[llvm-dev] [cfe-dev] RFC: Emitting empty invariant group for vtable loads
Hal Finkel via llvm-dev
llvm-dev at lists.llvm.org
Thu Jan 26 06:41:17 PST 2017
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
> <mailto:richard at metafoo.co.uk>>:
>
> On 25 January 2017 at 15:03, Hal Finkel via cfe-dev
> <cfe-dev at lists.llvm.org <mailto: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
>
>
> 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 <mailto: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
>> <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
>> <http://lists.llvm.org/pipermail/cfe-dev/2015-July/044227.html>
>>
>> My talk from LLVM Dev Meeting
>>
>> http://llvm.org/devmtg/2016-11/#talk6
>> <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:
>>
>> voidg(A& a){ a.foo();}
>>
>> define void at _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:
>>
>> structA { A();virtualvoidfoo();};structB :
>> A{ B();virtualvoidfoo();};voidg(A&
>> a){ a.foo(); a.foo();}voidclobber(A&);voidf() { 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.
>>
>> _______________________________________________
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> _______________________________________________ cfe-dev
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>
--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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