[cfe-dev] RFC: Devirtualization v2
Piotr Padlewski via cfe-dev
cfe-dev at lists.llvm.org
Thu Mar 29 06:12:04 PDT 2018
Hi John,
2018-03-28 23:23 GMT+02:00 John McCall <rjmccall at apple.com>:
>
>
> On Mar 19, 2018, at 7:27 PM, Piotr Padlewski via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Hi folks,
>
> here is a link to the proposal that we've been working on lately:
> https://docs.google.com/document/d/16GVtCpzK8sIHNc2qZz6RN8am
> ICNBtvjWUod2SujZVEo/edit?usp=sharing
>
> But for the record, I also paste it here. Feedback will be really
> appreciated!
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> *RFC: C++ Devirtualization v2Piotr Padlewski - piotr.padlewski at gmail.com
> <piotr.padlewski at gmail.com>Krzysztof Pszeniczny -
> krzysztof.pszeniczny at gmail.com <krzysztof.pszeniczny at gmail.com>Jakub
> Kuderski - kubakuderski at gmail.com <kubakuderski at gmail.com>Richard Smith -
> RichardSmith at google.com <RichardSmith at google.com>This proposal describes a
> new model of representing pointers to dynamic objects as “fat pointers”. It
> is designed to solve the hole in the previous devirtualization model that
> could cause miscompilation. We believe that solving this is very important,
> especially with the mitigation of the recent Spectre vulnerability -
> retpolines.Introduction to previous devirtualizationIn the previous model
> we introduced invariant.group to LLVM as a way of saying “this load will
> produce the same value if given the same argument”, which was applied on
> loads from virtual pointer to communicate that virtual pointer (or in other
> words, dynamic type) does not change during the lifetime of an object.
> Because virtual pointer might be set multiple times during construction and
> destruction of derived types, and because it is possible to change the
> dynamic type of an object using placement new, invariant.group.barrier was
> introduced. It is used in every place where the dynamic type might change.
> To turn devirtualization on user had to specify the
> -fstrict-vtable-pointers flag for clang. If you want to learn more about
> the previous model, check out [0][1][2][3]. Although the previous model is
> not sound for C++, languages like Java or Scala can safely use it. This is
> because virtual pointer in Java is set only once for every dynamic type,
> and you can’t do things like placement new, which means 2 aliasing pointers
> will always have the same dynamic type (also because every pointer still
> points to a valid object).The problemThe old model miscompiles on the
> following example:A *a = new A;a->foo(); A *b = new(a) B;if (a == b)
> b->foo(); // This call could be devirtualized to A::foo()The problem is
> that GVN and other pass replaces the SSA value of b with a based on the
> a==b comparison, which is a totally legal optimization in LLVM and we
> surely still want to do it. In C++ however, If you replaced b with a inside
> the if statement’s body, then you would introduce UB. The problem arises
> because after changing %b to %a in LLVM, the load from virtual pointer can
> be devirtualized to different type. %vtable_a = load %a,
> !invariant.group;;;; %a == %b%bool = icmp %a, %bbr %bool, %if, %afterif:;
> if this will be changed to %a, then the optimizer will be able to replace ;
> %vtable_b with %vtable_a%vtable_b = load %b, !invariant.groupFor other
> corner cases check out appendix.SolutionThe proposed solution is to model
> pointers to dynamic types as “fat pointers”. We can think of them as
> pointers that also store the current dynamic type. Virtual calls would
> consist of virtual pointer load from fat pointer. We will still use
> invariant.groups for the devirtualization. The difference is that accessing
> class field, or comparing pointers to dynamic objects would require a call
> to a new intrinsic - i8* llvm.strip.invariant.group(i8* ) to firstly get
> pointer without information about dynamic type. Creation (or reloading) of
> fat pointer would be done by call to new intrinsic - i8*
> llvm.launder.invariant.group(i8*) that replaces
> llvm.invariant.group.barrier.The pointer comparison will now not be an
> issue, because the optimizer will not be able to replace the operand of a
> virtual pointer load with another pointer:%vtable_a = load i8* %a,
> !invariant.group !{};;;; %a == %b%addr_a = call i8*
> @llvm.strip.invariant.group(i8* %a)%addr_b = call i8*
> @llvm.strip.invariant.group(i8* %b)%bool = icmp %addr_a, %addr_bbr %bool,
> %if, %afterif:; This will not be able to change %b to %a%vtable_b = load
> i8* %b, !invariant.group !{}It is important to notice that vtable load of
> %b cannot be replaced with a load of %addr_b Although the optimizer could
> potentially figure out that %b and %addr_b are aliasing, the aliasing rules
> are not strong enough to make this transformation valid. One counterexample
> could be that although both pointers are pointing to the same memory, one
