[?] Fix gdb build with gcc-4.8.x
Commit Message
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.
Comments
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
From 493f67826232f0c4dab3a6bf69f851d01616e75f Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
Date: Sun, 16 Feb 2020 22:55:55 +0100
Subject: [PATCH] Fix build with gcc-4.8.x
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Make displaced_step_closure_up a wrapper class around
unique_ptr<displaced_step_closure> to avoid compiler
bugs with implicit casting from derived
unique_ptr<arch-specific data> to displaced_step_closure_up
observed with with gcc-4.8.4:
../../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>&&’
gdb:
2020-02-16 Bernd Edlinger <bernd.edlinger@hotmail.de>
* infrun (displaced_step_closure_up): Convert to a wrapper class.
---
gdb/infrun.h | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)
@@ -265,7 +265,23 @@ struct displaced_step_closure
virtual ~displaced_step_closure () = 0;
};
-using displaced_step_closure_up = std::unique_ptr<displaced_step_closure>;
+/* A wrapper class around std::unique_ptr<displaced_step_closure>.
+ Technically unnecessary, but helps avoiding a compiler bug. */
+
+struct displaced_step_closure_up : std::unique_ptr<displaced_step_closure>
+{
+ displaced_step_closure_up ()
+ {}
+
+ template <typename T>
+ displaced_step_closure_up (std::unique_ptr<T> &up)
+ : std::unique_ptr<displaced_step_closure> (up.release ())
+ {}
+
+ displaced_step_closure_up (displaced_step_closure *up)
+ : std::unique_ptr<displaced_step_closure> (up)
+ {}
+};
/* A simple displaced step closure that contains only a byte buffer. */
--
1.9.1