Re: [XEN PATCH v1 2/3] ocaml/libs: Fill build failure due to unused variable in ocaml macro

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/10/2024 6:47 pm, Andrew Cooper wrote:
> On 17/10/2024 5:20 pm, Javi Merino wrote:
>> On Fedora 40, the build fails with:
>>
>>     gcc -m64 -DBUILD_ID -fno-strict-aliasing -std=gnu99 -Wall 
>> -Wstrict-prototypes -Wno-unused-but-set-variable -Wno-unused-local-typedefs  
>>  -Werror -O2 -fomit-frame-pointer 
>> -D__XEN_INTERFACE_VERSION__=__XEN_LATEST_INTERFACE_VERSION__ -MMD -MP -MF 
>> .domain_getinfo_stubs_v1.o.d -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE   
>> -fPIC -I/usr/lib64/ocaml -I 
>> /build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../libs -I 
>> /build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../libs/xsd_glue
>>  
>> -I/build/tools/ocaml/libs/xsd_glue/domain_getinfo_plugin_v1/../../../../../tools/include
>>  -D__XEN_TOOLS__ 
>> -I/build/tools/ocaml/libs\/xsd_glue/domain_getinfo_plugin_v1/../../../../../tools/include
>>   -c -o domain_getinfo_stubs_v1.o domain_getinfo_stubs_v1.c
>>     In file included from domain_getinfo_stubs_v1.c:10:
>>     domain_getinfo_stubs_v1.c: In function 'xsd_glue_failwith':
>>     /usr/lib64/ocaml/caml/memory.h:275:29: error: unused variable 
>> 'caml__frame' [-Werror=unused-variable]
>>       275 |   struct caml__roots_block *caml__frame = *caml_local_roots_ptr
>>           |                             ^~~~~~~~~~~
>>     domain_getinfo_stubs_v1.c:48:9: note: in expansion of macro 'CAMLparam0'
>>       48 |         CAMLparam0();
>>          |         ^~~~~~~~~~
>>     cc1: all warnings being treated as errors
>>
>> The CAMLparam0 macro is defined in /usr/lib64/ocaml/caml/memory.h:255 as:
>>
>>     #define CAMLparam0()                                                    \
>>       struct caml__roots_block** caml_local_roots_ptr =                     \
>>         (DO_CHECK_CAML_STATE ? Caml_check_caml_state() : (void)0,           \
>>          &CAML_LOCAL_ROOTS);                                                \
>>       struct caml__roots_block *caml__frame = *caml_local_roots_ptr
>>
>> We can't modify the macro.  Mark the xsd_glue_failwith() function with
>> ignore "-Wunused-variable" to prevent gcc from failing the build due
>> to the unused variable.
>>
>> Fixes: a6576011a4d2 ("ocaml/libs: Implement a dynamically-loaded plugin for 
>> Xenctrl.domain_getinfo")
>> Signed-off-by: Javi Merino <javi.merino@xxxxxxxxx>
> That's horrible...
>
> I note the Ocaml manual even says:
>
> "Some C compilers give bogus warnings about unused variables
> caml__dummy_xxx at each use of CAMLparam and CAMLlocal. You should
> ignore them."  which a brave claim to make...
>
>
> The problem with pragma gcc is that we support Clang too.
>
> Having had a play, this works too
>
> @@ -45,7 +45,9 @@ static struct custom_operations xsd_glue_xenctrl_ops = {
>  static void Noreturn xsd_glue_failwith(
>         xc_interface *xch, const char *func, unsigned int line)
>  {
> +#define caml__frame __attribute__((unused)) caml__frame
>         CAMLparam0();
> +#undef caml__frame
>         CAMLlocal1(msg);
>         const xc_error *error = xch ? xc_get_last_error(xch) : NULL;
>         char *str = NULL;
>
> and is rather more selective.
>
>
> However, looking through other bits of memory.h, there's this gem:
>
> #define CAMLnoreturn ((void) caml__frame)
>
> which has existed since db3745919 "suppression des warnings "unused
> variable" de gcc" in 2004.
>
> So, I think this is a better fix:
>
> @@ -69,6 +69,7 @@ static void Noreturn xsd_glue_failwith(
>         free(str);
>  
>         caml_raise_with_arg(*caml_named_value("xsg.error_v1"), msg);
> +       CAMLnoreturn;
>  }
>  #define xsd_glue_failwith(xch) xsd_glue_failwith(xch, __func__, __LINE__)
>  
> and F40 seem happy with the result.

And https://github.com/ocaml/ocaml/pull/13561 to update Ocaml's
documentation on the matter.

~Andrew