[LLVMbugs] [Bug 23034] New: GCC ABI incompatibility visible in forward_as_tuple

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23034

            Bug ID: 23034
           Summary: GCC ABI incompatibility visible in forward_as_tuple
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Axel.Naumann at cern.ch
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Created attachment 14103
  --> https://llvm.org/bugs/attachment.cgi?id=14103&action=edit
Reproducer. Apologies for its preprocessed size :-(

With $ gcc --version
gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2
(and other 4.8.1 from RedHat)

and $ clang --version
clang version 3.7.0 (trunk 233280)


I get
clang++ -std=c++11 -c clanggcc.cxx -o clang.o
g++ -std=c++11 clanggcc.cxx clang.o -o a.out
./a.out
gcc: &rval=0x7fff3c1b2700
gcc: &c_str()=0xcec028
tpl: &c_str()=0xcec028
tpl:  c_str()=a
clang: &rval=0x7fff3c1b2718
clang: &c_str()=0xcec028
make: *** [test] Segmentation fault

with attached code.

The linker uses GCC's version of forward_as_tuple. GCC sets %rsi and %rdi to
call it; clang only sets %rdi. The implementation of forward_as_tuple compiled
by GCC needs both %rsi and %rdi.

And yes - not part of this test but clang's implementation of forward_as_tuple
only needs %rdi - so clang is consistent but incompatible with GCC.

Assembly of the relevant part of the GCC call:
   0x00000000004009e6 <+86>:    callq  0x400ab1
<std::move<std::string&>(std::string&)>
   0x00000000004009eb <+91>:    mov    %rax,%rdx
   0x00000000004009ee <+94>:    lea    -0x10(%rbp),%rax
   0x00000000004009f2 <+98>:    mov    %rdx,%rsi
   0x00000000004009f5 <+101>:    mov    %rax,%rdi
   0x00000000004009f8 <+104>:    callq  0x400b94
<std::forward_as_tuple<std::string>(std::string&&)>

...and for clang:
   0x0000000000400d4f <+95>:    callq  0x400ab1
<std::move<std::string&>(std::string&)>
   0x0000000000400d54 <+100>:    mov    %rax,%rdi
   0x0000000000400d57 <+103>:    callq  0x400b94
<std::forward_as_tuple<std::string>(std::string&&)>


with GCC's forward_as_tuple being:
=> 0x0000000000400b94 <+0>:    push   %rbp
   0x0000000000400b95 <+1>:    mov    %rsp,%rbp
   0x0000000000400b98 <+4>:    sub    $0x10,%rsp
   0x0000000000400b9c <+8>:    mov    %rdi,-0x8(%rbp)
   0x0000000000400ba0 <+12>:    mov    %rsi,-0x10(%rbp)
   0x0000000000400ba4 <+16>:    mov    -0x10(%rbp),%rax
   0x0000000000400ba8 <+20>:    mov    %rax,%rdi
   0x0000000000400bab <+23>:    callq  0x400aea
<std::forward<std::string>(std::remove_reference<std::string>::type&)>
   0x0000000000400bb0 <+28>:    mov    %rax,%rdx
   0x0000000000400bb3 <+31>:    mov    -0x8(%rbp),%rax
   0x0000000000400bb7 <+35>:    mov    %rdx,%rsi
   0x0000000000400bba <+38>:    mov    %rax,%rdi
   0x0000000000400bbd <+41>:    callq  0x400b64
<std::tuple<std::string&&>::tuple<std::string, void>(std::string&&)>
   0x0000000000400bc2 <+46>:    mov    -0x8(%rbp),%rax
   0x0000000000400bc6 <+50>:    leaveq 
   0x0000000000400bc7 <+51>:    retq   


Help!

Cheers, Axel.

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