[PATCH] D131677: [clang][RISCV][WIP] Fix incorrect ABI lowering for inherited structs with hard-float ABIs

[Alex Bradbury via Phabricator via llvm-commits]

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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 <https://github.com/llvm/llvm-project/issues/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.

WIP as upstream-quality tests not yet implemented. Sharing early as this is a very nasty bug that we should fix ASAP, and it would be good to avoid duplicated effort.


https://reviews.llvm.org/D131677

Files:
  clang/lib/CodeGen/TargetInfo.cpp


Index: clang/lib/CodeGen/TargetInfo.cpp
===================================================================
--- clang/lib/CodeGen/TargetInfo.cpp
+++ clang/lib/CodeGen/TargetInfo.cpp
@@ -10979,6 +10979,15 @@
     // Unions aren't eligible unless they're empty (which is caught above).
     if (RD->isUnion())
       return false;
+    // If this is a C++ record, check the bases first.
+    if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
+      for (const CXXBaseSpecifier &B : CXXRD->bases()) {
+        bool Ret = detectFPCCEligibleStructHelper(
+            B.getType(), CurOff, Field1Ty, Field1Off, Field2Ty, Field2Off);
+        if (!Ret)
+          return false;
+      }
+    }
     int ZeroWidthBitFieldCount = 0;
     for (const FieldDecl *FD : RD->fields()) {
       const ASTRecordLayout &Layout = getContext().getASTRecordLayout(RD);


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