[PATCH] D12247: [libc++] remove possible trailing padding from aligned_storage

[Eric Fiselier via cfe-commits]

EricWF added a comment.

In http://reviews.llvm.org/D12247#231444, @yiranwang wrote:

> A test case is as following. It has to be build by GCC 4.9 -O3 (maybe or later), with latest libc++, and for AARCH64+ANDROID target. 
>  AARCH64 requires 128 bit alignment for aligned_storage and 64 bit pointers, while gcc 4.9 alias analysis will do field-sensitive points-to analysis. But this could happen for other ISA+ABI.
>  The fundamental issue is that for this combination, std::function has member __buf_ declared as 
>  aligned_storage<3*sizoef(void*)>::type __buf_;
>  Basically, it is
>  aligned_storage<24>::type;
>  This will generate aligned_storage of, _Len==24 and _Align==16;
>  While std::function will use the __buf_ to sizeof(__buf_) bytes (at line 1593 and 1628 of <functional>), which is 32. Basically, the pointer to "tbool' will be stored at "&__buf_+24". 
>  This is not a well defined memory area, and GCC alias analysis is going to ignore the "ESCAPE" of address of "tbool". Basically, the function "test_simple" would always return "false".
>
> #include <functional>
>  extern void external_test(std::function<bool()>fn);
>  extern bool test_simple(){
>
>   bool tbool = false;
>   int a, b;
>   external_test([&a, &b, &tbool](){
>     tbool = true;
>     return true;
>   });
>   return tbool;
>
> }


What GCC flags do I need to build it in this configuration?


http://reviews.llvm.org/D12247