Correctly detect shifts out of range

get_shift_range was incorrectly communicating that it couldn't calculate
a range when the shift values was always out fo range.  Fix this and
alwasy return [0, 0] when the shift value is always out of range.

	PR tree-optimization/108306
	gcc/
	* range-op.cc (operator_lshift::fold_range): Return [0, 0] not
	varying for shifts that are always out of void range.
	(operator_rshift::fold_range): Return [0, 0] not
	varying for shifts that are always out of void range.

	gcc/testsuite/
	* gcc.dg/pr108306.c: New.

2 files changed, 31 insertions, 2 deletions

diff --git a/gcc/range-op.cc b/gcc/range-op.cc
index ec75e07..6e5754e 100644
--- a/gcc/range-op.cc
+++ b/gcc/range-op.cc
@@ -2166,7 +2166,7 @@ operator_lshift::fold_range (irange &r, tree type,
       if (op2.undefined_p ())
 	r.set_undefined ();
       else
-	r.set_varying (type);
+	r.set_zero (type);
       return true;
     }
 
@@ -2440,7 +2440,7 @@ operator_rshift::fold_range (irange &r, tree type,
       if (op2.undefined_p ())
 	r.set_undefined ();
       else
-	r.set_varying (type);
+	r.set_zero (type);
       return true;
     }
 
diff --git a/gcc/testsuite/gcc.dg/pr108306.c b/gcc/testsuite/gcc.dg/pr108306.c
new file mode 100644
index 0000000..1044c64
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr108306.c
@@ -0,0 +1,29 @@
+/* { dg-do compile } */
+/* { dg-options "-O2  -fno-strict-overflow -fsanitize=shift -Warray-bounds" } */
+
+enum psi_task_count {
+	NR_IOWAIT,
+	NR_PSI_TASK_COUNTS = 4,
+};
+
+unsigned int tasks[NR_PSI_TASK_COUNTS];
+
+static void psi_group_change(unsigned int set)
+{
+	unsigned int t;
+	unsigned int state_mask = 0;
+
+	for (t = 0; set; set &= ~(1 << t), t++)
+		if (set & (1 << t))
+			tasks[t]++;
+}
+
+void psi_task_switch(int sleep)
+{
+	int set = 0;
+
+	if (sleep)
+		set |= (1 << NR_IOWAIT);
+
+	psi_group_change(set);
+}