> could be a pointer to mmap’ed memory with no write/load permission.The
> Solution formalizedFormally, to virtually mark pointers as optionally
> belonging to invariant groups subject to the following rules: - alloca and
> library malloc-like functions return pointers belonging to fresh invariant
> groups- belonging to an invariant group is preserved by bitcasts- an
> intrinsic, i8* llvm.strip.invariant.group(i8*) returns its argument with
> the invariant group virtual metadata stripped- an intrinsic, i8*
> llvm.launder.invariant.group(i8*) returns its argument with a fresh
> invariant group virtual metadata, i.e. it starts a new invariant group.
> This is similar to C++'s std::launder.- every load and store marked
> !invariant.group from/to pointers belonging to the same invariant group
> must load/store the same value- the behaviour of a load or store marked
> !invariant.group from/to pointers not belonging to any invariant group
> (e.g. obtained from llvm.strip.invariant.group) is undefined- constructors
> may assume the this pointer passed to them belongs to a fresh invariant
> groupFrom those rules one easily gets: - llvm.strip.invariant.group is a
> pure function, i.e. its value depends only on its argument. Its llvm
> attributes include at least: readnone speculatable nounwind. Its results
> must alias its argument.- llvm.launder.invariant.group is not a pure
> function: it creates a fresh invariant group each time it's called. One may
> mentally model this as getting a fresh invariant group identifier somewhere
> from a 'magic' memory inaccessible to no-one else, so its llvm attributes
> include at least: inaccessiblememonly speculatable nounwind. Its results
> must alias its argument.- strip({strip,launder}(X)) = strip(X). This is
> because we do not care which particular invariant group metadata is
> stripped.- launder({strip,launder}(X)) may be replaced with launder(X) by
> the optimiser. Note that this not mean that launder(launder(X)) =
> launder(X)in the IR itself, but only on a metalanguage level: launder is
> inherently nondeterministic, so no two invocations of it ever return the
> same value.This allows to compile the following C++ constructs: - vtable
> loads required to perform virtual calls are marked with !invariant.group,
> as before- constructors of derived classes need to launder the this pointer
> before passing it to the constructors of base classes: they may
> subsequently operate on the original this pointer- likewise, destructors of
> derived classes need to launder the this pointer before passing it to the
> destructors of base classes- placement new and std::launder need to call
> launder- accesses to union members need to call launder, because the active
> member of the union may have changed since the last visible access-
> whenever two pointers are to be compared, they must be stripped first,
> because we want the comparison to provide information about the address
> equality, and not about invariant group equality- likewise, whenever a
> pointer is to be cast to an integer type, it must be stripped first-
> whenever an integer is cast to a pointer type, the result must be laundered
> before it is used, to prevent reasoning about the equality of integers to
> provide any information about equality of invariant groupsNote that adding
> calls to strip and launder related to pointer comparisons and
> integer<->pointer conversions will not cause any semantic information to be
> lost: if any piece of information could be inferred by the optimiser about
> some collection of variables (e.g. that two pointers are equal) can be
> inferred now about their stripped versions, no matter how many strip and
> launder calls have been made to obtain them in the IR. As an example, the
> C++ expression ptr == std::launder(ptr) will be optimised to true, because
> it will compare strip(ptr) with strip(launder(ptr)), which are indeed equal
> according to our rules.*
>
>
> This proposal sounds great, even if it still doesn't solve some of the
> problems I personally need to solve with invariant loads. :)
>
> I take it that the actual devirtualization here is ultimately still done
> by forwarding a visible store of the v-table pointer to an invariant load,
> just by noticing that they occur to the same laundered pointer and
> therefore must involve the same value. There's no way of saying "I know
> what the value of the v-table pointer is even if you can't see a store"
> when creating a laundered pointer. For example, in Swift we have
> constructor functions that are known to return a complete object of a
> specific type, even if we can't necessarily see the implementation of that
> function; there's no way for us to say anything about that function pointer
>
> I think we have already solved that problem with calls to llvm.assume
intrinsic. After calling the constructor, we load virtual pointer (with
invariant group) and compare it with the vtable it should point to and then
pass it to the assume.
