rewrite undefined overflow to defined in ifcombine

When we make stmts to execute unconditionally in ifcombine we have
to make sure to rewrite stmts that can invoke undefined behavior
on overflow into a form with defined overflow.  That's possible
for all but signed division for which we have to avoid the transform.

2022-04-04  Richard Biener  <rguenther@suse.de>

	* tree-ssa-ifcombine.cc (bb_no_side_effects_p): Avoid executing
	divisions with undefined overflow unconditionally.
	(pass_tree_ifcombine::execute): Rewrite stmts with undefined
	overflow to defined.

diff --git a/gcc/tree-ssa-ifcombine.cc b/gcc/tree-ssa-ifcombine.cc
index 3a4ab694b7172e4c5cdb2a26cd8517437aa40f47..cb86cc1ea5fe742c5c58433a38482163542ced52 100644 (file)
--- a/gcc/tree-ssa-ifcombine.cc
+++ b/gcc/tree-ssa-ifcombine.cc
@@ -125,10 +125,26 @@ bb_no_side_effects_p (basic_block bb)
       if (is_gimple_debug (stmt))
        continue;
 
+      gassign *ass;
+      enum tree_code rhs_code;
       if (gimple_has_side_effects (stmt)
          || gimple_uses_undefined_value_p (stmt)
          || gimple_could_trap_p (stmt)
          || gimple_vuse (stmt)
+         /* We need to rewrite stmts with undefined overflow to use
+            unsigned arithmetic but cannot do so for signed division.  */
+         || ((ass = dyn_cast <gassign *> (stmt))
+             && INTEGRAL_TYPE_P (TREE_TYPE (gimple_assign_lhs (ass)))
+             && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (gimple_assign_lhs (ass)))
+             && ((rhs_code = gimple_assign_rhs_code (ass)), true)
+             && (rhs_code == TRUNC_DIV_EXPR
+                 || rhs_code == CEIL_DIV_EXPR
+                 || rhs_code == FLOOR_DIV_EXPR
+                 || rhs_code == ROUND_DIV_EXPR)
+             /* We cannot use expr_not_equal_to since we'd have to restrict
+                flow-sensitive info to whats known at the outer if.  */
+             && (TREE_CODE (gimple_assign_rhs2 (ass)) != INTEGER_CST
+                 || !integer_minus_onep (gimple_assign_rhs2 (ass))))
          /* const calls don't match any of the above, yet they could
             still have some side-effects - they could contain
             gimple_could_trap_p statements, like floating point
@@ -847,6 +863,19 @@ pass_tree_ifcombine::execute (function *fun)
            /* Clear range info from all stmts in BB which is now executed
               conditional on a always true/false condition.  */
            reset_flow_sensitive_info_in_bb (bb);
+           for (gimple_stmt_iterator gsi = gsi_start_bb (bb); !gsi_end_p (gsi);
+                gsi_next (&gsi))
+             {
+               gassign *ass = dyn_cast <gassign *> (gsi_stmt (gsi));
+               if (!ass)
+                 continue;
+               tree lhs = gimple_assign_lhs (ass);
+               if ((INTEGRAL_TYPE_P (TREE_TYPE (lhs))
+                    || POINTER_TYPE_P (TREE_TYPE (lhs)))
+                   && arith_code_with_undefined_signed_overflow
+                        (gimple_assign_rhs_code (ass)))
+                 rewrite_to_defined_overflow (ass, true);
+             }
            cfg_changed |= true;
          }
     }