[clang] [clang][CodeGen] Zero init unspecified fields in initializers in C (PR #97121)

[Eli Friedman via cfe-commits]

================
@@ -1745,13 +1748,48 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
       // Default-initialize to null.
       EmitNullInitializationToLValue(FieldLoc);
     }
+    if (ZeroInitPadding) {
+      CharUnits TotalSize =
+          Dest.getPreferredSize(CGF.getContext(), DestLV.getType());
+      CharUnits FieldSize =
+          CGF.getContext().getTypeSizeInChars(FieldLoc.getType());
+      if (FieldSize < TotalSize) {
+        CharUnits LeftSize = TotalSize - FieldSize;
+        llvm::Constant *LeftSizeVal =
+            CGF.Builder.getInt64(LeftSize.getQuantity());
+        Address BaseLoc = Dest.getAddress().withElementType(CGF.Int8Ty);
+        Address LeftLoc =
+            CGF.Builder.CreateConstGEP(BaseLoc, LeftSize.getQuantity());
+        CGF.Builder.CreateMemSet(LeftLoc, CGF.Builder.getInt8(0), LeftSizeVal,
----------------
efriedma-quic wrote:

If we don't have an initializer, instead of explicitly init'ing the padding, we 
can just `EmitNullInitializationToLValue(DestLV)`.

https://github.com/llvm/llvm-project/pull/97121
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