AArch64: Do not increase the vect reduction latency by multiplying count [PR110625]

The new costs should only count reduction latency by multiplying count for
single_defuse_cycle.  For other situations, this will increase the reduction
latency a lot and miss vectorization opportunities.

Tested on aarch64-linux-gnu.

gcc/ChangeLog:

	PR target/110625
	* config/aarch64/aarch64.cc (count_ops): Only '* count' for
	single_defuse_cycle while counting reduction_latency.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr110625_1.c: New testcase.
	* gcc.target/aarch64/pr110625_2.c: New testcase.

3 files changed, 83 insertions, 4 deletions

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index 560e543..d4d7602 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -16767,6 +16767,22 @@ aarch64_adjust_stmt_cost (vect_cost_for_stmt kind, stmt_vec_info stmt_info,
   return stmt_cost;
 }
 
+/* Return true if STMT_INFO is part of a reduction that has the form:
+
+      r = r op ...;
+      r = r op ...;
+
+   with the single accumulator being read and written multiple times.  */
+static bool
+aarch64_force_single_cycle (vec_info *vinfo, stmt_vec_info stmt_info)
+{
+  if (!STMT_VINFO_LIVE_P (stmt_info))
+    return false;
+
+  auto reduc_info = info_for_reduction (vinfo, stmt_info);
+  return STMT_VINFO_FORCE_SINGLE_CYCLE (reduc_info);
+}
+
 /* COUNT, KIND and STMT_INFO are the same as for vector_costs::add_stmt_cost
    and they describe an operation in the body of a vector loop.  Record issue
    information relating to the vector operation in OPS.  */
@@ -16788,10 +16804,13 @@ aarch64_vector_costs::count_ops (unsigned int count, vect_cost_for_stmt kind,
     {
       unsigned int base
 	= aarch64_in_loop_reduction_latency (m_vinfo, stmt_info, m_vec_flags);
-
-      /* ??? Ideally we'd do COUNT reductions in parallel, but unfortunately
-	 that's not yet the case.  */
-      ops->reduction_latency = MAX (ops->reduction_latency, base * count);
+      if (aarch64_force_single_cycle (m_vinfo, stmt_info))
+	/* ??? Ideally we'd use a tree to reduce the copies down to 1 vector,
+	   and then accumulate that, but at the moment the loop-carried
+	   dependency includes all copies.  */
+	ops->reduction_latency = MAX (ops->reduction_latency, base * count);
+      else
+	ops->reduction_latency = MAX (ops->reduction_latency, base);
     }
 
   /* Assume that multiply-adds will become a single operation.  */
diff --git a/gcc/testsuite/gcc.target/aarch64/pr110625_1.c b/gcc/testsuite/gcc.target/aarch64/pr110625_1.c
new file mode 100644
index 0000000..0965cac
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr110625_1.c
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast -mcpu=neoverse-n2 -fdump-tree-vect-details -fno-tree-slp-vectorize" } */
+/* { dg-final { scan-tree-dump-not "reduction latency = 8" "vect" } } */
+
+/* Do not increase the vector body cost due to the incorrect reduction latency
+    Original vector body cost = 51
+    Scalar issue estimate:
+      ...
+      reduction latency = 2
+      estimated min cycles per iteration = 2.000000
+      estimated cycles per vector iteration (for VF 2) = 4.000000
+    Vector issue estimate:
+      ...
+      reduction latency = 8      <-- Too large
+      estimated min cycles per iteration = 8.000000
+    Increasing body cost to 102 because scalar code would issue more quickly
+      ...
+    missed:  cost model: the vector iteration cost = 102 divided by the scalar iteration cost = 44 is greater or equal to the vectorization factor = 2.
+    missed:  not vectorized: vectorization not profitable.  */
+
+typedef struct
+{
+  unsigned short m1, m2, m3, m4;
+} the_struct_t;
+typedef struct
+{
+  double m1, m2, m3, m4, m5;
+} the_struct2_t;
+
+double
+bar (the_struct2_t *);
+
+double
+foo (double *k, unsigned int n, the_struct_t *the_struct)
+{
+  unsigned int u;
+  the_struct2_t result;
+  for (u = 0; u < n; u++, k--)
+    {
+      result.m1 += (*k) * the_struct[u].m1;
+      result.m2 += (*k) * the_struct[u].m2;
+      result.m3 += (*k) * the_struct[u].m3;
+      result.m4 += (*k) * the_struct[u].m4;
+    }
+  return bar (&result);
+}
diff --git a/gcc/testsuite/gcc.target/aarch64/pr110625_2.c b/gcc/testsuite/gcc.target/aarch64/pr110625_2.c
new file mode 100644
index 0000000..7a84aa8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr110625_2.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-Ofast -mcpu=neoverse-n2 -fdump-tree-vect-details -fno-tree-slp-vectorize" } */
+/* { dg-final { scan-tree-dump "reduction latency = 8" "vect" } } */
+
+/* The reduction latency should be multiplied by the count for
+   single_defuse_cycle.  */
+
+long
+f (long res, short *ptr1, short *ptr2, int n)
+{
+  for (int i = 0; i < n; ++i)
+    res += (long) ptr1[i] << ptr2[i];
+  return res;
+}