tree-optimization/98117 - fix range set by vectorization on niter IVs

This avoids the degenerate case of a TYPE_MAX_VALUE latch iteration
count value causing wrong range info for the vector IV.  There's
still the case of VF == 1 where if we don't know whether we hit the
above case we cannot emit a range.

2020-12-07  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98117
	* tree-vect-loop-manip.c (vect_gen_vector_loop_niters):
	Properly handle degenerate niter when setting the vector
	loop IV range.

	* gcc.dg/torture/pr98117.c: New testcase.

2 files changed, 41 insertions, 6 deletions

diff --git a/gcc/testsuite/gcc.dg/torture/pr98117.c b/gcc/testsuite/gcc.dg/torture/pr98117.c
new file mode 100644
index 0000000..f216025
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr98117.c
@@ -0,0 +1,19 @@
+/* { dg-do run } */
+/* { dg-additional-options "-fno-tree-scev-cprop" } */
+
+unsigned char c;
+void __attribute__((noipa))
+e()
+{
+  do
+    {
+    }
+  while (++c);
+}
+int main()
+{
+  e();
+  if (c != 0)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-loop-manip.c b/gcc/tree-vect-loop-manip.c
index 3617918..2370b87 100644
--- a/gcc/tree-vect-loop-manip.c
+++ b/gcc/tree-vect-loop-manip.c
@@ -2034,13 +2034,29 @@ vect_gen_vector_loop_niters (loop_vec_info loop_vinfo, tree niters,
       niters_vector = force_gimple_operand (niters_vector, &stmts, true, var);
       gsi_insert_seq_on_edge_immediate (pe, stmts);
       /* Peeling algorithm guarantees that vector loop bound is at least ONE,
-	 we set range information to make niters analyzer's life easier.  */
+	 we set range information to make niters analyzer's life easier.
+	 Note the number of latch iteration value can be TYPE_MAX_VALUE so
+	 we have to represent the vector niter TYPE_MAX_VALUE + 1 >> log_vf.  */
       if (stmts != NULL && log_vf)
-	set_range_info (niters_vector, VR_RANGE,
-			wi::to_wide (build_int_cst (type, 1)),
-			wi::to_wide (fold_build2 (RSHIFT_EXPR, type,
-						  TYPE_MAX_VALUE (type),
-						  log_vf)));
+	{
+	  if (niters_no_overflow)
+	    set_range_info (niters_vector, VR_RANGE,
+			    wi::one (TYPE_PRECISION (type)),
+			    wi::rshift (wi::max_value (TYPE_PRECISION (type),
+						       TYPE_SIGN (type)),
+					exact_log2 (const_vf),
+					TYPE_SIGN (type)));
+	  /* For VF == 1 the vector IV might also overflow so we cannot
+	     assert a minimum value of 1.  */
+	  else if (const_vf > 1)
+	    set_range_info (niters_vector, VR_RANGE,
+			    wi::one (TYPE_PRECISION (type)),
+			    wi::rshift (wi::max_value (TYPE_PRECISION (type),
+						       TYPE_SIGN (type))
+					- (const_vf - 1),
+					exact_log2 (const_vf), TYPE_SIGN (type))
+			    + 1);
+	}
     }
   *niters_vector_ptr = niters_vector;
   *step_vector_ptr = step_vector;