[clang] [Clang] Implement P2809: Trivial infinite loops are not Undefined Behavior (PR #90066)

[Eli Friedman via cfe-commits]

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@@ -908,6 +908,69 @@ void CodeGenFunction::EmitIfStmt(const IfStmt &S) {
     incrementProfileCounter(&S);
 }
 
+bool CodeGenFunction::checkIfLoopMustProgress(const Expr *ControllingExpression,
+                                              bool HasEmptyBody) {
+  if (CGM.getCodeGenOpts().getFiniteLoops() ==
+      CodeGenOptions::FiniteLoopsKind::Never)
+    return false;
+
+  // Now apply rules for plain C (see  6.8.5.6 in C11).
+  // Loops with constant conditions do not have to make progress in any C
+  // version.
+  // As an extension, we consisider loops whose constant expression
+  // can be constant-folded.
+  Expr::EvalResult Result;
+  bool CondIsConstInt =
----------------
efriedma-quic wrote:

One case to consider:

```
int main() {
  while (!__builtin_is_constant_evaluated()) {}
}
```

If you treat the condition of the loop as manifestly constant-evaluated, the loop is finite: the condition evaluates to false.  If we say it's not manifestly constant-evaluated, but do some sort of constant-evaluation anyway, it's a well-defined infinite loop.  If we somehow treat it as manifestly constant-evaluated for the constant evaluation, but not at runtime, it's UB.

https://github.com/llvm/llvm-project/pull/90066