vect: Tweak comparisons with existing epilogue loops

This patch uses a more accurate scalar iteration estimate when
comparing the epilogue of a constant-iteration loop with a candidate
replacement epilogue.

In the testcase, the patch prevents a 1-to-3-element SVE epilogue
from seeming better than a 64-bit Advanced SIMD epilogue.

gcc/
	* tree-vect-loop.c (vect_better_loop_vinfo_p): Detect cases in
	which old_loop_vinfo is an epilogue loop that handles a constant
	number of iterations.

gcc/testsuite/
	* gcc.target/aarch64/sve/cost_model_12.c: New test.

2 files changed, 28 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c
new file mode 100644
index 0000000..4c5226e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c
@@ -0,0 +1,19 @@
+/* { dg-options "-O3 -mtune=neoverse-512tvb" } */
+
+void
+f (float x[restrict 10][1024],
+   float y[restrict 10][1024], float z)
+{
+  for (int i = 0; i < 10; ++i)
+    {
+#pragma GCC unroll 10
+      for (int j = 0; j < 10; ++j)
+	x[j][i] = y[j][i] * z;
+    }
+}
+
+/* We should unroll the outer loop, with 2x 16-byte vectors and 1x
+   8-byte vectors.  */
+/* { dg-final { scan-assembler-not {\tptrue\t} } } */
+/* { dg-final { scan-assembler {\tv[0-9]+\.4s,} } } */
+/* { dg-final { scan-assembler {\tv[0-9]+\.2s,} } } */
diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index 48a54b0..1e21fe6 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -2772,7 +2772,15 @@ vect_better_loop_vinfo_p (loop_vec_info new_loop_vinfo,
 
   /* Limit the VFs to what is likely to be the maximum number of iterations,
      to handle cases in which at least one loop_vinfo is fully-masked.  */
-  HOST_WIDE_INT estimated_max_niter = likely_max_stmt_executions_int (loop);
+  HOST_WIDE_INT estimated_max_niter;
+  loop_vec_info main_loop = LOOP_VINFO_ORIG_LOOP_INFO (old_loop_vinfo);
+  unsigned HOST_WIDE_INT main_vf;
+  if (main_loop
+      && LOOP_VINFO_NITERS_KNOWN_P (main_loop)
+      && LOOP_VINFO_VECT_FACTOR (main_loop).is_constant (&main_vf))
+    estimated_max_niter = LOOP_VINFO_INT_NITERS (main_loop) % main_vf;
+  else
+    estimated_max_niter = likely_max_stmt_executions_int (loop);
   if (estimated_max_niter != -1)
     {
       if (known_le (estimated_max_niter, new_vf))