tree-optimization/111000 - restrict invariant motion of shifts

The following restricts moving variable shifts to when they are
always executed in the loop as we currently do not have an efficient
way to rewrite them to something that is unconditionally
well-defined and value range analysis will otherwise compute
invalid ranges for the shift operand.

	PR tree-optimization/111000
	* stor-layout.h (element_precision): Move ..
	* tree.h (element_precision): .. here.
	* tree-ssa-loop-im.cc (movement_possibility_1): Restrict
	motion of shifts and rotates.

	* gcc.dg/torture/pr111000.c: New testcase.

4 files changed, 40 insertions, 1 deletions

diff --git a/gcc/stor-layout.h b/gcc/stor-layout.h
index e776892..589ce33 100644
--- a/gcc/stor-layout.h
+++ b/gcc/stor-layout.h
@@ -36,7 +36,6 @@ extern void place_field (record_layout_info, tree);
 extern void compute_record_mode (tree);
 extern void finish_bitfield_layout (tree);
 extern void finish_record_layout (record_layout_info, int);
-extern unsigned int element_precision (const_tree);
 extern void finalize_size_functions (void);
 extern void fixup_unsigned_type (tree);
 extern void initialize_sizetypes (void);
diff --git a/gcc/testsuite/gcc.dg/torture/pr111000.c b/gcc/testsuite/gcc.dg/torture/pr111000.c
new file mode 100644
index 0000000..e6821e1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr111000.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+
+volatile int a = 68;
+int b, d, e;
+int main()
+{
+  int t = a;
+  for (; d <= 6; d++) {
+    for (b = 0; b <= 6; b++) {
+      if (t >= 31)
+        e = d;
+      else if (d > (647 >> t))
+        e = d;
+      else
+        e = 0;
+    }
+  }
+  if (e != 6)
+    __builtin_abort();
+  return 0;
+}
diff --git a/gcc/tree-ssa-loop-im.cc b/gcc/tree-ssa-loop-im.cc
index 49aeb68..396963b 100644
--- a/gcc/tree-ssa-loop-im.cc
+++ b/gcc/tree-ssa-loop-im.cc
@@ -400,6 +400,24 @@ movement_possibility_1 (gimple *stmt)
       || gimple_could_trap_p (stmt))
     return MOVE_PRESERVE_EXECUTION;
 
+  if (is_gimple_assign (stmt))
+    {
+      auto code = gimple_assign_rhs_code (stmt);
+      tree type = TREE_TYPE (gimple_assign_rhs1 (stmt));
+      /* For shifts and rotates and possibly out-of-bound shift operands
+	 we currently cannot rewrite them into something unconditionally
+	 well-defined.  */
+      if ((code == LSHIFT_EXPR
+	   || code == RSHIFT_EXPR
+	   || code == LROTATE_EXPR
+	   || code == RROTATE_EXPR)
+	  && (TREE_CODE (gimple_assign_rhs2 (stmt)) != INTEGER_CST
+	      /* We cannot use ranges at 'stmt' here.  */
+	      || wi::ltu_p (wi::to_wide (gimple_assign_rhs2 (stmt)),
+			    element_precision (type))))
+	ret = MOVE_PRESERVE_EXECUTION;
+    }
+
   /* Non local loads in a transaction cannot be hoisted out.  Well,
      unless the load happens on every path out of the loop, but we
      don't take this into account yet.  */
diff --git a/gcc/tree.h b/gcc/tree.h
index 31ea52a..781297c 100644
--- a/gcc/tree.h
+++ b/gcc/tree.h
@@ -2235,6 +2235,7 @@ class auto_suppress_location_wrappers
 #define SET_TYPE_MODE(NODE, MODE) \
   (TYPE_CHECK (NODE)->type_common.mode = (MODE))
 
+extern unsigned int element_precision (const_tree);
 extern machine_mode element_mode (const_tree);
 extern machine_mode vector_type_mode (const_tree);
 extern unsigned int vector_element_bits (const_tree);