Re: [PATCH 1/2] nat/amd64-linux-siginfo.c: Move align attribute from typedef to struct

[Pedro Alves <pedro@palves.net>]

Hi John,

On 2021-06-05 10:20 p.m., John Baldwin wrote:
> On 6/4/21 5:14 PM, Pedro Alves wrote:
>> diff --git a/gdb/nat/amd64-linux-siginfo.c b/gdb/nat/amd64-linux-siginfo.c
>> index e2d2db6e112..9ff9361487a 100644
>> --- a/gdb/nat/amd64-linux-siginfo.c
>> +++ b/gdb/nat/amd64-linux-siginfo.c
>> @@ -206,7 +206,7 @@ typedef struct compat_siginfo
>>   /* For x32, clock_t in _sigchld is 64bit aligned at 4 bytes.  */
>>   typedef long __attribute__ ((__aligned__ (4))) compat_x32_clock_t;
>>   -typedef struct compat_x32_siginfo
>> +typedef struct __attribute__ ((__aligned__ (8))) compat_x32_siginfo
>>   {
>>     int si_signo;
>>     int si_errno;
>> @@ -263,7 +263,7 @@ typedef struct compat_x32_siginfo
>>         int _fd;
>>       } _sigpoll;
>>     } _sifields;
>> -} compat_x32_siginfo_t __attribute__ ((__aligned__ (8)));
>> +} compat_x32_siginfo_t;
>>     /* To simplify usage of siginfo fields.  */
> 
> Looks good to me.  Note that alignas() is part of C++11, so you could
> perhaps spell this a bit shorter as "alignas (8)" instead.
> 

That sounded like a good idea at first, however, if we were to do that,
I think we should do it to all __aligned__ attributes in the file.  But note
the one seen quoted above.  Here again:

  typedef long __attribute__ ((__aligned__ (4))) compat_x32_clock_t;

We can't use alignas for that, because alignas doesn't work with typedefs.  

GCC:

gdb/nat/amd64-linux-siginfo.c:207:14: error: attribute ignored [-Werror=attributes]
  207 | typedef long alignas (4) compat_x32_clock_t;
      |              ^~~~~~~

Clang:

gdb/nat/amd64-linux-siginfo.c:207:14: error: 'alignas' attribute cannot be applied to types
typedef long alignas (4) compat_x32_clock_t;
             ^

Clang attempt 2:

gdb/nat/amd64-linux-siginfo.c:207:33: error: 'alignas' attribute only applies to variables, data members and tag types
typedef long compat_x32_clock_t alignas (4);
                                ^


See answer 4 here:

 https://stackoverflow.com/questions/15788947/where-can-i-use-alignas-in-c11

  "These are the only places where the standard says an alignment-specifier (alignas(...)) may be applied. 
   Note that this does not include typedef declarations nor alias-declarations."

So for consistency I'm leaving the struct attribute as is too.

I'm going to merge the patches now.

Thanks,
Pedro Alves