tree-optimization/110474 - Vect: select small VF for epilog of unrolled loop

If a loop is unrolled during vectorization (i.e. suggested_unroll_factor > 1),
the VFs of both main and epilog loop are enlarged.  The epilog vect loop is
specific for a loop with small iteration counts, so a large VF may hurt
performance.

This patch unscales the main loop VF by suggested_unroll_factor while selecting
the epilog loop VF, so that it will be the same as vectorized loop without
unrolling (i.e. suggested_unroll_factor = 1).

gcc/ChangeLog:

	PR tree-optimization/110474
	* tree-vect-loop.cc (vect_analyze_loop_2): unscale the VF by suggested
	unroll factor while selecting the epilog vect loop VF.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr110474.c: New testcase.

2 files changed, 47 insertions, 6 deletions

diff --git a/gcc/testsuite/gcc.target/aarch64/pr110474.c b/gcc/testsuite/gcc.target/aarch64/pr110474.c
new file mode 100644
index 0000000..e548416
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr110474.c
@@ -0,0 +1,37 @@
+/* { dg-do compile } */
+/* { dg-options "-O3 -mtune=neoverse-n2 -mcpu=neoverse-n1 -fdump-tree-vect-details --param aarch64-vect-unroll-limit=2" } */
+/* { dg-final { scan-tree-dump "Choosing vector mode V8HI"  "vect" } } */
+/* { dg-final { scan-tree-dump "Choosing epilogue vector mode V8QI"  "vect" } } */
+
+/* Do not increase the the vector factor of the epilog vectorized loop
+   for a loop with suggested_unroll_factor > 1.
+
+   before (suggested_unroll_factor=1):
+     if N >= 16:
+         main vect loop
+     if N >= 8:
+         epilog vect loop
+     scalar code
+
+   before (suggested_unroll_factor=2):
+     if N >= 32:
+         main vect loop
+     if N >= 16:  // May fail to execute vectorized code (e.g. N is 8)
+         epilog vect loop
+     scalar code
+
+   after  (suggested_unroll_factor=2):
+     if N >= 32:
+         main vect loop
+     if N >= 8:  // The same VF as suggested_unroll_factor=1
+         epilog vect loop
+     scalar code  */
+
+int
+foo (short *A, char *B, int N)
+{
+  int sum = 0;
+  for (int i = 0; i < N; ++i)
+    sum += A[i] * B[i];
+  return sum;
+}
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 3b46c58..4d9abd0 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -3021,12 +3021,16 @@ start_over:
      to be able to handle fewer than VF scalars, or needs to have a lower VF
      than the main loop.  */
   if (LOOP_VINFO_EPILOGUE_P (loop_vinfo)
-      && !LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo)
-      && maybe_ge (LOOP_VINFO_VECT_FACTOR (loop_vinfo),
-		   LOOP_VINFO_VECT_FACTOR (orig_loop_vinfo)))
-    return opt_result::failure_at (vect_location,
-				   "Vectorization factor too high for"
-				   " epilogue loop.\n");
+      && !LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo))
+    {
+      poly_uint64 unscaled_vf
+	= exact_div (LOOP_VINFO_VECT_FACTOR (orig_loop_vinfo),
+		     orig_loop_vinfo->suggested_unroll_factor);
+      if (maybe_ge (LOOP_VINFO_VECT_FACTOR (loop_vinfo), unscaled_vf))
+	return opt_result::failure_at (vect_location,
+				       "Vectorization factor too high for"
+				       " epilogue loop.\n");
+    }
 
   /* Decide whether this loop_vinfo should use partial vectors or peeling,
      assuming that the loop will be used as a main loop.  We will redo