[gcc r12-6740] vect: Fix epilogue mode skipping

[Andre Simoes Dias Vieira <avieira@gcc.gnu.org>]

https://gcc.gnu.org/g:f4ca0a53be18dfc7162fd5dcc1e73c4203805e14

commit r12-6740-gf4ca0a53be18dfc7162fd5dcc1e73c4203805e14
Author: Andre Vieira <andre.simoesdiasvieira@arm.com>
Date:   Wed Jan 19 14:11:32 2022 +0000

    vect: Fix epilogue mode skipping
    
    gcc/ChangeLog:
    
            PR tree-optimization/103997
            * tree-vect-loop.cc (vect_analyze_loop): Fix mode skipping for epilogue
            vectorization.

Diff:
---
 gcc/tree-vect-loop.cc | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 0b2785a5ed6..4860bfd3344 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -3004,6 +3004,12 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
   unsigned int mode_i = 0;
   unsigned HOST_WIDE_INT simdlen = loop->simdlen;
 
+  /* Keep track of the VF for each mode.  Initialize all to 0 which indicates
+     a mode has not been analyzed.  */
+  auto_vec<poly_uint64, 8> cached_vf_per_mode;
+  for (unsigned i = 0; i < vector_modes.length (); ++i)
+    cached_vf_per_mode.safe_push (0);
+
   /* First determine the main loop vectorization mode, either the first
      one that works, starting with auto-detecting the vector mode and then
      following the targets order of preference, or the one with the
@@ -3011,6 +3017,10 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
   while (1)
     {
       bool fatal;
+      unsigned int last_mode_i = mode_i;
+      /* Set cached VF to -1 prior to analysis, which indicates a mode has
+	 failed.  */
+      cached_vf_per_mode[last_mode_i] = -1;
       opt_loop_vec_info loop_vinfo
 	= vect_analyze_loop_1 (loop, shared, &loop_form_info,
 			       NULL, vector_modes, mode_i,
@@ -3020,6 +3030,12 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
 
       if (loop_vinfo)
 	{
+	  /*  Analyzis has been successful so update the VF value.  The
+	      VF should always be a multiple of unroll_factor and we want to
+	      capture the original VF here.  */
+	  cached_vf_per_mode[last_mode_i]
+	    = exact_div (LOOP_VINFO_VECT_FACTOR (loop_vinfo),
+			 loop_vinfo->suggested_unroll_factor);
 	  /* Once we hit the desired simdlen for the first time,
 	     discard any previous attempts.  */
 	  if (simdlen
@@ -3100,12 +3116,10 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
     {
       /* If the target does not support partial vectors we can shorten the
 	 number of modes to analyze for the epilogue as we know we can't pick a
-	 mode that has at least as many NUNITS as the main loop's vectorization
-	 factor, since that would imply the epilogue's vectorization factor
-	 would be at least as high as the main loop's and we would be
-	 vectorizing for more scalar iterations than there would be left.  */
+	 mode that would lead to a VF at least as big as the
+	 FIRST_VINFO_VF.  */
       if (!supports_partial_vectors
-	  && maybe_ge (GET_MODE_NUNITS (vector_modes[mode_i]), first_vinfo_vf))
+	  && maybe_ge (cached_vf_per_mode[mode_i], first_vinfo_vf))
 	{
 	  mode_i++;
 	  if (mode_i == vector_modes.length ())