[analyzer] EvalBinOpLL should return Unknown less often (#114222)

SValBuilder::getKnownValue, getMinValue, getMaxValue use
SValBuilder::simplifySVal.

simplifySVal does repeated simplification until a fixed-point is
reached. A single step is done by SimpleSValBuilder::simplifySValOnce,
using a Simplifier visitor. That will basically decompose SymSymExprs,
and apply constant folding using the constraints we have in the State.
Once it decomposes a SymSymExpr, it simplifies both sides and then uses
the SValBuilder::evalBinOp to reconstruct the same - but now simpler -
SymSymExpr, while applying some caching to remain performant.

This decomposition, and then the subsequent re-composition poses new
challenges to the SValBuilder::evalBinOp, which is built to handle
expressions coming from real C/C++ code, thus applying some implicit
assumptions.

One previous assumption was that nobody would form an expression like
"((int*)0) - q" (where q is an int pointer), because it doesn't really
makes sense to write code like that.

However, during simplification, we may end up with a call to evalBinOp
similar to this.

To me, simplifying a SymbolRef should never result in Unknown or Undef,
unless it was Unknown or Undef initially or, during simplification we
realized that it's a division by zero once we did the constant folding,
etc.

In the following case the simplified SVal should not become UnknownVal:
```c++
void top(char *p, char *q) {
  int diff = p - q; // diff: reg<p> - reg<q>
  if (!p) // p: NULL
    simplify(diff); // diff after simplification should be: 0(loc) - reg<q>
}
```

Returning Unknown from the simplifySVal can weaken analysis precision in
other places too, such as in SValBuilder::getKnownValue, getMinValue, or
getMaxValue because we call simplifySVal before doing anything else.

For nonloc::SymbolVals, this loss of precision is critical, because for
those the SymbolRef carries an accurate type of the encoded computation,
thus we should at least have a conservative upper or lower bound that we
could return from getMinValue or getMaxValue - yet we would just return
nullptr.

```c++
const llvm::APSInt *SimpleSValBuilder::getKnownValue(ProgramStateRef state,
                                                      SVal V) {
  return getConstValue(state, simplifySVal(state, V));
}

const llvm::APSInt *SimpleSValBuilder::getMinValue(ProgramStateRef state,
                                                    SVal V) {
  V = simplifySVal(state, V);

  if (const llvm::APSInt *Res = getConcreteValue(V))
    return Res;

  if (SymbolRef Sym = V.getAsSymbol())
    return state->getConstraintManager().getSymMinVal(state, Sym);

  return nullptr;
}
```

For now, I don't plan to make the simplification bullet-proof, I'm just
explaining why I made this change and what you need to look out for in
the future if you see a similar issue.

CPP-5750

0 files changed, 0 insertions, 0 deletions