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.
Bootstrapped and tested on x86_64-unknown-linux-gnu.
This was developed as not working solution to fix PR105142 and
I do not have a testcase that experiences wrong-code (but I only
tried for about 10 minutes to construct one). Still the problem
is obviously latent and we should fix it.
Thus - OK for trunk?
Thanks,
Richard.
2022-04-04 Richard Biener <rguent...@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.
---
gcc/tree-ssa-ifcombine.cc | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)
diff --git a/gcc/tree-ssa-ifcombine.cc b/gcc/tree-ssa-ifcombine.cc
index 3a4ab694b71..cb86cc1ea5f 100644
--- 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;
}
}
--
2.35.3
