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Re: PR81136: ICE from inconsistent DR_MISALIGNMENTs


On Thu, Jun 22, 2017 at 1:30 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> The test case triggered this assert in vect_update_misalignment_for_peel:
>
>           gcc_assert (DR_MISALIGNMENT (dr) / dr_size ==
>                       DR_MISALIGNMENT (dr_peel) / dr_peel_size);
>
> We knew that the two DRs had the same misalignment at runtime, but when
> considered in isolation, one data reference guaranteed a higher compile-time
> base alignment than the other.
>
> In the test case this looks like a missed opportunity.  Both references
> are unconditional, so it should be possible to use the highest of the
> available base alignment guarantees when analyzing each reference.
> The patch does this.
>
> However, as the comment in the patch says, the base alignment guarantees
> provided by a conditional reference only apply if the reference occurs
> at least once.  In this case it would be legitimate for two references
> to have the same runtime misalignment and for one reference to provide a
> stronger compile-time guarantee than the other about what the misalignment
> actually is.  The patch therefore relaxes the assert to handle that case.

Hmm, but you don't actually check whether a reference occurs only conditional,
do you?  You just seem to say that for masked loads/stores the reference
is conditional (I believe that's not true).  But for a loop like

 for (;;)
   if (a[i])
     sum += b[j];

you still assume b[j] executes unconditionally?

The vectorizer of course only sees unconditionally executed stmts.

So - I'd simply not add this DR_IS_CONDITIONAL.  Did you run into
any real-world (testsuite) issues without this?

Note that the assert is to prevent bogus information.  Iff we aligned
DR with base alignment 8 and misalign 3 then if another same-align
DR has base alignment 16 we can't simply zero its DR_MISALIGNMENT
as it still can be 8 after aligning DR.

So I think it's wrong to put DRs with differing base-alignment into
the same-align-refs chain, those should get their DR_MISALIGNMENT
updated independenlty after peeling.

I'd rather not mix fixing this with the improvement to eventuall use a
larger align for the other DR if possible.

Thanks,
Richard.

> Tested on powerpc64-linux-gnu, aarch64-linux-gnu and x86_64-linux-gnu.
> OK to instal?
>
> Richard
>
>
> 2017-06-22  Richard Sandiford  <richard.sandiford@linaro.org>
>
> gcc/
>         PR tree-optimization/81136
>         * tree-vectorizer.h: Include tree-hash-traits.h.
>         (vec_base_alignments): New typedef.
>         (vec_info): Add a base_alignments field.
>         (vect_compute_base_alignments: Declare.
>         * tree-data-ref.h (data_reference): Add an is_conditional field.
>         (DR_IS_CONDITIONAL): New macro.
>         (create_data_ref): Add an is_conditional argument.
>         * tree-data-ref.c (create_data_ref): Likewise.  Use it to initialize
>         the is_conditional field.
>         (data_ref_loc): Add an is_conditional field.
>         (get_references_in_stmt): Set the is_conditional field.
>         (find_data_references_in_stmt): Update call to create_data_ref.
>         (graphite_find_data_references_in_stmt): Likewise.
>         * tree-ssa-loop-prefetch.c (determine_loop_nest_reuse): Likewise.
>         * tree-vect-data-refs.c (vect_analyze_data_refs): Likewise.
>         (vect_get_base_address): New function.
>         (vect_compute_base_alignments): Likewise.
>         (vect_compute_base_alignment): Likewise, split out from...
>         (vect_compute_data_ref_alignment): ...here.  Use precomputed
>         base alignments.  Only compute a new base alignment here if the
>         reference is conditional.
>         (vect_update_misalignment_for_peel): Allow the compile-time
>         DR_MISALIGNMENTs of two references with the same runtime alignment
>         to be different if one of the references is conditional.
>         (vect_find_same_alignment_drs): Compare base addresses instead
>         of base objects.
>         (vect_compute_data_ref_alignment): Call vect_compute_base_alignments.
>         * tree-vect-slp.c (vect_slp_analyze_bb_1): Likewise.
>         (new_bb_vec_info): Initialize base_alignments.