call void @_ZN1AC1Ev(%struct.A* %a) ; call ctor
%3 = load {...} %a ; Load vptr
%4 = icmp eq %3, @_ZTV1A ; compare vptr with vtable
call void @llvm.assume(i1 %4)
(from http://blog.llvm.org/2017/03/devirtualization-in-llvm-and-clang.html )
If I understand it correctly, you should be able to use the same technique
for the constructor-like functions in Swift :)
>
>
> *LLVMBecause LTO between a module with and without devirtualization will
> be invalid, we will need to break LLVM level ABI. This is however already
> implemented, because LTO between modules with invariant.group.barriers and
> without is also invalid. This also means that if we don’t want to break ABI
> between modules with and without optimizations, we will need to have
> invariant.barriers and fatpointer.create/strip turned on all the time. For
> the users it will means that when switching to new compiler, they will have
> to recompile all of the generated object files for LTO builds.*
>
>
> Is there really no way to have this degrade more gracefully? I continue
> to be very concerned about frontend interworking here, either between
> different versions of a single frontend (e.g. clang 6 vs. clang 8), or
> between different invocations of a single frontend with different language
> options set (e.g. clang vs. clang++), or even between different frontends
> that produce IR that gets linked together (e.g. clang vs. swift).
>
> How about this approach:
> - Instead of taking a meaningless !{} argument, invariant.group takes a
> string argument which identifies a metadata-dependent optimization. In
> your case, it would be something like !"clang.cxx_devirtualization".
> - Functions have a "supported optimizations" list which declares all the
> metadata-reliant optimizations they promise to have correct metadata for.
> So e.g. clang++ would list "clang.cxx_devirtualization" on every single
> function it compiled, regardless of whether that function actually needed
> any metadata. I'm pretty sure metadata are optimized so that identical
> lists of options like this don't take up more space just because they're
> added to every single function in the module.
> - Interprocedural optimizations — which mostly means inlining — are
> required to be aware of the supported-optimizations list. The inliner
> would intersect the supported-optimizations lists and then strip
> metadata/intrinsics that don't belong anymore.
>
> But the idea that every single metadata-dependent optimization is going to
> create a new "IR ABI break" just seems unacceptable to me. Compiler
> optimization IRs are not stable things; compiler engineers constantly find
> new things that they want to express.
>
> John.
>
I haven't thought about LTO between different languages, thanks for
bringing that!
Can you actually use C++ objects without going through C interface? If it
is possible, then that is heavy.
To clarify how it works right now - if you would do LTO between IR compiled
with -fstrict-vtable-pointers and without, then the linker would throw an
error. I can see it right now, that it pretty much stops you from doing any
LTO between different languages.
The other idea that we had, was to actually strip all the invariant.groups
when linking with module that does not have them. This, opposed to the
first idea would let us link whatever we want, but we could silently loose
some optimizations.
I like the idea that you proposed - it is somewhere between these two
ideas, as you limit the potential loses to only some functions and in the
same time you can link whaterver IR you like.
However, if you agree that the option 2 - stripping invariant.groups from
whole modules - addresses all of your concerns, then I would propose to
firstly go with this idea and then optimize it if we would find a problem
with it.
