Re: Type-punning warnings [was Re: PATCH: silence warnings in unwind-dw2-fde.c]

[Andrew Haley <aph at redhat dot com>]

Dave Korn wrote:
> Ben Elliston wrote:
>> This patch silences the following warnings when building libgcc:
>>
>> unwind-dw2-fde.c:321: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules
> 
>> -  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin;
>> -  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin;
>> +  _Unwind_Ptr x_ptr, y_ptr;
>> +  memcpy (&x_ptr, x->pc_begin, sizeof (_Unwind_Ptr));
>> +  memcpy (&y_ptr, y->pc_begin, sizeof (_Unwind_Ptr));
> 
>   Say, I've been in the habit in the past (GCC 3.x) of avoiding these warnings
> by doing like:
> 
>> -  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin;
>> -  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin;
>> +  const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) (void *) x->pc_begin;
>> +  const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) (void *) y->pc_begin;
> 
> ... which I see no longer works.  Was that ever correct, or was I just losing
> information during the cast-to-void that meant GCC just couldn't diagnose a
> potential problem, but it was still there?  I believed that casting through
> void would make the result could-alias-anything, which I thought would prevent
> any untoward consequences.

Casting via void* just shuts up a warning; the bad code is still there.
But this case should be fine AFAICS, because pc_begin is char[].  I'm not
sure why we get the warning.

Andrew.