[clang][RISCV] Fix incorrect ABI lowering for inherited structs under hard-float ABIs

The hard float ABIs have a rule that if a flattened struct contains
either a single fp value, or an int+fp, or fp+fp then it may be passed
in a pair of registers (if sufficient GPRs+FPRs are available).
detectFPCCEligibleStruct and the helper it calls,
detectFPCCEligibleStructHelper examine the type of the argument/return
value to determine if it complies with the requirements for this ABI
rule.

As reported in bug #57084, this logic produces incorrect results for C++
structs that inherit from other structs. This is because only the fields
of the struct were examined, but enumerating RD->fields misses any
fields in inherited C++ structs. This patch corrects that issue by
adding appropriate logic to enumerate any included base structs.

Differential Revision: https://reviews.llvm.org/D131677

(cherry picked from commit bc538320809fb52af12ec0366118c82201af4f40)

1 files changed, 14 insertions, 1 deletions

diff --git a/clang/lib/CodeGen/TargetInfo.cpp b/clang/lib/CodeGen/TargetInfo.cpp
index 195ad8c..36e10e4df 100644
--- a/clang/lib/CodeGen/TargetInfo.cpp
+++ b/clang/lib/CodeGen/TargetInfo.cpp
@@ -11002,9 +11002,22 @@ bool RISCVABIInfo::detectFPCCEligibleStructHelper(QualType Ty, CharUnits CurOff,
     // Unions aren't eligible unless they're empty (which is caught above).
     if (RD->isUnion())
       return false;
+    const ASTRecordLayout &Layout = getContext().getASTRecordLayout(RD);
+    // If this is a C++ record, check the bases first.
+    if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+      for (const CXXBaseSpecifier &B : CXXRD->bases()) {
+        const auto *BDecl =
+            cast<CXXRecordDecl>(B.getType()->castAs<RecordType>()->getDecl());
+        CharUnits BaseOff = Layout.getBaseClassOffset(BDecl);
+        bool Ret = detectFPCCEligibleStructHelper(B.getType(), CurOff + BaseOff,
+                                                  Field1Ty, Field1Off, Field2Ty,
+                                                  Field2Off);
+        if (!Ret)
+          return false;
+      }
+    }
     int ZeroWidthBitFieldCount = 0;
     for (const FieldDecl *FD : RD->fields()) {
-      const ASTRecordLayout &Layout = getContext().getASTRecordLayout(RD);
       uint64_t FieldOffInBits = Layout.getFieldOffset(FD->getFieldIndex());
       QualType QTy = FD->getType();
       if (FD->isBitField()) {