[Bug analyzer/109266] Wanalyzer-null-dereference does not warn when struct is at null

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109266

--- Comment #3 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
(In reply to Jonny Grant from comment #2)
> Thank you for your reply David. Your analyzer is very good already.
> 
> I played around a bit, a base of nullptr doesn't give a warning. But
> changing to 0x10 does give array-bounds warning.
> cc1plus: note: source object is likely at address zero
> <source>:13:13: warning: array subscript 0 is outside array bounds of 'a_t
> [0]' [-Warray-bounds=]
> 
> https://godbolt.org/z/PhhT48xxP

FWIW, note that [-Warray-bounds=] is separate from -fanalyzer.

> 
> Found Andrew Pinski comment says 4096 is not accessible:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106699#c1

Aha - thanks for the link!  I think that's the thing that I was
half-remembering (well, its dup, PR 99578), and that it was, in fact, in GCC.

Looks like I should extend -Wanalyzer-null-dereference to warn about accesses
to constant addresses, but have it respect --param=min-pagesize=
(see r11-9731-g91f7d7e1bb6827bf8e0b7ba7eb949953a5b1bd18).  This would have to
wait for GCC 14 at this point in the release cycle.

> 
> I wondered if you know how to turn on that "cc1plus: note: source object is
> likely at address zero? It seems different from normal warnings.

Grepping the sources shows it's from gcc/pointer-query.cc:
access_ref::inform_access; I think it's one of the middle-end warnings that
triggers that, but I'm not sure exactly which (the analyzer doesn't use that at
the moment).


> It would be fantastic if there was a way for me to specify on the gcc
> command line an address range I didn't want read and/or writable. That would
> be great to get build warnings from those addresses if the compiler could
> see them being accessed.

Is this always for stuff near the 0 address, or are there other use cases? 
(embedded?)  Are you able to post an example here of what the input might look
like?


> 
> At the moment, I always need to use the JTAG debugger to set some hw
> breakpoints on read from various addresses to catch those accesses (as they
> are mapped to the interrupt vector from 0x0). On Windows I've had various
> crashes where the access was address 0x10 so felt like that was probably a
> struct offset too
> 
> I don't know very much about gcc internals. I did wonder if the analyzer can
> see the base address of the struct being passed as 0x0 in the RTL file?
> I tried -fdump-rtl-all but couldn't see the 0x0 address, or when I changed
> to 0x10 either

The analyzer works on the gimple-ssa representation, which is before it become
RTL.  If you want to see the gory details, have a look in:
  https://gcc.gnu.org/onlinedocs/gccint/Analyzer-Internals.html
in the gcc internal docs, and:
  https://gcc-newbies-guide.readthedocs.io/en/latest/inside-cc1.html
in the guide for new gcc contributors I wrote.