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.

1 files changed, 19 insertions, 0 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;
+}