[llvm-dev] [cfe-dev] RFC: Emitting empty invariant group for vtable loads
Piotr Padlewski via llvm-dev
llvm-dev at lists.llvm.org
Sat Jan 28 08:36:18 PST 2017
2017-01-26 15:41 GMT+01:00 Hal Finkel <hfinkel at anl.gov>:
>
> On 01/26/2017 06:44 AM, Piotr Padlewski wrote:
>
>
>
> 2017-01-26 3:28 GMT+01:00 Richard Smith <richard at metafoo.co.uk>:
>
>> On 25 January 2017 at 15:03, Hal Finkel via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hi Piotr,
>>>
>>> I think makes sense. Modulo bitcasts, the invariant is identified by a
>>> particular pointer SSA value. Given that you can't sensibly have two
>>> nonequivalent invariants associated with the same pointer SSA value
>>> simultaneously, there's no need to also identify the invariant with a
>>> metadata string as well. When we need a new "identifier" for the pointed-to
>>> value, we get one using invariant.group.barrier.
>>>
>>
>> As I recall, the original motivation for the identifier was to support
>> cases where the "invariant" region's value changes and then changes back
>> (remember that invariant.group does not imply the storage doesn't change,
>> just that a particular set of loads and stores witness the same value):
>>
>> struct A { void *vptr; /*...*/ };
>> struct B { void *vptr; /*...*/ };
>> union U { A a; B b; };
>>
>> void f(U *u) {
>> // #1, load A vptr
>> load u->a.vptr, !invariant "A::vptr"
>> // #2, change union member to B ...
>> store u->b.vptr, !invariant "B::vptr"
>> // #3, change union member back to A ...
>> f(u); // performs: store u->a.vptr, !invariant "A::vptr"
>> // #4, load A vptr again, can be forwarded from #1 but not from #2
>> load u->a.vptr, !invariant "A::vptr"
>> }
>>
>> Excellent point, I forgot that one don't have to call placement new in
> order to emplace different type.
> I discussed it with Krzysztof and we belive the best way to solve it is to
> introduce barriers before every use of union if it contains any polymorphic
> class (recursively for each class in union). This might look very bad, but
> assuming we will be able skip barrier for optimizations not relying on
> !invariant.group, then it will not pessimize anything.
>
>
> In light of this, can we go back and reevaluate your list of proposed
> solutions? You pointed out two problems:
> 1. "This works well if the pointer type doesn’t change, but when it does,
> devirtualization might not happen" (e.g. a conversion to a base-class
> pointer/reference).
> 2. "The other problem is that when we combine 2 instructions with
> different invariant.group metadata, then we pick one of them, because for
> now we can have only single !invariant.group metadata."
>
> Given what's been said, I'm leaning toward favoring solution (b), "having
> sub invariant groups - like inheritance, so we could figure out that one
> group is subgroup of another" because 1) it seems to solve both problems
> and 2) it seems like we could get all of the non-trivial logic (for
> generation, comparison, and merging) from our TBAA implementation (we might
> even be able to directly reuse the same metadata).
>
> -Hal
>
>
I was thinking about this, and as long as I think about it I am convincing
myself that different groups, or even sub groups that you have mentioned,
will have no use for const memory propagation. In the future we probably
would like to decorate const member loads and
stores with* !invariant.group* (or *!invariant* if groups will be removed
[if name will change :P]).
In order to decorate const members, we will need to insert barrier before
*every* use of union member.
So I was thinking about this example: https://godbolt.org/g/iSUjKD but it
doesn't compile, but Krzysztof figured out this one:
https://godbolt.org/g/lYQFBd (BTW it seems that clang is pretty bad at
cleaning out iostreams)
struct A {
const int x;
};
union B {
A a1;
A a2;
B() : a1{42} { }
void switch2(int x) {
a1.~A();
new (&a2) A{x};
}
};
int main() {
B b;
std::cout << b.a1.x << std::endl;
b.switch2(50);
std::cout << b.a2.x << std::endl;
}
As he checked in standard, it seems to be valid C++11 code by *9.5.4*.
So as you can see different groups won't gonna work in case of const
members inside union, like:
int main() {
B b;
std::cout << b.a1.x << std::endl;
b.switch2(50);
std::cout << b.a2.x << std::endl;
b.switch2(51);
std::cout << b.a2.x << std::end; // This is not 50, as we would
thought based on invariant.groups
}
I am not familiar with the TBAA implementation, but after digging more int
MSSA, I have other reason to not like the sub groups.
It might be pretty hard to handle them efficiently in many algorithms
including MSSA.