>         * tree-vect-loop.c (new_loop_vec_info): Likewise.
>         * tree-vectorizer.c (vect_destroy_datarefs): Release base_alignments.
>
> gcc/testsuite/
>         PR tree-optimization/81136
>         * gcc.dg/vect/pr81136.c: New test.
>
> Index: gcc/tree-vectorizer.h
> ===================================================================
> --- gcc/tree-vectorizer.h       2017-06-08 08:51:43.347264181 +0100
> +++ gcc/tree-vectorizer.h       2017-06-22 12:23:21.288421018 +0100
> @@ -22,6 +22,7 @@ Software Foundation; either version 3, o
>  #define GCC_TREE_VECTORIZER_H
>
>  #include "tree-data-ref.h"
> +#include "tree-hash-traits.h"
>  #include "target.h"
>
>  /* Used for naming of new temporaries.  */
> @@ -84,6 +85,10 @@ struct stmt_info_for_cost {
>
>  typedef vec<stmt_info_for_cost> stmt_vector_for_cost;
>
> +/* Maps base addresses to the largest alignment that we've been able
> +   to calculate for them.  */
> +typedef hash_map<tree_operand_hash, unsigned int> vec_base_alignments;
> +
>  /************************************************************************
>    SLP
>   ************************************************************************/
> @@ -156,6 +161,10 @@ struct vec_info {
>    /* All data references.  */
>    vec<data_reference_p> datarefs;
>
> +  /* Maps the base addresses of all data references in DATAREFS to the
> +     largest alignment that we've been able to calculate for them.  */
> +  vec_base_alignments base_alignments;
> +
>    /* All data dependences.  */
>    vec<ddr_p> ddrs;
>
> @@ -1117,6 +1126,7 @@ extern bool vect_prune_runtime_alias_tes
>  extern bool vect_check_gather_scatter (gimple *, loop_vec_info,
>                                        gather_scatter_info *);
>  extern bool vect_analyze_data_refs (vec_info *, int *);
> +extern void vect_compute_base_alignments (vec_info *);
>  extern tree vect_create_data_ref_ptr (gimple *, tree, struct loop *, tree,
>                                       tree *, gimple_stmt_iterator *,
>                                       gimple **, bool, bool *,
> Index: gcc/tree-data-ref.h
> ===================================================================
> --- gcc/tree-data-ref.h 2017-06-08 08:51:43.349263895 +0100
> +++ gcc/tree-data-ref.h 2017-06-22 12:23:21.285421180 +0100
> @@ -119,6 +119,10 @@ struct data_reference
>    /* True when the data reference is in RHS of a stmt.  */
>    bool is_read;
>
> +  /* True when the data reference is conditional, i.e. if it might not
> +     occur even when the statement runs to completion.  */
> +  bool is_conditional;
> +
>    /* Behavior of the memory reference in the innermost loop.  */
>    struct innermost_loop_behavior innermost;
>
> @@ -138,6 +142,7 @@ #define DR_ACCESS_FN(DR, I)        DR_AC
>  #define DR_NUM_DIMENSIONS(DR)      DR_ACCESS_FNS (DR).length ()
>  #define DR_IS_READ(DR)             (DR)->is_read
>  #define DR_IS_WRITE(DR)            (!DR_IS_READ (DR))
> +#define DR_IS_CONDITIONAL(DR)      (DR)->is_conditional
>  #define DR_BASE_ADDRESS(DR)        (DR)->innermost.base_address
>  #define DR_OFFSET(DR)              (DR)->innermost.offset
>  #define DR_INIT(DR)                (DR)->innermost.init
> @@ -350,7 +355,8 @@ extern bool graphite_find_data_reference
>                                                    vec<data_reference_p> *);
>  tree find_data_references_in_loop (struct loop *, vec<data_reference_p> *);
>  bool loop_nest_has_data_refs (loop_p loop);
> -struct data_reference *create_data_ref (loop_p, loop_p, tree, gimple *, bool);
> +struct data_reference *create_data_ref (loop_p, loop_p, tree, gimple *, bool,
> +                                       bool);
>  extern bool find_loop_nest (struct loop *, vec<loop_p> *);
>  extern struct data_dependence_relation *initialize_data_dependence_relation
>       (struct data_reference *, struct data_reference *, vec<loop_p>);
> Index: gcc/tree-data-ref.c
> ===================================================================
> --- gcc/tree-data-ref.c 2017-06-08 08:51:43.349263895 +0100
> +++ gcc/tree-data-ref.c 2017-06-22 12:23:21.284421233 +0100
> @@ -1053,15 +1053,18 @@ free_data_ref (data_reference_p dr)
>    free (dr);
>  }
>
> -/* Analyzes memory reference MEMREF accessed in STMT.  The reference
> -   is read if IS_READ is true, write otherwise.  Returns the
> -   data_reference description of MEMREF.  NEST is the outermost loop
> -   in which the reference should be instantiated, LOOP is the loop in
> -   which the data reference should be analyzed.  */
> +/* Analyze memory reference MEMREF, which is accessed in STMT.  The reference
> +   is a read if IS_READ is true, otherwise it is a write.  IS_CONDITIONAL
> +   indicates that the reference is conditional, i.e. that it might not
> +   occur every time that STMT runs to completion.
