[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 =
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efriedma-quic wrote:

So... we treat it as a manifestly constant-evaluated for the purpose of checking whether the loop is trivial, but then flips to not manifestly constant-evaluated for the actual evaluation at runtime?  The wording could use some clarification...

Due to the way Sema::CheckForImmediateInvocation works, once we decide an expression is not manifestly constant-evaluated, we can't actually constant-evaluate it, I think; the AST is modified.  Maybe we can run constant-evaluation before Sema::CheckForImmediateInvocation runs, though.

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