[clang] [analyzer] Don't assume third iteration in loops (PR #119388)

[Balazs Benics via cfe-commits]

================
@@ -2801,8 +2799,35 @@ void ExprEngine::processBranch(const Stmt *Condition,
     if (StTrue && StFalse)
       assert(!isa<ObjCForCollectionStmt>(Condition));
 
-    if (StTrue)
-      Builder.generateNode(StTrue, true, PredN);
+    if (StTrue) {
+      // If we are processing a loop condition where two iterations have
+      // already been completed and the the false branch is also feasible, then
+      // don't assume a third iteration, because it is a redundant execution
+      // path (unlikely to be different from earlier loop exits) and can cause
+      // false positives if e.g. the loop iterates over a two-element structure
+      // with an opaque condition.
+      //
+      // The iteration count "2" is hardcoded because it's the natural limit:
+      // * the fact that the programmer wrote a loop (and not just an `if`)
+      //   implies that they thought that the loop body may be executed twice;
+      // * however, there are situations where the programmer knows that there
+      //   are at most two iterations, but writes a loop that appears to be
+      //   generic, because there is no special syntax for "loop with at most
+      //   two iterations". (This pattern is common in FFMPEG and appears in
+      //   many other projects as well.)
+      bool CompletedTwoIterations = IterationsCompletedInLoop.value_or(0) >= 2;
+      bool FalseAlsoFeasible =
+          StFalse ||
+          didEagerlyAssumeBifurcateAt(PrevState, dyn_cast<Expr>(Condition));
+      bool SkipTrueBranch = CompletedTwoIterations && FalseAlsoFeasible;
+
+      // FIXME: This "don't assume third iteration" heuristic partially
+      // conflicts with the widen-loop analysis option (which is off by
+      // default). If we intend to support and stabilize the loop widening,
+      // we'll need to ensure that it 'plays nicely' with this logic.
----------------
steakhal wrote:

```suggestion
      // we must ensure that it 'plays nicely' with this logic.
```

https://github.com/llvm/llvm-project/pull/119388