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

[Nick Desaulniers via cfe-commits]

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@@ -12410,12 +12420,22 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   case Builtin::BI__builtin_ctz:
   case Builtin::BI__builtin_ctzl:
   case Builtin::BI__builtin_ctzll:
-  case Builtin::BI__builtin_ctzs: {
+  case Builtin::BI__builtin_ctzs:
+  case Builtin::BI__builtin_ctzg: {
     APSInt Val;
     if (!EvaluateInteger(E->getArg(0), Val, Info))
       return false;
-    if (!Val)
+
+    if (!Val) {
+      if (BuiltinOp == Builtin::BI__builtin_ctzg && E->getNumArgs() > 1) {
+        APSInt Fallback;
+        if (!EvaluateInteger(E->getArg(1), Fallback, Info))
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nickdesaulniers wrote:

Reuse `Val`?

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