I feel that it might be an overkill to implement it on the first go,
especially that we are not even in the point of thinking about turing
-fstrict-vtable-pointers on by default.
What do you think about that?
Piotr
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> *ClangClang will require a couple of minor changes in CodeGen for
> constructors and destructors.AcknowledgmentSpecial thanks for the
> initiators of this idea - Richard Smith, Chandler Carruth and Sanjoy Das
> and also for all the other people who took a part in helping with
> devirtualization v1 including: Daniel Berlin, Reid Kleckner, David
> Majnemer, John McCall.References[0] - Devirtualization in LLVM, Proceeding
> SPLASH Companion 2017 Proceedings Companion of the 2017 ACM SIGPLAN
> International Conference on Systems, Programming, Languages, and
> Applications: Software for Humanity -
> https://dl.acm.org/citation.cfm?id=3135947
> <https://dl.acm.org/citation.cfm?id=3135947>[1] - RFC: Devirtualization in
> LLVM (ver 1) -
> https://docs.google.com/document/d/1f2SGa4TIPuBGm6y6YO768GrQsA8awNfGEJSBFukLhYA/edit?usp=sharing
> <https://docs.google.com/document/d/1f2SGa4TIPuBGm6y6YO768GrQsA8awNfGEJSBFukLhYA/edit?usp=sharing>[2]
> - Devirtualization in LLVM - LLVM Dev meeting 2016 -
> https://www.youtube.com/watch?v=qMhV6d3B1Vk
> <https://www.youtube.com/watch?v=qMhV6d3B1Vk>[3] - Devirtualization in LLVM
> and Clang - llvm blog-
> http://blog.llvm.org/2017/03/devirtualization-in-llvm-and-clang.html
> <http://blog.llvm.org/2017/03/devirtualization-in-llvm-and-clang.html>AppendixHere
> are some other corner cases that we found.UnionsBecause union can have
> dynamic types as members, this means that they would have the same address.
> To solve this we emit launder.invariant.group before getting any dynamic
> type. It is possible that we could somehow avoid it, but because virtually
> no one is using unions this way, this should not cause any problems.struct
> A { virtual void foo();};struct B : A { void foo() override;};union U {
> A a; B b; U() : b() {}};void do_call(A &a) {
> a.foo();}__attribute__((noinline)) void init_B(U &u) { new(&u.b)
> B;}void union_test(U &u) { do_call(u.b); new(&u.a) A; do_call(u.a);
> init_B(u); do_call(u.b);}int main() { U u; union_test(u);}Ptr to
> intIn this example, instead of comparing the pointers we compare the
> addresses stored in integers. Because ptrtoint conversion will also require
> call to strip.invariant.group, this will also work.void ptr_to_int() { A
> *a = new A; a->foo(); A *b = new(a) B; auto av = (uintptr_t)a; auto
> bv = (uintptr_t)b; if (av == bv) b->foo();}int to ptrHere because
> we are creating a fat pointer from integer, we need to use launder:void
> foo(Base *base) { uintptr_t base_int = (uintptr_t)base; /* base_int =
> ptrtoint (strip base) */ Base *base_int_ptr = (Base*)base_int; /*
> base_int_ptr = inttoptr base_int = inttoptr (ptrtoint (strip base)) = strip
> base */ base_int_ptr->vfun(); ... Derived *derived = std::launder(base);
> uintptr_t derived_int = (uintptr_t)derived; /* derived_int = ptrtoint
> (strip derived) */ /* Note: base_int == derived_int */ .... Base
> *derived_int_ptr = (Base*)derived_int; /* derived_int_ptr = inttoptr
> (ptrtoint (strip derived)) */ /* Note: base_int_ptr == derived_int_ptr */
> derived_int_ptr->vfun(); /* BUG: But it's different than
> base_int_ptr->vfun()! */}*
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20180329/8e7ddd82/attachment.html>
More information about the cfe-dev
mailing list