[clang] Reapply "[clang][CodeGen] Zero init unspecified fields in initializers in C" (#109898) (PR #110051)

[Eli Friedman via cfe-commits]

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@@ -1774,6 +1795,54 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
       }
     }
   }
+  if (ZeroInitPadding) {
+    uint64_t TotalSize = CGF.getContext().toBits(
+        Dest.getPreferredSize(CGF.getContext(), DestLV.getType()));
+    DoZeroInitPadding(PaddingStart, TotalSize, nullptr);
+  }
+}
+
+void AggExprEmitter::DoZeroInitPadding(uint64_t &PaddingStart,
+                                       uint64_t PaddingEnd,
+                                       const FieldDecl *NextField) {
+
+  auto InitBytes = [&](uint64_t StartBit, uint64_t EndBit) {
+    CharUnits Start = CGF.getContext().toCharUnitsFromBits(StartBit);
+    CharUnits End = CGF.getContext().toCharUnitsFromBits(EndBit);
+    Address Addr = Dest.getAddress().withElementType(CGF.CharTy);
+    if (!Start.isZero())
+      Addr = Builder.CreateConstGEP(Addr, Start.getQuantity());
+    llvm::Constant *SizeVal = Builder.getInt64((End - Start).getQuantity());
+    CGF.Builder.CreateMemSet(Addr, Builder.getInt8(0), SizeVal, false);
+  };
+
+  if (NextField != nullptr && NextField->isBitField()) {
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efriedma-quic wrote:

I had to spend a little time checking this actually works... but I think you're fine here: we should do initialization in field order, and any out-of-order access is undefined behavior.

https://github.com/llvm/llvm-project/pull/110051