> +
> +   Return the data_reference description of MEMREF.  NEST is the outermost
> +   loop in which the reference should be instantiated, LOOP is the loop
> +   in which the data reference should be analyzed.  */
>
>  struct data_reference *
>  create_data_ref (loop_p nest, loop_p loop, tree memref, gimple *stmt,
> -                bool is_read)
> +                bool is_read, bool is_conditional)
>  {
>    struct data_reference *dr;
>
> @@ -1076,6 +1079,7 @@ create_data_ref (loop_p nest, loop_p loo
>    DR_STMT (dr) = stmt;
>    DR_REF (dr) = memref;
>    DR_IS_READ (dr) = is_read;
> +  DR_IS_CONDITIONAL (dr) = is_conditional;
>
>    dr_analyze_innermost (dr, nest);
>    dr_analyze_indices (dr, nest, loop);
> @@ -4446,6 +4450,10 @@ struct data_ref_loc
>
>    /* True if the memory reference is read.  */
>    bool is_read;
> +
> +  /* True if the data reference is conditional, i.e. if it might not
> +     occur even when the statement runs to completion.  */
> +  bool is_conditional;
>  };
>
>
> @@ -4512,6 +4520,7 @@ get_references_in_stmt (gimple *stmt, ve
>         {
>           ref.ref = op1;
>           ref.is_read = true;
> +         ref.is_conditional = false;
>           references->safe_push (ref);
>         }
>      }
> @@ -4539,6 +4548,7 @@ get_references_in_stmt (gimple *stmt, ve
>               type = TREE_TYPE (gimple_call_arg (stmt, 3));
>             if (TYPE_ALIGN (type) != align)
>               type = build_aligned_type (type, align);
> +           ref.is_conditional = true;
>             ref.ref = fold_build2 (MEM_REF, type, gimple_call_arg (stmt, 0),
>                                    ptr);
>             references->safe_push (ref);
> @@ -4558,6 +4568,7 @@ get_references_in_stmt (gimple *stmt, ve
>             {
>               ref.ref = op1;
>               ref.is_read = true;
> +             ref.is_conditional = false;
>               references->safe_push (ref);
>             }
>         }
> @@ -4571,6 +4582,7 @@ get_references_in_stmt (gimple *stmt, ve
>      {
>        ref.ref = op0;
>        ref.is_read = false;
> +      ref.is_conditional = false;
>        references->safe_push (ref);
>      }
>    return clobbers_memory;
> @@ -4635,8 +4647,8 @@ find_data_references_in_stmt (struct loo
>
>    FOR_EACH_VEC_ELT (references, i, ref)
>      {
> -      dr = create_data_ref (nest, loop_containing_stmt (stmt),
> -                           ref->ref, stmt, ref->is_read);
> +      dr = create_data_ref (nest, loop_containing_stmt (stmt), ref->ref,
> +                           stmt, ref->is_read, ref->is_conditional);
>        gcc_assert (dr != NULL);
>        datarefs->safe_push (dr);
>      }
> @@ -4665,7 +4677,8 @@ graphite_find_data_references_in_stmt (l
>
>    FOR_EACH_VEC_ELT (references, i, ref)
>      {
> -      dr = create_data_ref (nest, loop, ref->ref, stmt, ref->is_read);
> +      dr = create_data_ref (nest, loop, ref->ref, stmt, ref->is_read,
> +                           ref->is_conditional);
>        gcc_assert (dr != NULL);
>        datarefs->safe_push (dr);
>      }
> Index: gcc/tree-ssa-loop-prefetch.c
> ===================================================================
> --- gcc/tree-ssa-loop-prefetch.c        2017-06-07 21:58:55.928557601 +0100
> +++ gcc/tree-ssa-loop-prefetch.c        2017-06-22 12:23:21.285421180 +0100
> @@ -1633,7 +1633,7 @@ determine_loop_nest_reuse (struct loop *
