[clang] [clang][Interp] Add basic support for _BitInt (PR #68069)

[Timm Baeder via cfe-commits]

================
@@ -1601,6 +1617,46 @@ bool CastFloatingIntegral(InterpState &S, CodePtr OpPC) {
   }
 }
 
+static inline bool CastFloatingIntegralAP(InterpState &S, CodePtr OpPC,
+                                          uint32_t BitWidth) {
+  const Floating &F = S.Stk.pop<Floating>();
+
+  APSInt Result(BitWidth, /*IsUnsigned=*/true);
+  auto Status = F.convertToInteger(Result);
+
+  // Float-to-Integral overflow check.
+  if ((Status & APFloat::opStatus::opInvalidOp) && F.isFinite()) {
+    const Expr *E = S.Current->getExpr(OpPC);
+    QualType Type = E->getType();
+
+    S.CCEDiag(E, diag::note_constexpr_overflow) << F.getAPFloat() << Type;
+    return S.noteUndefinedBehavior();
+  }
+
+  S.Stk.push<IntegralAP<true>>(IntegralAP<true>(Result));
+  return CheckFloatResult(S, OpPC, F, Status);
+}
+
+static inline bool CastFloatingIntegralAPS(InterpState &S, CodePtr OpPC,
+                                           uint32_t BitWidth) {
+  const Floating &F = S.Stk.pop<Floating>();
+
+  APSInt Result(BitWidth, /*IsUnsigned=*/false);
+  auto Status = F.convertToInteger(Result);
+
+  // Float-to-Integral overflow check.
+  if ((Status & APFloat::opStatus::opInvalidOp) && F.isFinite()) {
+    const Expr *E = S.Current->getExpr(OpPC);
+    QualType Type = E->getType();
+
+    S.CCEDiag(E, diag::note_constexpr_overflow) << F.getAPFloat() << Type;
+    return S.noteUndefinedBehavior();
+  }
+
+  S.Stk.push<IntegralAP<true>>(IntegralAP<true>(Result));
+  return CheckFloatResult(S, OpPC, F, Status);
+}
+
----------------
tbaederr wrote:

`CastFloatingIntegralAP(S)` needs a helper function to get rid of the code duplication.

https://github.com/llvm/llvm-project/pull/68069