[clang] [libc++] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

[Eli Friedman via cfe-commits]

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@@ -2456,6 +2461,139 @@ static RValue EmitHipStdParUnsupportedBuiltin(CodeGenFunction *CGF,
   return RValue::get(CGF->Builder.CreateCall(UBF, Args));
 }
 
+template <class T>
+void RecursivelyClearPaddingImpl(CodeGenFunction &CGF, Value *Ptr, QualType Ty, size_t CurrentStartOffset, size_t &RunningOffset, T&& WriteZeroAtOffset);
+
+template <class T>
+void ClearPaddingStruct(CodeGenFunction &CGF, Value *Ptr, QualType Ty, StructType *ST, 
+                        size_t CurrentStartOffset, size_t &RunningOffset, T&& WriteZeroAtOffset) {
+  std::cerr << "\n struct! " << ST->getName().data() << std::endl;
+  auto SL = CGF.CGM.getModule().getDataLayout().getStructLayout(ST);
+
+  auto R = dyn_cast<CXXRecordDecl>(Ty->getAsRecordDecl());
+  if(!R) {
+    std::cerr << "\n not a CXXRecordDecl" << std::endl;
+
+  }
+  const ASTRecordLayout &ASTLayout = CGF.getContext().getASTRecordLayout(R);
+  for (auto Base : R->bases()) {
+    std::cerr << "\n\n base!"  << std::endl;
+    // Zero padding between base elements.
+    auto BaseRecord = cast<CXXRecordDecl>(Base.getType()->getAsRecordDecl());
+    auto Offset = static_cast<size_t>(
+        ASTLayout.getBaseClassOffset(BaseRecord).getQuantity());
+    // Recursively zero out base classes.
+    auto Index = SL->getElementContainingOffset(Offset);
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efriedma-quic wrote:

I'm a bit skeptical it's a good idea to rely on the LLVM StructLayout for this; lowering from a clang type to an LLVM type involves a bunch of transforms.  Can we use information from the clang RecordLayout instead?

https://github.com/llvm/llvm-project/pull/75371