Re: [PR72835] Incorrect arithmetic optimization involving bitfield arguments

[kugan <kugan.vivekanandarajah@linaro.org>]

Hi Richard,
Thanks for the review.

On 19/09/16 23:40, Richard Biener wrote:
> On Sun, Sep 18, 2016 at 10:21 PM, kugan
> <kugan.vivekanandarajah@linaro.org> wrote:
>> Hi Richard,
>>
>>
>> On 14/09/16 21:31, Richard Biener wrote:
>>>
>>> On Fri, Sep 2, 2016 at 10:09 AM, Kugan Vivekanandarajah
>>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>>
>>>> Hi Richard,
>>>>
>>>> On 25 August 2016 at 22:24, Richard Biener <richard.guenther@gmail.com>
>>>> wrote:
>>>>>
>>>>> On Thu, Aug 11, 2016 at 1:09 AM, kugan
>>>>> <kugan.vivekanandarajah@linaro.org> wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>>
>>>>>> On 10/08/16 20:28, Richard Biener wrote:
>>>>>>>
>>>>>>>
>>>>>>> On Wed, Aug 10, 2016 at 10:57 AM, Jakub Jelinek <jakub@redhat.com>
>>>>>>> wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Aug 10, 2016 at 08:51:32AM +1000, kugan wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> I see it now. The problem is we are just looking at (-1) being in
>>>>>>>>> the
>>>>>>>>> ops
>>>>>>>>> list for passing changed to rewrite_expr_tree in the case of
>>>>>>>>> multiplication
>>>>>>>>> by negate.  If we have combined (-1), as in the testcase, we will
>>>>>>>>> not
>>>>>>>>> have
>>>>>>>>> the (-1) and will pass changed=false to rewrite_expr_tree.
>>>>>>>>>
>>>>>>>>> We should set changed based on what happens in
>>>>>>>>> try_special_add_to_ops.
>>>>>>>>> Attached patch does this. Bootstrap and regression testing are
>>>>>>>>> ongoing.
>>>>>>>>> Is
>>>>>>>>> this OK for trunk if there is no regression.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> I think the bug is elsewhere.  In particular in
>>>>>>>> undistribute_ops_list/zero_one_operation/decrement_power.
>>>>>>>> All those look problematic in this regard, they change RHS of
>>>>>>>> statements
>>>>>>>> to something that holds a different value, while keeping the LHS.
>>>>>>>> So, generally you should instead just add a new stmt next to the old
>>>>>>>> one,
>>>>>>>> and adjust data structures (replace the old SSA_NAME in some ->op
>>>>>>>> with
>>>>>>>> the new one).  decrement_power might be a problem here, dunno if all
>>>>>>>> the
>>>>>>>> builtins are const in all cases that DSE would kill the old one,
>>>>>>>> Richard, any preferences for that?  reset flow sensitive info + reset
>>>>>>>> debug
>>>>>>>> stmt uses, or something different?  Though, replacing the LHS with a
>>>>>>>> new
>>>>>>>> anonymous SSA_NAME might be needed too, in case it is before SSA_NAME
>>>>>>>> of
>>>>>>>> a
>>>>>>>> user var that doesn't yet have any debug stmts.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> I'd say replacing the LHS is the way to go, with calling the
>>>>>>> appropriate
>>>>>>> helper
>>>>>>> on the old stmt to generate a debug stmt for it / its uses (would need
>>>>>>> to look it
>>>>>>> up here).
>>>>>>>
>>>>>>
>>>>>> Here is an attempt to fix it. The problem arises when in
>>>>>> undistribute_ops_list, we linearize_expr_tree such that NEGATE_EXPR is
>>>>>> added
>>>>>> (-1) MULT_EXPR (OP). Real problem starts when we handle this in
>>>>>> zero_one_operation. Unlike what was done earlier, we now change the
>>>>>> stmt
>>>>>> (with propagate_op_to_signle use or by directly) such that the value
>>>>>> computed by stmt is no longer what it used to be. Because of this, what
>>>>>> is
>>>>>> computed in undistribute_ops_list and rewrite_expr_tree are also
>>>>>> changed.
>>>>>>
>>>>>> undistribute_ops_list already expects this but rewrite_expr_tree will
>>>>>> not if
>>>>>> we dont pass the changed as an argument.
