Re: [PATCH] warn for more impossible null pointer tests

[Koning, Paul via Gcc-patches]

> On Sep 1, 2021, at 3:35 PM, Iain Sandoe <idsan...@googlemail.com> wrote:
> 
> 
> [EXTERNAL EMAIL] 
> 
> Hi Paul,
> 
>> ...
>> If so, then I would think that ignoring it for this patch as well is 
>> reasonable.  If in a given target a pointer that C thinks of as NULL is in 
>> fact a valid object pointer, then all sorts of optimizations are incorrect.  
>> If the target really cares, it can use a different representation for the 
>> null pointer.  (Does GCC give us a way to do that?)  For example, pdp11 
>> could use the all-ones bit pattern to represent an invalid pointer.
> 
> regardless of whether GCC supports it or not - trying to use a non-0 NULL 
> pointer is likely to break massive amounts of code in the wild.

It depends on what you mean by "non-0 NULL pointer".  The constant written as 0 
in pointer context doesn't represent the all-zeroes bit pattern but rather 
whatever is a null pointer on that target.  Most code would not notice that.  
The two places I can think of where this would break is (a) if you cast a 
pointer to int or look at it via a pointer/int union and expect to see integer 
zero, and (b) if you initialize pointers by using bzero.  The former seems 
rather unlikely, the latter is somewhat common.  Can GCC detect the bzero case? 
 It would make a good check for -Wpedantic on the usual platforms that use all 
zero bits as NULL.

> It might, OTOH, be possible to use a non-0 special value to represent the 
> valid 0 address-use (providing that there is somewhere in the address space 
> you can steal that from).

That would be really ugly, because every pointer reference would have to do the 
address translation at run time.

        paul