[clang] [clang] Implement constexpr support for __builtin_{clzg, ctzg} (PR #86577)

[Nick Desaulniers via cfe-commits]

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@@ -12367,8 +12368,17 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
                            BuiltinOp != Builtin::BI__lzcnt &&
                            BuiltinOp != Builtin::BI__lzcnt64;
 
-    if (ZeroIsUndefined && !Val)
-      return Error(E);
+    if (!Val) {
+      if (BuiltinOp == Builtin::BI__builtin_clzg && E->getNumArgs() > 1) {
+        APSInt Fallback;
+        if (!EvaluateInteger(E->getArg(1), Fallback, Info))
+          return false;
+        return Success(Fallback, E);
+      }
+
+      if (ZeroIsUndefined)
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nickdesaulniers wrote:

Sink the definition of `ZeroIsUndefined` closer to the use to reduce its scope. We might not need to evaluate that expression.

https://github.com/llvm/llvm-project/pull/86577