[cfe-dev] Clang issue with __declspec(property)

[Reid Kleckner]

David took a look at this. If you could file a PR for it, that'd be great.

On Thu, Jul 9, 2015 at 9:38 AM, chen xu <chenxuz at outlook.com> wrote:

> When using clang (3.6.0, on Ubuntu 15.04) to build a C++ class with
> __declspec(property), we observed a strange behavior, please see sample
> code below.
>
> 00  #include <stdio.h>
> 01  class Test1 {
> 02  private:
> 03      int x_;
> 04  public:
> 05      Test1(int x) : x_(x) {}
> 06      __declspec(property(get=get_x)) int X;
> 07      int get_x() const { return x_; }
> 08      static Test1* GetTest1() { printf("create, "); return new
> Test1(10); }
> 09  };
> 10  int main(int argc, char** argv) {
> 11      int x = Test1::GetTest1()->X;
> 12      printf("x=%d\n",x);
> 13  }
>
> The execution output is like:
> create, create, x=10
>
> Basically, the compiler builds the code line 11 into
> 0x000000000040077f call 0x4007c0 <Test1::GetTest1()>
> 0x0000000000400784 mov QWORD PTR [rbp-0x20],rax
> 0x0000000000400788 call 0x4007c0 <Test1::GetTest1()>
> 0x000000000040078d mov rdi,rax
> 0x0000000000400790 call 0x400830 <Test1::get_x() const>
> 0x0000000000400795 movabs rdi,0x4008e4
> 0x000000000040079f mov DWORD PTR [rbp-0x14],eax
>
> As we notice, Test1::GetTest1() is called twice. The result of the 1st
> call is stored into a compiler-allocated stack variable, and seems never
> used later; The result of the 2nd call is passed as "this" pointer to
> Test1::get_x(). Is this an expected behavior? Since our real GetTest1() is
> not idempotent, being called twice is causing incorrect program behavior.
> Could any folk working on __declspec(property) feature of clang take a
> look? Thanks a lot in advance.
>
>
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>
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