>      for (ref = gr->refs; ref; ref = ref->next)
>        {
>         dr = create_data_ref (nest, loop_containing_stmt (ref->stmt),
> -                             ref->mem, ref->stmt, !ref->write_p);
> +                             ref->mem, ref->stmt, !ref->write_p, false);
>
>         if (dr)
>           {
> Index: gcc/tree-vect-data-refs.c
> ===================================================================
> --- gcc/tree-vect-data-refs.c   2017-06-08 08:51:43.350263752 +0100
> +++ gcc/tree-vect-data-refs.c   2017-06-22 12:23:21.286421126 +0100
> @@ -646,6 +646,102 @@ vect_slp_analyze_instance_dependence (sl
>    return res;
>  }
>
> +/* If DR is nested in a loop that is being vectorized, return the base
> +   address in the context of the vectorized loop (rather than the
> +   nested loop).  Otherwise return the base address in the context
> +   of the containing statement.  */
> +
> +static tree
> +vect_get_base_address (data_reference *dr)
> +{
> +  gimple *stmt = DR_STMT (dr);
> +  stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
> +  loop_vec_info loop_vinfo = STMT_VINFO_LOOP_VINFO (stmt_info);
> +  struct loop *loop = loop_vinfo != NULL ? LOOP_VINFO_LOOP (loop_vinfo) : NULL;
> +  if (loop && nested_in_vect_loop_p (loop, stmt))
> +    return STMT_VINFO_DR_BASE_ADDRESS (stmt_info);
> +  else
> +    return DR_BASE_ADDRESS (dr);
> +}
> +
> +/* Compute and return the alignment of base address BASE_ADDR in DR.  */
> +
> +static unsigned int
> +vect_compute_base_alignment (data_reference *dr, tree base_addr)
> +{
> +  /* To look at the alignment of the base we have to preserve an inner
> +     MEM_REF as that carries the alignment information of the actual
> +     access.  */
> +  tree base = DR_REF (dr);
> +  while (handled_component_p (base))
> +    base = TREE_OPERAND (base, 0);
> +  unsigned int base_alignment = 0;
> +  unsigned HOST_WIDE_INT base_bitpos;
> +  get_object_alignment_1 (base, &base_alignment, &base_bitpos);
> +
> +  /* As data-ref analysis strips the MEM_REF down to its base operand
> +     to form DR_BASE_ADDRESS and adds the offset to DR_INIT we have to
> +     adjust things to make base_alignment valid as the alignment of
> +     DR_BASE_ADDRESS.  */
> +  if (TREE_CODE (base) == MEM_REF)
> +    {
> +      /* Note all this only works if DR_BASE_ADDRESS is the same as
> +        MEM_REF operand zero, otherwise DR/SCEV analysis might have factored
> +        in other offsets.  We need to rework DR to compute the alingment
> +        of DR_BASE_ADDRESS as long as all information is still available.  */
> +      if (operand_equal_p (TREE_OPERAND (base, 0), base_addr, 0))
> +       {
> +         base_bitpos -= mem_ref_offset (base).to_short_addr () * BITS_PER_UNIT;
> +         base_bitpos &= (base_alignment - 1);
> +       }
> +      else
> +       base_bitpos = BITS_PER_UNIT;
> +    }
> +  if (base_bitpos != 0)
> +    base_alignment = base_bitpos & -base_bitpos;
> +
> +  /* Also look at the alignment of the base address DR analysis
> +     computed.  */
> +  unsigned int base_addr_alignment = get_pointer_alignment (base_addr);
> +  if (base_addr_alignment > base_alignment)
> +    base_alignment = base_addr_alignment;
> +
> +  return base_alignment;
> +}
> +
> +/* Compute alignments for the base addresses of all datarefs in VINFO.  */
> +
> +void
