Pool alignment information for common bases

This patch is a follow-on to the fix for PR81136.  The testcase for that
PR shows that we can (correctly) calculate different base alignments
for two data_references but still tell that their misalignments wrt the
vector size are equal.  This is because we calculate the base alignments
for each dr individually, without looking at the other drs, and in
general the alignment we calculate is only guaranteed if the dr's DR_REF
actually occurs.

This is working as designed, but it does expose a missed opportunity.
We know that if a vectorised loop is reached, all statements in that
loop execute at least once, so it should be safe to pool the alignment
information for all the statements we're vectorising.  The only catch is
that DR_REFs for masked loads and stores only occur if the mask value is
nonzero.  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 guarantee that ptr points to a "struct s" if at least
one c[i] is true.

This patch adds a DR_IS_CONDITIONAL_IN_STMT flag to record whether
the DR_REF is guaranteed to occur every time that the statement
executes to completion.  It then pools the alignment information
for references that aren't conditional in this sense.

2017-08-04  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_record_base_alignments): Declare.
	* tree-data-ref.h (data_reference): Add an is_conditional_in_stmt
	field.
	(DR_IS_CONDITIONAL_IN_STMT): New macro.
	(create_data_ref): Add an is_conditional_in_stmt argument.
	* tree-data-ref.c (create_data_ref): Likewise.  Use it to initialize
	the is_conditional_in_stmt field.
	(data_ref_loc): Add an is_conditional_in_stmt field.
	(get_references_in_stmt): Set the is_conditional_in_stmt 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_record_base_alignment): New function.
	(vect_record_base_alignments): Likewise.
	(vect_compute_data_ref_alignment): Adjust base_addr and aligned_to
	for nested statements even if we fail to compute a misalignment.
	Use pooled base alignments for unconditional references.
	(vect_find_same_alignment_drs): Compare base addresses instead
	of base objects.
	(vect_analyze_data_refs_alignment): Call vect_record_base_alignments.
	* tree-vect-slp.c (vect_slp_analyze_bb_1): Likewise.

gcc/testsuite/
	PR tree-optimization/81136
	* gcc.dg/vect/pr81136.c: Add scan test.

From-SVN: r250870

1 files changed, 11 insertions, 0 deletions

diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index e8acf66..4ee3c3f 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -22,6 +22,7 @@ along with GCC; see the file COPYING3.  If not see
 #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 +85,11 @@ struct stmt_info_for_cost {
 
 typedef vec<stmt_info_for_cost> stmt_vector_for_cost;
 
+/* Maps base addresses to an innermost_loop_behavior that gives the maximum
+   known alignment for that base.  */
+typedef hash_map<tree_operand_hash,
+		 innermost_loop_behavior *> vec_base_alignments;
+
 /************************************************************************
   SLP
  ************************************************************************/
@@ -169,6 +175,10 @@ struct vec_info {
   /* All data references.  Freed by free_data_refs, so not an auto_vec.  */
   vec<data_reference_p> datarefs;
 
+  /* Maps base addresses to an innermost_loop_behavior that gives the maximum
+     known alignment for that base.  */
+  vec_base_alignments base_alignments;
+
   /* All data dependences.  Freed by free_dependence_relations, so not
      an auto_vec.  */
   vec<ddr_p> ddrs;
@@ -1162,6 +1172,7 @@ extern bool vect_prune_runtime_alias_test_list (loop_vec_info);
 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_record_base_alignments (vec_info *);
 extern tree vect_create_data_ref_ptr (gimple *, tree, struct loop *, tree,
 				      tree *, gimple_stmt_iterator *,
 				      gimple **, bool, bool *,