[gcc r14-4786] tree-optimization/111000 - restrict invariant motion of shifts

[gcc r14-4786] tree-optimization/111000 - restrict invariant motion of shifts

[Richard Biener]

https://gcc.gnu.org/g:d118738e71cf4615f170fff8dabe66442206d008

commit r14-4786-gd118738e71cf4615f170fff8dabe66442206d008
Author: Richard Biener <rguenther@suse.de>
Date:   Fri Oct 20 11:54:07 2023 +0200

    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.

Diff:
---
 gcc/stor-layout.h                       |  1 -
 gcc/testsuite/gcc.dg/torture/pr111000.c | 21 +++++++++++++++++++++
 gcc/tree-ssa-loop-im.cc                 | 18 ++++++++++++++++++
 gcc/tree.h                              |  1 +
 4 files changed, 40 insertions(+), 1 deletion(-)

diff --git a/gcc/stor-layout.h b/gcc/stor-layout.h
index e7768921c22d..589ce33c950e 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 000000000000..e6821e1618da
--- /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 49aeb6857734..396963b6754c 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 31ea52a5d6bc..781297c7e770 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);