Re: Building with recent GCC versions: gdbsupport/gdb_assert.h:35:4: error: 'nonnull' argument 'filename' compared to NULL [-Werror=nonnull-compare]

On 7/27/21 1:35 PM, Andrew Burgess wrote:
> * Tom de Vries <[email protected]> [2021-07-27 12:44:10 +0200]:
> 
>> On 7/27/21 12:03 PM, Andrew Burgess wrote:
>>> * Jan-Benedict Glaw <[email protected]> [2021-07-26 23:11:01 +0200]:
>>>
>>>> Hi!
>>>>
>>>> I'm running some CI builds and noticed that, when building GDB with
>>>> quite recent GCC versions, it breaks.
>>>>
>>>> With ie. this "gcc-snapshot" GCC from Debian:
>>>>
>>>> /usr/lib/gcc-snapshot/bin/gcc --version
>>>> gcc (Debian 20210630-1) 12.0.0 20210630 (experimental) [master revision 6bf383c37e6:93c270320bb:35da8a98026849bd20d16bbf9210ac1d0b44ea6a]
>>>>
>>>> we see:
>>>>
>>>> ./configure --target=i686-linux --prefix=/tmp/gdb-i686-linux
>>>> [...]
>>>> all make V=1 all-gdb
>>>> [...]
>>>> [all 2021-07-26 20:39:22] /usr/lib/gcc-snapshot/bin/g++ -x c++    -I. -I. -I./config -DLOCALEDIR="\"/tmp/gdb-i686-linux/share/locale\"" -DHAVE_CONFIG_H -I./../include/opcode -I./../readline/readline/.. -I./../zlib -I../bfd -I./../bfd -I./../include -I../libdecnumber -I./../libdecnumber  -I./../gnulib/import -I../gnulib/import -I./.. -I..  -DTUI=1    -I./.. -pthread  -Wall -Wpointer-arith -Wno-unused -Wunused-value -Wunused-variable -Wunused-function -Wno-switch -Wno-char-subscripts -Wempty-body -Wunused-but-set-parameter -Wunused-but-set-variable -Wno-sign-compare -Wno-error=maybe-uninitialized -Wno-mismatched-tags -Wsuggest-override -Wimplicit-fallthrough=3 -Wduplicated-cond -Wshadow=local -Wdeprecated-copy -Wdeprecated-copy-dtor -Wredundant-move -Wmissing-declarations -Wstrict-null-
 sentinel -Wformat -Wformat-nonliteral -Werror -g -O2   -c -o compile/compile.o -MT compile/compile.o -MMD -MP -MF compile/.deps/compile.Tpo compile/compile.c
>>>> [all 2021-07-26 20:39:26] In file included from ./../gdbsupport/common-defs.h:126,
>>>> [all 2021-07-26 20:39:26]                  from ./defs.h:28,
>>>> [all 2021-07-26 20:39:26]                  from compile/compile.c:20:
>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_unlinker.h: In constructor 'gdb::unlinker::unlinker(const char*)':
>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_assert.h:35:4: error: 'nonnull' argument 'filename' compared to NULL [-Werror=nonnull-compare]
>>>> [all 2021-07-26 20:39:26]    35 |   ((void) ((expr) ? 0 :                                                       \
>>>> [all 2021-07-26 20:39:26]       |   ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> [all 2021-07-26 20:39:26]    36 |            (gdb_assert_fail (#expr, __FILE__, __LINE__, FUNCTION_NAME), 0)))
>>>> [all 2021-07-26 20:39:26]       |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>> [all 2021-07-26 20:39:26] ./../gdbsupport/gdb_unlinker.h:38:5: note: in expansion of macro 'gdb_assert'
>>>> [all 2021-07-26 20:39:26]    38 |     gdb_assert (filename != NULL);
>>>> [all 2021-07-26 20:39:26]       |     ^~~~~~~~~~
>>>> [all 2021-07-26 20:39:27] cc1plus: all warnings being treated as errors
>>>> [all 2021-07-26 20:39:27] make[1]: *** [Makefile:1642: compile/compile.o] Error 1
>>>> [all 2021-07-26 20:39:27] make[1]: Leaving directory '/var/lib/laminar/run/gdb-i686-linux/4/binutils-gdb/gdb'
>>>> [all 2021-07-26 20:39:27] make: *** [Makefile:11410: all-gdb] Error 2
>>>>
>>>>
>>>> I also discussed this on the GCC patches mailing list
>>>> (https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575568.html),
>>>> where Martin suggested that this should be fixed here in GDB.
>>>>
>>>> Any thoughts about this?
>>>
>>> As I understand it the nonnull attribute only provides compile time
>>> protection against explicitly passing NULL, there's no compiled in
>>> non-null check (well, maybe with -fisolate-erroneous-paths-attribute,
>>> but the assert might give a better error message).
>>>
>>> This means its still possible to pass NULL to a nonnull function, its
>>> just the behaviour of the program is undefined in that case.
>>>
>>> So, it doesn't seem crazy that we might want to both (a) have a
>>> function declared nonnull, to prevent explicitly passing NULL, and (b)
>>> have a NULL check inside the function to catch logic bugs that result
>>> in NULL being passed.
>>>
>>> We could, of course, push the assert outside of the function, but that
>>> would really suck due to code duplication, and the risk of missing an
>>> assert, so that seems like a non-starter.
>>>
>>> We could drop either the assert, or the nonnull attribute, but that
>>> would suck as both give a valuable, but different form of protection.
>>>
>>> After some experimenting, I suspect that the assert is being optimised
>>> away anyway, which kind of makes sense, as we're telling the compiler
>>> it can assume that the pointer is non-null.
>>>
>>
>> Yes, in fact that's what the nonnull-compare warning specifically warns
>> against: there's some code that may be optimized away, due to the
>> nonnull attribute.
>>
>>> So, what we probably want is someway to tell (or trick) GCC into
>>> including the null check even in the nonnull function....
>>>
>>> ... here's what I came up with, add this somewhere:
>>>
>>>  template<typename T>
>>>  bool __attribute__ ((noinline))
>>>  nonnull_arg_is_really_not_null (const T *ptr)
>>>  {
>>>    return ptr != nullptr;
>>>  }
>>>
>>> then change the assert to:
>>>
>>>  gdb_assert (nonnull_arg_is_really_not_null (filename));
>>>
>>> Seems to keep the assert, and silence the warning.  Thoughts?
>>>
>>
>> I understand why it works, but it seems fragile to me.  At some point
>> some compiler may get smart enough to also optimize this case, and then
>> we're back in the same situation.
> 
> Good point.
> 
> The GCC documentation for noinline[1] suggests we can avoid the call
> being removed by adding 'asm ("");' into the function:
> 
>   template<typename T>
>   bool __attribute__ ((noinline))
>   nonnull_arg_is_really_not_null (const T *ptr)
>   {
>     asm ("");
>     return ptr != nullptr;
>   }
> 
> I'm not really arguing for this approach over any other, just sharing
> what I discovered.
> 

Ack, understood.  Note that the added asm doesn't stop a compiler from
doing:
...
gdb_assert (nonnull_arg_is_really_not_null (filename));
...
->
...
nonnull_arg_is_really_not_null (filename);
gdb_assert (true);
...

Thanks,
- Tom

> Thanks,
> Andrew
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Common-Function-Attributes.html#Common-Function-Attributes
> 
>>
>> I wonder whether using volatile is a better idea (can't try this out
>> right now).
>>
>> Thanks,
>> - Tom