[PATCH ?] Fix gdb build with gcc-4.8.x

[Simon Marchi]

On 2020-02-18 2:06 p.m., Bernd Edlinger wrote:
> Hi,
> 
> I noticed that gdb cannot be built any more with gcc-4.8.4
> since Simon's patch which introduced the
> std::unique_ptr<displaced_step_closure>.
> 
> The failure mode is as follows:
> 
>   CXX    amd64-tdep.o
> ../../binutils-gdb/gdb/amd64-tdep.c: In function ‘displaced_step_closure_up amd64_displaced_step_copy_insn(gdbarch*, CORE_ADDR, CORE_ADDR, regcache*)’:
> ../../binutils-gdb/gdb/amd64-tdep.c:1514:10: error: cannot bind ‘std::unique_ptr<amd64_displaced_step_closure>’ lvalue to ‘std::unique_ptr<amd64_displaced_step_closure>&&’
>    return dsc;
>           ^
> In file included from /usr/include/c++/4.8/memory:81:0,
>                  from ../../binutils-gdb/gdb/../gdbsupport/common-exceptions.h:25,
>                  from ../../binutils-gdb/gdb/../gdbsupport/common-defs.h:140,
>                  from ../../binutils-gdb/gdb/defs.h:28,
>                  from ../../binutils-gdb/gdb/amd64-tdep.c:22:
> /usr/include/c++/4.8/bits/unique_ptr.h:169:2: error:   initializing argument 1 of ‘std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Up, _Ep>&&) [with _Up = amd64_displaced_step_closure; _Ep = std::default_delete<amd64_displaced_step_closure>; <template-parameter-2-3> = void; _Tp = displaced_step_closure; _Dp = std::default_delete<displaced_step_closure>]’
>   unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept
>   ^
> ../../binutils-gdb/gdb/amd64-tdep.c:1515:1: error: control reaches end of non-void function [-Werror=return-type]
>  }
>  ^
> cc1plus: all warnings being treated as errors
> 
> 
> It continues to work with gcc-5.4.0, though.  I don't know what
> is with gcc-4.9.x.
> 
> I have two possible workarounds for this attached to this as
> variant-1 patch and variant-2 patch respectively.  I personally
> would prefer variant-2 which makes displaced_step_closure_up a
> wrapper class around unique_ptr<displaced_step_closure> and
> avoids to trigger this compiler bug by being slightly simpler as
> the original, I think the issue always starts when the argument
> to displaced_step_closure_up (std::unique_ptr<T> &up) is
> using double-ampersand.
> 
> So we have three possible ways to deal with this:
> variant-1: simplify the code where the type cast happens,
> variant-2: use a simplified wrapper clase, and
> variant-3: do nothing about it, and document that gcc-5.4.0 is
> or newer is required.
> 
> What do you think?
> 
> 
> Thanks
> Bernd.
> 

Hi Bernd,

I don't like variant 2, because it changes the API/contract of
std::unique_ptr.  It allows doing

  std::unique_ptr<amd64_displaced_step_closure> dsc;
  displaced_step_closure_up hello (dsc);

Which would not be possible if displaced_step_closure_up was
a simple typedef.  In our code base, the types that end with _up
are known to be typedefs to std::unique_ptr, and I don't think it
would be a good idea to provide such a type with a semantic that
differs from std::unique_ptr.

So I would prefer something along the lines of variant 1, but with
a small comment at each site saying that this is to work around
a problem with g++ 4.8.

Thanks,

Simon