[PR middle-end/79123] cast false positive in -Walloca-larger-than=

[Aldy Hernandez <aldyh at redhat dot com>]

In the attached testcase, we have a clearly bounded case of alloca which 
is being incorrectly reported:

void g (int *p, int *q)
{
   size_t n = (size_t)(p - q);

   if (n < 10)
     f (__builtin_alloca (n));
}

The problem is that VRP gives us an anti-range for `n' which may be out 
of range:

  # RANGE ~[2305843009213693952, 16140901064495857663]
   n_9 = (long unsigned int) _4;

We do a less than stellar job with casts and VR_ANTI_RANGE's, mostly 
because we're trying various heuristics to make up for the fact that we 
have crappy range info from VRP.  More specifically, we're basically 
punting on an VR_ANTI_RANGE and ignoring that the casted result (n_9) 
has a bound later on.

Luckily, we already have code to check simple ranges coming into the 
alloca by looking into all the basic blocks feeding it.  The attached 
patch delays the final decision on anti ranges until we have examined 
the basic blocks and determined for that we are definitely out of range.

I expect all this to disappear with Andrew's upcoming range info overhaul.

OK for trunk?

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