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

[Koning, Paul via Gcc-patches]

> On Sep 1, 2021, at 3:08 PM, Jeff Law via Gcc-patches 
> <gcc-patches@gcc.gnu.org> wrote:
> 
> 
> 
> On 9/1/2021 12:57 PM, Koning, Paul wrote:
>> 
>>> On Sep 1, 2021, at 1:35 PM, Jeff Law via Gcc-patches 
>>> <gcc-patches@gcc.gnu.org> wrote:
>>> 
>>> Generally OK.  There's some C++ front-end bits that Jason ought to take a 
>>> quick looksie at.   Second, how does this interact with targets that allow 
>>> objects at address 0?   We have a few targets like that and that makes me 
>>> wonder if we should be suppressing some, if not all, of these warnings for 
>>> targets that turn on -fno-delete-null-pointer-checks?
>> But in C, the pointer constant 0 represents the null (invalid) pointer, not 
>> the actual address zero necessarily.
>> 
>> If a target supports objects at address zero, how does it represent the 
>> pointer value 0 (which we usually refer to as NULL)?  Is the issue simply 
>> ignored?  It seems to me it is in pdp11, which I would guess is one of the 
>> targets for which objects at address 0 make sense.
> The issue is ignored to the best of my knowledge.

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.

        paul