[clang] [clang][Interp] Implement __builtin_bit_cast (PR #68288)

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Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>,
Timm =?utf-8?q?Bäder?= <tbaeder at redhat.com>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/68288 at github.com>


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@@ -73,6 +73,74 @@ template <class Emitter> class OptionScope final {
 } // namespace interp
 } // namespace clang
 
+//  This function is constexpr if and only if To, From, and the types of
+//  all subobjects of To and From are types T such that...
+//  (3.1) - is_union_v<T> is false;
+//  (3.2) - is_pointer_v<T> is false;
+//  (3.3) - is_member_pointer_v<T> is false;
+//  (3.4) - is_volatile_v<T> is false; and
+//  (3.5) - T has no non-static data members of reference type
+template <class Emitter>
+bool ByteCodeExprGen<Emitter>::emitBuiltinBitCast(const CastExpr *E) {
+  const Expr *SubExpr = E->getSubExpr();
+  QualType FromType = SubExpr->getType();
+  QualType ToType = E->getType();
+  std::optional<PrimType> ToT = classify(ToType);
+
+  // FIXME: This is wrong. We need to do the bitcast and then
+  //   throw away the result, so we still get the diagnostics.
+  if (DiscardResult)
+    return this->discard(SubExpr);
+
+  if (ToType->isNullPtrType()) {
+    if (!this->discard(SubExpr))
+      return false;
+
+    return this->emitNullPtr(E);
+  }
+
+  if (FromType->isNullPtrType() && ToT) {
+    if (!this->discard(SubExpr))
+      return false;
+
+    return visitZeroInitializer(*ToT, ToType, E);
+  }
+  assert(!ToType->isReferenceType());
+
+  // Get a pointer to the value-to-cast on the stack.
+  if (!this->visit(SubExpr))
+    return false;
+
+  if (!ToT || ToT == PT_Ptr) {
+    // Conversion to an array or record type.
+    return this->emitBitCastPtr(E);
+  }
+
+  assert(ToT);
+
+  // Conversion to a primitive type. FromType can be another
+  // primitive type, or a record/array.
+  //
+  // Same thing for floats, but we need the target
+  // semantics here.
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cor3ntin wrote:

Does it actually make sense to have a complete different codepath? we could have a single function and pass a null pointer in the non floating point case.

https://github.com/llvm/llvm-project/pull/68288