[Bug c++/108646] nonnull attribute does not detect variables that are NULL being passed

["jg at jguk dot org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108646

--- Comment #3 from Jonny Grant <jg at jguk dot org> ---
(In reply to Jonathan Wakely from comment #2)
> It already does detect it:
> 
> n.c: In function ‘test’:
> n.c:6:2: warning: argument 1 null where non-null expected [-Wnonnull]
>     6 |  mem2(dest);
>       |  ^~~~~~~~~~
> n.c:2:8: note: in a call to function ‘mem2’ declared ‘nonnull’
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
> 
> You need to enable optimization, otherwise there is no data flow analysis.
> 
> Without optimization, it detects it with -fanalyzer
> 
> n.c: In function ‘void test()’:
> n.c:6:6: warning: use of NULL ‘dest’ where non-null expected [CWE-476]
> [-Wanalyzer-null-argument]
>     6 |  mem2(dest);
>       |  ~~~~^~~~~~
>   ‘void test()’: events 1-2
>     |
>     |    5 |  char *dest = NULL;
>     |      |        ^~~~
>     |      |        |
>     |      |        (1) ‘dest’ is NULL
>     |    6 |  mem2(dest);
>     |      |  ~~~~~~~~~~
>     |      |      |
>     |      |      (2) argument 1 (‘dest’) NULL where non-null expected
>     |
> n.c:2:8: note: argument 1 of ‘void* mem2(void*)’ must be non-null
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
> 
> 
> And it's also detected at runtime using -fsanitize=undefined:
> 
> n.c:6:6: runtime error: null pointer passed as argument 1, which is declared
> to never be null


That's great it works with optimization, I should remember to always do
that.(In reply to Jonathan Wakely from comment #2)
> It already does detect it:
> 
> n.c: In function ‘test’:
> n.c:6:2: warning: argument 1 null where non-null expected [-Wnonnull]
>     6 |  mem2(dest);
>       |  ^~~~~~~~~~
> n.c:2:8: note: in a call to function ‘mem2’ declared ‘nonnull’
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
> 
> You need to enable optimization, otherwise there is no data flow analysis.
> 
> Without optimization, it detects it with -fanalyzer
> 
> n.c: In function ‘void test()’:
> n.c:6:6: warning: use of NULL ‘dest’ where non-null expected [CWE-476]
> [-Wanalyzer-null-argument]
>     6 |  mem2(dest);
>       |  ~~~~^~~~~~
>   ‘void test()’: events 1-2
>     |
>     |    5 |  char *dest = NULL;
>     |      |        ^~~~
>     |      |        |
>     |      |        (1) ‘dest’ is NULL
>     |    6 |  mem2(dest);
>     |      |  ~~~~~~~~~~
>     |      |      |
>     |      |      (2) argument 1 (‘dest’) NULL where non-null expected
>     |
> n.c:2:8: note: argument 1 of ‘void* mem2(void*)’ must be non-null
>     2 | void * mem2(void *dest) __attribute__((nonnull));
>       |        ^~~~
> 
> 
> And it's also detected at runtime using -fsanitize=undefined:
> 
> n.c:6:6: runtime error: null pointer passed as argument 1, which is declared
> to never be null


That's great. I should have remembered to add -O2

Is it worth -Wnonnull emitting a warning message that it needs optimization to
get the needed data flow analysis?