Re: Uninit warnings due to optimizing short-circuit conditionals

[David Malcolm <dmalcolm@redhat.com>]

On Mon, 2022-02-14 at 17:57 +0100, Mark Wielaard wrote:
> Hi David,
> 
> On Mon, 2022-02-14 at 10:57 -0500, David Malcolm wrote:
> > [CCing Mark in the hopes of insight from the valgrind side of
> > things]
> 
> Adding Julian to CC so he can correct me if I say something silly.
> 
> > There is a false positive from -Wanalyzer-use-of-uninitialized-
> > value on
> > gcc.dg/analyzer/pr102692.c here:
> > 
> >   ‘fix_overlays_before’: events 1-3
> >     |
> >     |   75 |   while (tail
> >     |      |          ~~~~
> >     |   76 |          && (tem = make_lisp_ptr (tail, 5),
> >     |      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >     |      |          |
> >     |      |          (1) following ‘false’ branch (when ‘tail’ is
> > NULL)...
> >     |   77 |              (end = marker_position (XOVERLAY (tem)-
> > >end)) >= pos))
> >     |      |             
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >     |......
> >     |   82 |   if (!tail || end < prev || !tail->next)
> >     |      |       ~~~~~    ~~~~~~~~~~
> >     |      |       |            |
> >     |      |       |            (3) use of uninitialized value
> > ‘end’ here
> >     |      |       (2) ...to here
> >     |
> > 
> > The issue is that inner || of the conditionals have been folded
> > within the
> > frontend from a chain of control flow:
> > 
> >    5   │   if (tail == 0B) goto <D.1986>; else goto <D.1988>;
> >    6   │   <D.1988>:
> >    7   │   if (end < prev) goto <D.1986>; else goto <D.1989>;
> >    8   │   <D.1989>:
> >    9   │   _1 = tail->next;
> >   10   │   if (_1 == 0B) goto <D.1986>; else goto <D.1987>;
> >   11   │   <D.1986>:
> > 
> > to an OR expr (and then to a bitwise-or by the gimplifier):
> > 
> >    5   │   _1 = tail == 0B;
> >    6   │   _2 = end < prev;
> >    7   │   _3 = _1 | _2;
> >    8   │   if (_3 != 0) goto <D.1986>; else goto <D.1988>;
> >    9   │   <D.1988>:
> >   10   │   _4 = tail->next;
> >   11   │   if (_4 == 0B) goto <D.1986>; else goto <D.1987>;
> > 
> > This happens for sufficiently simple conditionals in
> > fold_truth_andor.
> > In particular, the (end < prev) is short-circuited without
> > optimization,
> > but is evaluated with optimization, leading to the false positive.
> > 
> > Given how early this folding occurs, it seems the simplest fix is
> > to
> > try to detect places where this optimization appears to have
> > happened,
> > and suppress uninit warnings within the statement that would have
> > been short-circuited (and thus e.g. ignoring them when evaluating
> > _2
> > above for the case where _1 is known to be true at the (_1 | _2) ,
> > and
> > thus _2 being redundant).
> > 
> > Attached is a patch that implements this.
> > 
> > There are some more details in the patch, but I'm wondering if this
> > is a
> > known problem, and how e.g. valgrind copes with such code.  My
> > patch
> > feels like something of a hack, but I'm not sure of any other way
> > around
> > it given that the conditional is folded directly within the
> > frontend.
> 
> As far as I know this is what valgrind memcheck also does with an
> bitwise or. It knows that _3 is defined and true if either _1 or _2
> is
> defined and true. Or more generically that the result bits of a
> bitwise
> or are defined for those bits that are both defined or where one is
> defined and has the value 1.

Aha - thanks.  I think the distinction here is that:

* GCC's -fanalyzer complains about uninitialized values immediately
when it sees one being fetched for use in any expression (replacing the
value with a safe one to avoid further complaints), without considering
how they are going to be used in the expression, whereas

* it sounds like valgrind keeps track of uninitialized bits, propagates
the "uninit-ness" of the bits, and complains at certain times when
uninitialized bits are used in certain ways.

Dave