_Dependent_ptr problems in RTL passes

_Dependent_ptr problems in RTL passes

[Akshat Garg]

Hi all,

We are working on making memory_order_consume not get promoted to
memory_order_acquire. Here is a little background on the work we are doing
https://gcc.gnu.org/ml/gcc/2019-07/msg00038.html

We are able to parse _Dependent_ptr from C front-end. The patch files are
given here.
https://github.com/AKG001/gcc/commit/2accdd2b43100abae937c714eb4c8e385940b5c7
https://github.com/AKG001/gcc/commit/fb4187bc3872a50880159232cf336f0a03505fa8

Currently, we are working over the pointers only.
As discussed earlier,  there are certain passes, on the tree and RTL level,
that may break the dependencies specified by the user. We are interested to
know about the problems that could arise during the RTL passes. For that,
we have tried to skip the tree passes, by considering the _Dependent_ptr as
volatile. The patch for this here.
https://github.com/AKG001/gcc/commit/e4ffd77f62ace986c124a70b90a662c083c570ba

We are trying to find all the passes where the dependencies could get
broken. We have experimented on certain examples, and we have some doubts
regarding an example. Hoping community people can help us.  The example is
figure 20 from here (
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0190r4.pdf).

The example:  (https://github.com/AKG001/rtl_opt/blob/master/p0190r4_fig20.c).

The .optimized code:
https://github.com/AKG001/rtl_opt/blob/master/p0190r4_fig20.c.231t.optimized
The .expand code:
https://github.com/AKG001/rtl_opt/blob/master/p0190r4_fig20.c.233r.expand
The .cse1 code:
https://github.com/AKG001/rtl_opt/blob/master/p0190r4_fig20.c.239r.cse1
The .final code:
https://github.com/AKG001/rtl_opt/blob/master/p0190r4_fig20.c.317r.final

In the .expand code, I believe there are no dependencies that gets broken.
Hoping someone could also verify. But, in .cse1 code instruction at line at
line 231, as shown below,

1.  (insn 10 9 11 2 (set (reg:CCZ 17 flags)
2.   (compare:CCZ (reg/f:DI 84 [ p.2_3 ])
3.          (const_int 0 [0]))) "p0190r4_fig20.c":44:6 8  {*cmpdi_ccno_1}
4.  (nil))

and instruction at line 402, as shown below
5.  (insn 10 9 11 2 (set (reg:CCZ 17 flags)
6.      (compare:CCZ (reg:DI 82 [ _1 ])
7.            (const_int 0 [0]))) "p0190r4_fig20.c":44:6 8 {*cmpdi_ccno_1}
8.      (expr_list:REG_DEAD (reg/f:DI 84 [ p.2_3 ])
9.  (nil)))
dependencies get broken.

The instruction starting at line 1 gets changed to instruction starting at
line 5 and starts referring to variable _1 which is defined as " long
unsigned int _1;" in the .optimized code in thread1(), which is a temporary
variable. I believe, this breaks the dependencies specified by the user and
for that we need to put some code inside cse.c file.

Also, many versions for variable 'p' gets created shown in .optimized code,
they all should have _dependent_ptr qualification which they don't have
currently. I believe, simply bypassing the tree passes through volatile
checks won't mark them as dependent pointer qualified. For this, I believe,
we need to tweak the ssa generation pass(tree-ssa.c) somewhat.

Thank you all and let me know if anyone finds me wrong anywhere.
-Akshat