[PATCH] D35110: [Analyzer] Constraint Manager Negates Difference

[Balogh, Ádám via Phabricator via cfe-commits]

baloghadamsoftware added a comment.

In https://reviews.llvm.org/D35110#972430, @NoQ wrote:

> In https://reviews.llvm.org/D35110#969782, @baloghadamsoftware wrote:
>
> > Strange, but modifying the tests from `m <relation> n` to `m - n <relation> 0`  does not help. The statement `if (m - n <relation> 0)` does not store a range for `m - n` in the constraint manager. With the other patch which automatically changes `m <relation> n` to `m - n <relation> 0` the range is stored automatically.
>
>
> I guess we can easily assume how a `SymIntExpr` relates to a number by storing a range on the opaque left-hand-side symbol, no matter how complicated it is, but we cannot assume how a symbol relates to a symbol (there's no obvious range to store). That's just how `assumeSym` currently works.


Actually it happens because `m - n` evaluates to `Unknown`. The code part responsible for this is the beginning of `SValBuilder::makeSymExprValNN()`, which prevents `m - n`-like symbolic expression unless one of `m` or `n` is `Tainted`. Anna added this part 5-6 years ago because some kind of bug, but it seems that it still exists. If I try to remove it then one test executes for days (with loop max count 63 or 64) and two tests fail with an assert.


https://reviews.llvm.org/D35110