[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 +1805,50 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
       }
     }
   }
+  if (ZeroInitPadding) {
+    uint64_t TotalSize = CGF.getContext().toBits(
+        Dest.getPreferredSize(CGF.getContext(), DestLV.getType()));
+    DoZeroInitPadding(BaseAddr, LastFieldBitOffset, TotalSize);
+  }
+}
+
+void AggExprEmitter::DoZeroInitPadding(const Address BaseAddr,
+                                       uint64_t StartBitOffset,
+                                       uint64_t EndBitOffset) {
+  if (StartBitOffset >= EndBitOffset)
+    return;
+
+  auto InitBytes = [&](uint64_t Start, uint64_t End) {
+    Address Addr = CGF.Builder.CreateConstGEP(BaseAddr, Start);
+    llvm::Constant *SizeVal = CGF.Builder.getInt64(End - Start);
+    CGF.Builder.CreateMemSet(Addr, CGF.Builder.getInt8(0), SizeVal, false);
+  };
+  auto InitBits = [&](uint64_t Byte, uint64_t Start, uint64_t End) {
+    Address Addr = CGF.Builder.CreateConstGEP(BaseAddr, Byte);
+    llvm::Value *Val = Builder.CreateLoad(Addr);
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efriedma-quic wrote:

(Ideally, we'd use load/store operations with the same width as the bitfield, as computed by CGBitFieldInfo, to make the result easier for LLVM to optimize.)

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