + {
>>>> + if (dump_file)
>>>> + {
>>>> + fprintf (dump_file,
>>>> + "Hoist the load to outside of the loop:\n");
>>>> + print_gimple_stmt (dump_file, stmt, 0,
>>>> + TDF_VOPS|TDF_MEMSYMS);
>>>> + }
>>>> +
>>>> + basic_block preheader = loop_preheader_edge (loop)->src;
>>>> + gimple_stmt_iterator si_dst = gsi_last_bb (preheader);
>>>> + gsi_move_after (&si, &si_dst);
>>>> + }
>>>> + else
>>>> + {
>>>> + gimple_stmt_iterator si_dst =
>>>> + gsi_last_bb (single_exit (loop)->dest);
>>>> + gsi_move_after (&si, &si_dst);
>>>> + }
>>>> + continue;
>>>> + }
>>>> + else if (!dr)
>>>> + {
>>>> + bool hoist = true;
>>>> + for (size_t i = 0; i < gimple_num_ops (stmt); i++)
>>>> + {
>>>> + tree op = gimple_op (stmt, i);
>>>> + if (TREE_CODE (op) == INTEGER_CST
>>>> + || TREE_CODE (op) == REAL_CST)
>>>> + continue;
>>>> + if (TREE_CODE (op) == SSA_NAME)
>>>> + {
>>>> + gimple def = SSA_NAME_DEF_STMT (op);
>>>> + if (def == stmt
>>>> + || gimple_nop_p (def)
>>>> + || !flow_bb_inside_loop_p (loop, gimple_bb (def)))
>>>> + continue;
>>>> + }
>>>> + hoist = false;
>>>> + break;
>>>> + }
>>>> +
>>>> + if (hoist)
>>>> + {
>>>> + basic_block preheader = loop_preheader_edge (loop)->src;
>>>> + gimple_stmt_iterator si_dst = gsi_last_bb (preheader);
>>>> + gsi_move_after (&si, &si_dst);
>>>> + continue;
>>>> + }
>>>> + }
>>>> + gsi_next (&si);
>>>> + }
>>>> + }
>>>> +
>>>> /* End loop-exit-fixes after versioning. */
>>>>
>>>> if (cond_expr_stmt_list)
>>>> Index: gcc/ChangeLog
>>>> ===================================================================
>>>> --- gcc/ChangeLog (revision 202663)
>>>> +++ gcc/ChangeLog (working copy)
>>>> @@ -1,3 +1,8 @@
>>>> +2013-10-01 Cong Hou <co...@google.com>
>>>> +
>>>> + * tree-vect-loop-manip.c (vect_create_cond_for_alias_checks): Combine
>>>> + alias checks if it is possible to amortize the runtime overhead.
>>>> +
>>>>
>>>>
>>>
>>> --
>>> Richard Biener <rguent...@suse.de>
>>> SUSE / SUSE Labs
>>> SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
>>> GF: Jeff Hawn, Jennifer Guild, Felix Imend
