* Re: [PATCH ?] Fix gdb build with gcc-4.8.x
2020-02-18 20:27 ` [PATCH ?] Fix gdb build with gcc-4.8.x Simon Marchi
@ 2020-02-18 21:14 ` Bernd Edlinger
2020-02-18 21:17 ` Simon Marchi
0 siblings, 1 reply; 3+ messages in thread
From: Bernd Edlinger @ 2020-02-18 21:14 UTC (permalink / raw)
To: Simon Marchi, gdb-patches
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On 2/18/20 9:27 PM, Simon Marchi wrote:
> 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.
>
Okay, works for me, so then I added just one short comment at each site.
Is it OK for trunk?
Thanks
Bernd.
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From 43e6c14091dee9c32fe1dee62466450e57f6c13a Mon Sep 17 00:00:00 2001
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
Date: Sun, 16 Feb 2020 21:43:33 +0100
Subject: [PATCH] Fix build with gcc-4.8.x
Use an explicit conversion from unique_ptr<T> to
displaced_step_closure_up to avoid a compiler bug
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>
* aarch64-tdep.c (aarch64_displaced_step_copy_insn): Use an explicit
conversion.
* amd64-tdep.c (amd64_displaced_step_copy_insn): Likewise.
* arm-linux-tdep.c (arm_linux_displaced_step_copy_insn): Likewise.
* i386-tdep.c (i386_displaced_step_copy_insn): Likewise.
* rs6000-tdep.c (ppc_displaced_step_copy_insn): Likewise.
* s390-tdep.c (s390_displaced_step_copy_insn): Likewise.
---
gdb/aarch64-tdep.c | 3 ++-
gdb/amd64-tdep.c | 3 ++-
gdb/arm-linux-tdep.c | 3 ++-
gdb/i386-tdep.c | 3 ++-
gdb/rs6000-tdep.c | 3 ++-
gdb/s390-tdep.c | 3 ++-
6 files changed, 12 insertions(+), 6 deletions(-)
diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index bfacfb0..31b90c8 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3053,7 +3053,8 @@ aarch64_displaced_step_copy_insn (struct gdbarch *gdbarch,
dsc = NULL;
}
- return dsc;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (dsc.release ());
}
/* Implement the "displaced_step_fixup" gdbarch method. */
diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 9264fe4..5c56a97 100644
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -1511,7 +1511,8 @@ amd64_displaced_step_copy_insn (struct gdbarch *gdbarch,
displaced_step_dump_bytes (gdb_stdlog, buf, len);
}
- return dsc;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (dsc.release ());
}
static int
diff --git a/gdb/arm-linux-tdep.c b/gdb/arm-linux-tdep.c
index ccb556b..f60cb51 100644
--- a/gdb/arm-linux-tdep.c
+++ b/gdb/arm-linux-tdep.c
@@ -1131,7 +1131,8 @@ arm_linux_displaced_step_copy_insn (struct gdbarch *gdbarch,
arm_displaced_init_closure (gdbarch, from, to, dsc.get ());
- return dsc;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (dsc.release ());
}
/* Implementation of `gdbarch_stap_is_single_operand', as defined in
diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
index 9771421..19876c3 100644
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -831,7 +831,8 @@ i386_displaced_step_copy_insn (struct gdbarch *gdbarch,
displaced_step_dump_bytes (gdb_stdlog, buf, len);
}
- return closure;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (closure.release ());
}
/* Fix up the state of registers and memory after having single-stepped
diff --git a/gdb/rs6000-tdep.c b/gdb/rs6000-tdep.c
index 513ce6a..2c41e1c 100644
--- a/gdb/rs6000-tdep.c
+++ b/gdb/rs6000-tdep.c
@@ -894,7 +894,8 @@ ppc_displaced_step_copy_insn (struct gdbarch *gdbarch,
displaced_step_dump_bytes (gdb_stdlog, buf, len);
}
- return closure;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (closure.release ());
}
/* Fix up the state of registers and memory after having single-stepped
diff --git a/gdb/s390-tdep.c b/gdb/s390-tdep.c
index 51d0203..d8c28c7 100644
--- a/gdb/s390-tdep.c
+++ b/gdb/s390-tdep.c
@@ -477,7 +477,8 @@ s390_displaced_step_copy_insn (struct gdbarch *gdbarch,
displaced_step_dump_bytes (gdb_stdlog, buf, len);
}
- return closure;
+ /* This is a work around for a problem with g++ 4.8. */
+ return displaced_step_closure_up (closure.release ());
}
/* Fix up the state of registers and memory after having single-stepped
--
1.9.1
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