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

[Martin J. O'Riordan via cfe-dev]

Hi Anton,

Sorry, I did 'Reply' instead of 'Reply All'

Actually, I should've added that the '__shared_ptr_pointer::__get_deleter' is also similarly affected, but is not qualified by '_LIBCPP_BUILD_STATIC', and that the same is true for the 'functional implementation although I have no test cases yet which illustrate this as a problem.

Thanks,

	MartinO

-----Original Message-----
From: Martin J. O'Riordan [mailto:martin.oriordan at movidius.com] 
Sent: 30 April 2017 20:22
To: 'Anton Korobeynikov' <anton at korobeynikov.info>
Subject: RE: [cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'

Exactly.  When building the LibC++ library, '_LIBCPP_BUILD_STATIC' is defined.

But this means that the ABI for the 'vtable' is now a function of RTTI enabled vs disabled.

	MartinO

-----Original Message-----
From: Anton Korobeynikov [mailto:anton at korobeynikov.info]
Sent: 30 April 2017 18:39
To: Martin J. O'Riordan <martin.oriordan at movidius.com>
Cc: Clang Dev <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [LibC++] Bug in implementation of 'std::shared_ptr'

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_s
> haredEv
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_s
> hared_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_s
> haredEv
>
>     .word      _ZNKSt3__119__shared_weak_count13__get_deleterERKSt9type_info
>
>     .word
> _ZNSt3__120__shared_ptr_emplaceI4TestNS_9allocatorIS1_EEE21__on_zero_s
> hared_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