> +vect_compute_base_alignments (vec_info *vinfo)
> +{
> +  /* If the region we're going to vectorize is reached, all unconditional
> +     data references occur at least once.  We can therefore pool the base
> +     alignment guarantees from each unconditional reference.  */
> +  data_reference *dr;
> +  unsigned int i;
> +  FOR_EACH_VEC_ELT (vinfo->datarefs, i, dr)
> +    if (!DR_IS_CONDITIONAL (dr))
> +      {
> +       tree base_addr = vect_get_base_address (dr);
> +       unsigned int alignment = vect_compute_base_alignment (dr, base_addr);
> +       bool existed;
> +       unsigned int &entry
> +         = vinfo->base_alignments.get_or_insert (base_addr, &existed);
> +       if (!existed || entry < alignment)
> +         {
> +           entry = alignment;
> +           if (dump_enabled_p ())
> +             {
> +               dump_printf_loc (MSG_NOTE, vect_location,
> +                                "setting base alignment for ");
> +               dump_generic_expr (MSG_NOTE, TDF_SLIM, base_addr);
> +               dump_printf (MSG_NOTE, " to %d, based on ", alignment);
> +               dump_gimple_stmt (MSG_NOTE, TDF_SLIM, DR_STMT (dr), 0);
> +             }
> +         }
> +      }
> +}
> +
>  /* Function vect_compute_data_ref_alignment
>
>     Compute the misalignment of the data reference DR.
> @@ -663,6 +759,7 @@ vect_compute_data_ref_alignment (struct
>  {
>    gimple *stmt = DR_STMT (dr);
>    stmt_vec_info stmt_info = vinfo_for_stmt (stmt);
> +  vec_base_alignments *base_alignments = &stmt_info->vinfo->base_alignments;
>    loop_vec_info loop_vinfo = STMT_VINFO_LOOP_VINFO (stmt_info);
>    struct loop *loop = NULL;
>    tree ref = DR_REF (dr);
> @@ -699,6 +796,8 @@ vect_compute_data_ref_alignment (struct
>      {
>        tree step = DR_STEP (dr);
>
> +      base_addr = STMT_VINFO_DR_BASE_ADDRESS (stmt_info);
> +      aligned_to = STMT_VINFO_DR_ALIGNED_TO (stmt_info);
>        if (tree_fits_shwi_p (step)
>           && tree_to_shwi (step) % GET_MODE_SIZE (TYPE_MODE (vectype)) == 0)
>          {
> @@ -706,8 +805,6 @@ vect_compute_data_ref_alignment (struct
>              dump_printf_loc (MSG_NOTE, vect_location,
>                               "inner step divides the vector-size.\n");
>           misalign = STMT_VINFO_DR_INIT (stmt_info);
> -         aligned_to = STMT_VINFO_DR_ALIGNED_TO (stmt_info);
> -         base_addr = STMT_VINFO_DR_BASE_ADDRESS (stmt_info);
>          }
>        else
>         {
> @@ -738,39 +835,15 @@ vect_compute_data_ref_alignment (struct
>         }
>      }
>
> -  /* To look at alignment of the base we have to preserve an inner MEM_REF
> -     as that carries alignment information of the actual access.  */
> -  base = ref;
> -  while (handled_component_p (base))
> -    base = TREE_OPERAND (base, 0);
> +  /* Calculate the maximum of the pooled base address alignment and the
> +     alignment that we can compute for DR itself.  The latter should
> +     already be included in the former for unconditional references.  */
>    unsigned int base_alignment = 0;
> -  unsigned HOST_WIDE_INT base_bitpos;
> -  get_object_alignment_1 (base, &base_alignment, &base_bitpos);
> -  /* As data-ref analysis strips the MEM_REF down to its base operand
> -     to form DR_BASE_ADDRESS and adds the offset to DR_INIT we have to
> -     adjust things to make base_alignment valid as the alignment of
