[clang] [Clang][AArch64] Fix Pure Scalables Types argument passing and return (PR #112747)

[Eli Friedman via cfe-commits]

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@@ -533,11 +638,158 @@ bool AArch64ABIInfo::isZeroLengthBitfieldPermittedInHomogeneousAggregate()
   return true;
 }
 
+// Check if a type is a Pure Scalable Type as defined by AAPCS64. Return the
+// number of data vectors and the number of predicate vectors in the types, into
+// `NVec` and `NPred`, respectively. Upon return `CoerceToSeq` contains an
+// expanded sequence of LLVM IR types, one element for each non-composite
+// member. For practical purposes, limit the length of `CoerceToSeq` to about
+// 12, the maximum size that could possibly fit in registers.
+bool AArch64ABIInfo::isPureScalableType(
+    QualType Ty, unsigned &NVec, unsigned &NPred,
+    SmallVectorImpl<llvm::Type *> &CoerceToSeq) const {
+  if (const ConstantArrayType *AT = getContext().getAsConstantArrayType(Ty)) {
+    uint64_t NElt = AT->getZExtSize();
+    if (NElt == 0)
+      return false;
+
+    unsigned NV = 0, NP = 0;
+    SmallVector<llvm::Type *> EltCoerceToSeq;
+    if (!isPureScalableType(AT->getElementType(), NV, NP, EltCoerceToSeq))
+      return false;
+
+    for (uint64_t I = 0; CoerceToSeq.size() < 12 && I < NElt; ++I)
+      llvm::copy(EltCoerceToSeq, std::back_inserter(CoerceToSeq));
+
+    NVec += NElt * NV;
+    NPred += NElt * NP;
+    return true;
+  }
+
+  if (const RecordType *RT = Ty->getAs<RecordType>()) {
+    // If the record cannot be passed in registers, then it's not a PST.
+    if (CGCXXABI::RecordArgABI RAA = getRecordArgABI(RT, getCXXABI());
+        RAA != CGCXXABI::RAA_Default)
+      return false;
+
+    // Pure scalable types are never unions and never contain unions.
+    const RecordDecl *RD = RT->getDecl();
+    if (RD->isUnion())
+      return false;
+
+    // If this is a C++ record, check the bases bases.
+    if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+      for (const auto &I : CXXRD->bases()) {
+        if (isEmptyRecord(getContext(), I.getType(), true))
----------------
efriedma-quic wrote:

Does this handle empty unions correctly?

https://github.com/llvm/llvm-project/pull/112747