>>>>>>
>>>>>> The way I am fixing this now is, in linearize_expr_tree, I set
>>>>>> ops_changed
>>>>>> to true if we change NEGATE_EXPR to (-1) MULT_EXPR (OP). Then when we
>>>>>> call
>>>>>> zero_one_operation with ops_changed = true, I replace all the LHS in
>>>>>> zero_one_operation with the new SSA and replace all the uses. I also
>>>>>> call
>>>>>> the rewrite_expr_tree with changed = false in this case.
>>>>>>
>>>>>> Does this make sense? Bootstrapped and regression tested for
>>>>>> x86_64-linux-gnu without any new regressions.
>>>>>
>>>>>
>>>>> I don't think this solves the issue.  zero_one_operation associates the
>>>>> chain starting at the first *def and it will change the intermediate
>>>>> values
>>>>> of _all_ of the stmts visited until the operation to be removed is
>>>>> found.
>>>>> Note that this is independent of whether try_special_add_to_ops did
>>>>> anything.
>>>>>
>>>>> Even for the regular undistribution cases we get this wrong.
>>>>>
>>>>> So we need to back-track in zero_one_operation, replacing each LHS
>>>>> and in the end the op in the opvector of the main chain.  That's
>>>>> basically
>>>>> the same as if we'd do a regular re-assoc operation on the sub-chains.
>>>>> Take their subops, simulate zero_one_operation by
>>>>> appending the cancelling operation and optimizing the oplist, and then
>>>>> materializing the associated ops via rewrite_expr_tree.
>>>>>
>>>> Here is a draft patch which records the stmt chain when in
>>>> zero_one_operation and then fixes it when OP is removed. when we
>>>> update *def, that will update the ops vector. Does this looks sane?
>>>
>>>
>>> Yes.  A few comments below
>>>
>>> +  /* PR72835 - Record the stmt chain that has to be updated such that
>>> +     we dont use the same LHS when the values computed are different.  */
>>> +  auto_vec<gimple *> stmts_to_fix;
>>>
>>> use auto_vec<gimple *, 64> here so we get stack allocation only most
>>> of the times
>>
>> Done.
>>
>>>           if (stmt_is_power_of_op (stmt, op))
>>>             {
>>> +             make_new_ssa_for_all_defs (def, op, stmts_to_fix);
>>>               if (decrement_power (stmt) == 1)
>>>                 propagate_op_to_single_use (op, stmt, def);
>>>
>>> for the cases you end up with propagate_op_to_single_use its argument
>>> stmt is handled superfluosly in the new SSA making, I suggest to pop it
>>> from the stmts_to_fix vector in that case.  I suggest to break; instead
>>> of return in all cases and do the make_new_ssa_for_all_defs call at
>>> the function end instead.
>>>
>> Done.
>>
>>> @@ -1253,14 +1305,18 @@ zero_one_operation (tree *def, enum tree_code
>>> opcode, tree op)
>>>               if (gimple_assign_rhs1 (stmt2) == op)
>>>                 {
>>>                   tree cst = build_minus_one_cst (TREE_TYPE (op));
>>> +                 stmts_to_fix.safe_push (stmt2);
>>> +                 make_new_ssa_for_all_defs (def, op, stmts_to_fix);
>>>                   propagate_op_to_single_use (cst, stmt2, def);
>>>                   return;
>>>
>>> this safe_push should be unnecessary for the above reason (others are
>>> conditionally unnecessary).
>>>
>> Done.
>>
>> Bootstrapped and regression tested on X86_64-linux-gnu with no new
>> regression. Is this OK?
>
> +static void
> +make_new_ssa_for_all_defs (tree *def, tree op,
> +               auto_vec<gimple *, 64> &stmts_to_fix)
>
> I think you need to use vec<gimple *> &stmts_to_fix here AFAIK.
>

This is what I had. With that I get:
error: invalid initialization of reference of type ‘auto_vec<gimple*>&’ 
from expression of type ‘auto_vec<gimple*, 64ul>

Is this a bug?

Thanks,
Kugan