On Fri, Jun 23, 2017 at 2:05 PM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> Richard Biener <richard.guent...@gmail.com> writes:
>> On Thu, Jun 22, 2017 at 1:30 PM, Richard Sandiford
>> <richard.sandif...@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?
>
> Maybe the documentation isn't clear enough, but DR_IS_CONDITIONAL
> was supposed to mean "even if the containing statement executes
> and runs to completion, the reference might not actually occur".
> The example above isn't conditional in that sense because the
> reference to b[j] does occur if the store is reached and completes.
>
> Masked loads and stores are conditional in that sense though.
> The reference only occurs if the mask is nonzero; the memory
> isn't touched otherwise. The functions are used to if-convert
> things like:
>
> for (...)
> a[i] = b[i] ? c[i] : d[i];
>
> where there's no guarantee that it's safe to access c[i] when !b[i]
> (or d[i] when b[i]). No reference occurs for an all-false mask.
But as you touch generic data-ref code here you should apply more
sensible semantics to DR_IS_CONDITIONAL than just marking
masked loads/stores but not DRs occuring inside BBs only executed
conditionally ...
>> 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?
>
> Dropping DR_IS_CONDITIONAL would cause us to make invalid alignment
> assumptions in silly corner cases. I could add a scan test for it,
> for targets with masked loads and stores. It wouldn't trigger
> an execution failure though because we assume that targets with
> masked loads and stores allow unaligned accesses:
>
> /* For now assume all conditional loads/stores support unaligned
> access without any special code. */
> if (is_gimple_call (stmt)
> && gimple_call_internal_p (stmt)
> && (gimple_call_internal_fn (stmt) == IFN_MASK_LOAD
> || gimple_call_internal_fn (stmt) == IFN_MASK_STORE))
> return dr_unaligned_supported;
>
> So the worst that would happen is that we'd supposedly peel for
> alignment, but actually misalign everything instead, and so make
> things slower rather than quicker.
>
>> 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.
>
> DR_MISALIGNMENT is relative to the vector alignment rather than
> the base alignment though. So:
We seem to use it that way, yes (looking at set_ptr_info_alignment
uses). So why not fix the assert then by capping the alignment/misalignment
we compute at this value as well? (and document this in the header
around DR_MISALIGNMENT)
Ideally we'd do alignment analysis independent of the vector size
though (for those stupid targets with multiple vector sizes to consider...).
> a) when looking for references *A1 and *A2 with the same alignment,
> we simply have to prove that A1 % vecalign == A2 % vecalign.
> This doesn't require any knowledge about the base alignment.
> If we break the addresses down as:
>
> A1 = BASE1 + REST1, REST1 = INIT1 + OFFSET1 + X * STEP1
> A2 = BASE2 + REST2, REST2 = INIT2 + OFFSET2 + X * STEP2
>
> and can prove that BASE1 == BASE2, the alignment of that base
> isn't important. We simply need to prove that REST1 % vecalign
> == REST2 % vecalign for all X.
>
> b) In the assert, we've peeled the loop so that DR_PEEL is guaranteed
> to be vector-aligned. If DR_PEEL is A1 in the example above, we have
> A1 % vecalign == 0, so A2 % vecalign must be 0 too. This again doesn't
> rely on the base alignment being known.
>
> What a high base alignment for DR_PEEL gives us is the ability to know
> at compile how many iterations need to be peeled to make DR_PEEL aligned.
> But the points above apply regardless of whether we know that value at
> compile time or not.
>
> In examples like the test case, we would have known at compile time that
> VF-1 iterations would need to be peeled if we'd picked the store as the
> DR_PEEL, but would have treated the number of peels as variable if we'd
> picked the load. The value calculated at runtime would still have been
> VF-1, it's just that the code wouldn't have been as efficient.
>
> One of the benefits of pooling the alignments for unconditional references
> is that it no longer matters which DR we pick: the number of peels will
> be a compile-time constant both ways.
>
> Thanks,
> Richard
>
>> I'd rather not mix fixing this with the improvement to eventuall use a
>> larger align for the other DR if possible.
^^^
So can you fix the ICE with capping base alignment / DR_MISALIGNMENT?
Richard.
>> 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.sandif...@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];
>>> +}