> -     DR_BASE_ADDRESS.  */
> -  if (TREE_CODE (base) == MEM_REF)
> -    {
> -      /* Note all this only works if DR_BASE_ADDRESS is the same as
> -        MEM_REF operand zero, otherwise DR/SCEV analysis might have factored
> -        in other offsets.  We need to rework DR to compute the alingment
> -        of DR_BASE_ADDRESS as long as all information is still available.  */
> -      if (operand_equal_p (TREE_OPERAND (base, 0), base_addr, 0))
> -       {
> -         base_bitpos -= mem_ref_offset (base).to_short_addr () * BITS_PER_UNIT;
> -         base_bitpos &= (base_alignment - 1);
> -       }
> -      else
> -       base_bitpos = BITS_PER_UNIT;
> -    }
> -  if (base_bitpos != 0)
> -    base_alignment = base_bitpos & -base_bitpos;
> -  /* Also look at the alignment of the base address DR analysis
> -     computed.  */
> -  unsigned int base_addr_alignment = get_pointer_alignment (base_addr);
> -  if (base_addr_alignment > base_alignment)
> -    base_alignment = base_addr_alignment;
> +  if (DR_IS_CONDITIONAL (dr))
> +    base_alignment = vect_compute_base_alignment (dr, base_addr);
> +  if (unsigned int *entry = base_alignments->get (base_addr))
> +    base_alignment = MAX (base_alignment, *entry);
> +  gcc_assert (base_alignment != 0);
>
>    if (base_alignment >= TYPE_ALIGN (TREE_TYPE (vectype)))
>      DR_VECT_AUX (dr)->base_element_aligned = true;
> @@ -906,8 +979,29 @@ vect_update_misalignment_for_peel (struc
>      {
>        if (current_dr != dr)
>          continue;
> -      gcc_assert (DR_MISALIGNMENT (dr) / dr_size ==
> -                  DR_MISALIGNMENT (dr_peel) / dr_peel_size);
> +      /* Any alignment guarantees provided by a reference only apply if
> +        the reference actually occurs.  For example, in:
> +
> +           struct s __attribute__((aligned(32))) {
> +             int misaligner;
> +             int array[N];
> +           };
> +
> +           int *ptr;
> +           for (int i = 0; i < n; ++i)
> +             ptr[i] = c[i] ? ((struct s *) (ptr - 1))->array[i] : 0;
> +
> +        we can only assume that ptr is part of a struct s if at least one
> +        c[i] is true.  This in turn means that we have a higher base
> +        alignment guarantee for the read from ptr (if it occurs) than for
> +        the write to ptr, and we cannot unconditionally carry the former
> +        over to the latter.  We still know that the two address values
> +        have the same misalignment, so if peeling has forced one of them
> +        to be aligned, the other must be too.  */
> +      gcc_assert (DR_IS_CONDITIONAL (dr_peel)
> +                 || DR_IS_CONDITIONAL (dr)
> +                 || (DR_MISALIGNMENT (dr) / dr_size
> +                     == DR_MISALIGNMENT (dr_peel) / dr_peel_size));
>        SET_DR_MISALIGNMENT (dr, 0);
>        return;
>      }
> @@ -2117,8 +2211,7 @@ vect_find_same_alignment_drs (struct dat
>    if (dra == drb)
>      return;
>
> -  if (!operand_equal_p (DR_BASE_OBJECT (dra), DR_BASE_OBJECT (drb),
> -                       OEP_ADDRESS_OF)
> +  if (!operand_equal_p (DR_BASE_ADDRESS (dra), DR_BASE_ADDRESS (drb), 0)
>        || !operand_equal_p (DR_OFFSET (dra), DR_OFFSET (drb), 0)
>        || !operand_equal_p (DR_STEP (dra), DR_STEP (drb), 0))
>      return;
> @@ -2176,6 +2269,7 @@ vect_analyze_data_refs_alignment (loop_v