Hal, what are the things that you dislike about the emitting barriers
everywhere for unions?
It only seems to be very heavy, but I am pretty sure we can get to the
point where we will be able to
delete barriers when we can, and teach optimizations and analysis how to
skip them, so they will not pesimize any code.
*TL;DR *
different groups works for devirtualization, but it is seem to not be a
general way to go with "invariant" things.
Piotr
>
>
>
>
>> However, I don't immediately see a way in which the C++ object model
>> would require us to track multiple distinct groups of loads and stores, so
>> if it isn't useful to be able to do that outside of C++ vptr / const member
>> invariant tracking, I think we can remove it.
>>
>
> It should not make optimizations in LLVM any harder with group or without,
> so I will postpone the removal of groups for some time.
>
>
>> -Hal
>>>
>>> On 01/24/2017 01:39 PM, Piotr Padlewski via llvm-dev wrote:
>>>
>>> Hi,
>>> I would really like to hear some feedback about this.
>>>
>>> Piotr
>>>
>>> 2017-01-20 17:07 GMT+01:00 Piotr Padlewski <piotr.padlewski at gmail.com>:
>>>
>>>> Hi all,
>>>>
>>>> I would like to propose a new way clang would decorate vtable loads in
>>>> order to handle devirtualization better.
>>>>
>>>> I've added *llvm-dev* also, because this can start a discussion about
>>>> changing invariant.group to just invariant.
>>>>
>>>> PDF version of this RFC can be found here:
>>>>
>>>> https://drive.google.com/file/d/0B72TmzNsY6Z8ZmpOUnB5dDZfSFU
>>>> /view?usp=sharing
>>>>
>>>> Background:
>>>>
>>>> Initial old design:
>>>>
>>>> http://lists.llvm.org/pipermail/cfe-dev/2015-July/044227.html
>>>>
>>>> My talk from LLVM Dev Meeting
>>>>
>>>> http://llvm.org/devmtg/2016-11/#talk6
>>>> The problem
>>>>
>>>> Clang with -fstrict-vtable-pointers decorates vtable loads with
>>>> metadata corresponding to mangled pointer type name like:
>>>>
>>>> void g(A& a){ a.foo(); }
>>>>
>>>> define void @_Z1gR1A(%struct.A* dereferenceable(8) %a)
>>>> local_unnamed_addr #0 { entry: %0 = bitcast %struct.A* %a to void (%
>>>> struct.A*)*** %vtable = load void (%struct.A*)**, void (%struct.A*)***
>>>> %0, !invariant.group !7 %1 = load void (%struct.A*)*, void (%struct.A*)**
>>>> %vtable tail call void %1(%struct.A* nonnull %a) ret void } !7 = !{!
>>>> "_ZTS1A"}
>>>>
>>>> This works well if the pointer type doesn’t change, but when it does,
>>>> devirtualization might not happen like here:
>>>>
>>>> struct A { A(); virtual void foo(); }; struct B : A{ B();
>>>> virtual void foo(); }; void g(A& a){ a.foo(); a.foo(); } void
>>>> clobber(A&); void f() { B b; clobber(b); g(b); }
>>>>
>>>> The other problem is that when we combine 2 instructions with different
>>>> invariant.group metadata, then we pick one of them, because for now we can
>>>> have only single !invariant.group metadata.
>>>> The solution
>>>>
>>>> I had some initial ideas how it can be solved, like
>>>>
>>>> 1.
>>>>
>>>> introducing multi invariant groups
>>>> 2.
>>>>
>>>> having sub invariant groups - like inheritance, so we could figure
>>>> out that one group is subgroup of another
>>>> 3.
>>>>
>>>> decorating all loads with base pointer MD (doesn’t work with
>>>> multiple inheritance)
>>>>
>>>> I consulted my ideas with Krzysztof Pszeniczny, and he proposed
>>>> something much simpler: we can decorate every invariant.group md with empty
>>>> metadata.
>>>>
>>>> This should work because the lifetime of the object is strictly defined
>>>> by invariant.group.barrier.
>>>>
>>>> If this holds, we can start discussion about if it makes sense to keep
>>>> invariant groups, and instead have just “invariant”, that would be
>>>> equivalent to having invariant.group with the same metadata.
>>>>
>>>> Do you have some thoughts about this approach? I don’t have a
>>>> mathematical proof, but I am confident that it should be valid.
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>> --
>>> Hal Finkel
>>> Lead, Compiler Technology and Programming Languages
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>>>
>>> _______________________________________________ cfe-dev mailing list
>>> cfe-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/
>>> mailman/listinfo/cfe-dev
>>
>> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
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