[cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'

[Anton Korobeynikov via cfe-dev]

Martin,

How the '_LIBCPP_BUILD_STATIC' define got set in your case? It should
only appear when building libc++ itself.

On Thu, Apr 27, 2017 at 6:27 PM, Martin J. O'Riordan via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> I have a test that has been failing for a long time, and finally got around
> to investigating it.  The test is really simple:
>
>
>
> #include <iostream>
>
> #include <memory>
>
>
>
> struct Test {
>
>   int n;
>
>   Test(int x) : n(x) { std::cerr << "Creating " << n << std::endl; }
>
>   ~Test() { std::cerr << "Deleting " << n << std::endl; }
>
> };
>
>
>
> int main () {
>
>   std::shared_ptr<Test> x(std::make_shared<Test>(42));
>
>   x.reset();
>
>   std::cerr << "Completed " << std::endl;
>
> }
>
>
>
> but it crashes immediately after printing the message from the destructor,
> and before the message in ‘main’.
>
>
>
> After investigating, I found that the test works perfectly with RTTI
> enabled, but crashes if it is disabled which puzzled me.  My LibC++ library
> is built with RTTI enabled as instructed on ‘libcxx.llvm.org’ which says
> that the library must be built with RTTI enabled, though it may be used with
> RTTI disabled.
>
>
>
> What I found was that the ‘vtable’ for the ‘shared_ptr’ specialisation is
> different depending on whether RTTI is enabled or disabled.  With RTTI
> disabled it is:
>
>
>
> _ZTVNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE:
>
>     .word      0
>
>     .word      0
>
>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED1Ev
>
>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED0Ev
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE16__on_zero_sharedEv
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_shared_weakEv
>
>
>
> but with RTTI enabled it is:
>
>
>
> _ZTVNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE:
>
>     .word      0
>
>     .word      _ZTINSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEEE
>
>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED1Ev
>
>     .word      _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEED0Ev
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE16__on_zero_sharedEv
>
>     .word      _ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_shared_weakEv
>
>
>
> In the library, the implementation (in ‘memory.cpp’ compiler with ‘-frtti’)
> is attempting to call the function ‘__on_zero_shared_weak’, but using offset
> 24, and the ‘vtable’ emitted in the test case (compiled with ‘-fno-rtti’)
> has this function at offset 20.
>
>
>
> This is caused by the following lines in ‘<memory>’:
>
>
>
>     // Define the function out only if we build static libc++ without RTTI.
>
>     // Otherwise we may break clients who need to compile their projects
> with
>
>     // -fno-rtti and yet link against a libc++.dylib compiled
>
>     // without -fno-rtti.
>
> #if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
>
>     virtual const void* __get_deleter(const type_info&) const _NOEXCEPT;
>
> #endif
>
>
>
> and because the function is virtual, the layout of the ‘vtable’ is different
> between RTTI enabled and disabled (we are building a static library, so
> ‘_LIBCPP_BUILD_STATIC’ is true).
>
>
>
> There are a couple of places in ‘<memory>’ and ‘memory.cpp’ where this
> happens (always with ‘__get_deleter’), but after sanity checking the other
> headers, I see that ‘<functional>’ and ‘<__functional_03>’ also have similar
> issues where the layout of the ‘vtable’ is different depending on whether
> RTTI is enabled or not; though I don’t have any tests which show this.
>
>
>
> I don’t know what the best fix is for this because it was clearly introduced
> to address some issue with dynamic libraries, but locally I have decided to
> always define ‘__get_deleter’, and make its implementation return ‘nullptr’
> when RTTI is disabled.  This preserves the ‘vtable’ layout independent of
> RTTI.
>
>
>
> Thanks,
>
>
>
>             MartinO
>
>
>
>
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>



-- 
With best regards, Anton Korobeynikov
Department of Statistical Modelling, Saint Petersburg State University