[gcc r16-6133] vect: Relax known iteration number constraint

[Victor Do Nascimento]

https://gcc.gnu.org/g:ca8d97f8cb8d282c190183e9c297d5e0490bab4e

commit r16-6133-gca8d97f8cb8d282c190183e9c297d5e0490bab4e
Author: Victor Do Nascimento <victor.donascimento@arm.com>
Date:   Thu May 8 15:28:05 2025 +0100

    vect: Relax known iteration number constraint
    
    At present we reject uncounted loops outright when doing initial loop
    analysis in `vect_analyze_loop_form'.
    
    We have the following gating condition that causes rejection of a
    given loop:
    
     if (integer_zerop (info->assumptions)
          || !info->number_of_iterations
          || chrec_contains_undetermined (info->number_of_iterations))
    
    We can do away with this check altogether, but not without problems,
    allowing many malformed loops through which ought to be rejected as
    early as possible.
    
    We observe that a common thread running through these malformed loops
    is the absence of any scalar evolution between iterations.
    
    We have therefore adjusted the analysis replacing the checks on
    `niters' for a test for the presence of scalar evolution in the loop,
    which can be detected via the presence of phi nodes in the loop.
    
    gcc/ChangeLog:
    
            * tree-vect-loop.cc (vect_analyze_loop_form): Relax niters
            condition.

Diff:
---
 gcc/tree-vect-loop.cc | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 9b435c78c21b..00b21ecfc958 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -1473,6 +1473,20 @@ vect_analyze_loop_form (class loop *loop, gimple *loop_vectorized_call,
 				       "not vectorized:"
 				       " unsupported control flow in loop.\n");
       }
+
+  /* Check if we have any control flow that doesn't leave the loop.  */
+  bool has_phi = false;
+  for (unsigned i = 0; i < loop->num_nodes; i++)
+    if (!gimple_seq_empty_p (phi_nodes (bbs[i])))
+      {
+	has_phi = true;
+	break;
+      }
+  if (!has_phi)
+    return opt_result::failure_at (vect_location,
+				   "not vectorized:"
+				   " no scalar evolution detected in loop.\n");
+
   free (bbs);
 
   /* Different restrictions apply when we are considering an inner-most loop,
@@ -1597,9 +1611,20 @@ vect_analyze_loop_form (class loop *loop, gimple *loop_vectorized_call,
 	std::swap (info->conds[0], info->conds[i]);
     }
 
-  if (integer_zerop (info->assumptions)
-      || !info->number_of_iterations
-      || chrec_contains_undetermined (info->number_of_iterations))
+  if (chrec_contains_undetermined (info->number_of_iterations))
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location,
+			 "Loop being analyzed as uncounted.\n");
+      if (loop->inner)
+	return opt_result::failure_at
+	  (vect_location,
+	   "not vectorized: outer loop vectorization of uncounted loops"
+	   " is unsupported.\n");
+      return opt_result::success ();
+    }
+
+  if (integer_zerop (info->assumptions))
     return opt_result::failure_at
       (info->conds[0],
        "not vectorized: number of iterations cannot be computed.\n");