>    vec<data_reference_p> datarefs = vinfo->datarefs;
>    struct data_reference *dr;
>
> +  vect_compute_base_alignments (vinfo);
>    FOR_EACH_VEC_ELT (datarefs, i, dr)
>      {
>        stmt_vec_info stmt_info = vinfo_for_stmt (DR_STMT (dr));
> @@ -3374,7 +3468,8 @@ vect_analyze_data_refs (vec_info *vinfo,
>             {
>               struct data_reference *newdr
>                 = create_data_ref (NULL, loop_containing_stmt (stmt),
> -                                  DR_REF (dr), stmt, maybe_scatter ? false : true);
> +                                  DR_REF (dr), stmt, !maybe_scatter,
> +                                  DR_IS_CONDITIONAL (dr));
>               gcc_assert (newdr != NULL && DR_REF (newdr));
>               if (DR_BASE_ADDRESS (newdr)
>                   && DR_OFFSET (newdr)
> Index: gcc/tree-vect-slp.c
> ===================================================================
> --- gcc/tree-vect-slp.c 2017-06-07 21:58:56.336475882 +0100
> +++ gcc/tree-vect-slp.c 2017-06-22 12:23:21.288421018 +0100
> @@ -2367,6 +2367,7 @@ new_bb_vec_info (gimple_stmt_iterator re
>    gimple_stmt_iterator gsi;
>
>    res = (bb_vec_info) xcalloc (1, sizeof (struct _bb_vec_info));
> +  new (&res->base_alignments) vec_base_alignments ();
>    res->kind = vec_info::bb;
>    BB_VINFO_BB (res) = bb;
>    res->region_begin = region_begin;
> @@ -2741,6 +2742,8 @@ vect_slp_analyze_bb_1 (gimple_stmt_itera
>        return NULL;
>      }
>
> +  vect_compute_base_alignments (bb_vinfo);
> +
>    /* Analyze and verify the alignment of data references and the
>       dependence in the SLP instances.  */
>    for (i = 0; BB_VINFO_SLP_INSTANCES (bb_vinfo).iterate (i, &instance); )
> Index: gcc/tree-vect-loop.c
> ===================================================================
> --- gcc/tree-vect-loop.c        2017-06-22 12:22:57.734313143 +0100
> +++ gcc/tree-vect-loop.c        2017-06-22 12:23:21.287421072 +0100
> @@ -1157,6 +1157,7 @@ new_loop_vec_info (struct loop *loop)
>    LOOP_VINFO_VECT_FACTOR (res) = 0;
>    LOOP_VINFO_LOOP_NEST (res) = vNULL;
>    LOOP_VINFO_DATAREFS (res) = vNULL;
> +  new (&res->base_alignments) vec_base_alignments ();
>    LOOP_VINFO_DDRS (res) = vNULL;
>    LOOP_VINFO_UNALIGNED_DR (res) = NULL;
>    LOOP_VINFO_MAY_MISALIGN_STMTS (res) = vNULL;
> Index: gcc/tree-vectorizer.c
> ===================================================================
> --- gcc/tree-vectorizer.c       2017-06-22 12:22:57.732313220 +0100
> +++ gcc/tree-vectorizer.c       2017-06-22 12:23:21.288421018 +0100
> @@ -370,6 +370,8 @@ vect_destroy_datarefs (vec_info *vinfo)
>        }
>
>    free_data_refs (vinfo->datarefs);
> +
> +  vinfo->base_alignments.~vec_base_alignments ();
>  }
>
>  /* A helper function to free scev and LOOP niter information, as well as
> Index: gcc/testsuite/gcc.dg/vect/pr81136.c
> ===================================================================
> --- /dev/null   2017-06-22 07:43:14.805493307 +0100
> +++ gcc/testsuite/gcc.dg/vect/pr81136.c 2017-06-22 12:23:21.283421287 +0100
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +
> +struct __attribute__((aligned (32)))
> +{
> +  char misaligner;
> +  int foo[100];
> +  int bar[100];
> +} *a;
> +
> +void
> +fn1 (int n)
> +{
> +  int *b = a->foo;
> +  for (int i = 0; i < n; i++)
> +    a->bar[i] = b[i];
> +}


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