[llvm] [MachO] Preserve weak linkage for aliases (PR #198148)

[Folkert de Vries via llvm-commits]

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@@ -443,7 +443,10 @@ void MachObjectWriter::writeNlist(MachSymbolData &MSD, const MCAssembler &Asm) {
   // The Mach-O streamer uses the lowest 16-bits of the flags for the 'desc'
   // value.
   bool EncodeAsAltEntry = IsAlias && OrigSymbol.isAltEntry();
-  W.write<uint16_t>(Symbol->getEncodedFlags(EncodeAsAltEntry));
+  uint16_t Flags = Symbol->getEncodedFlags(EncodeAsAltEntry);
+  if (IsAlias)
+    Flags |= OrigSymbol.getEncodedFlags(EncodeAsAltEntry);
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folkertdev wrote:

Can you put this in a comment here, it's not obvious.

> When writing aliased symbols, keep the aliasee flags and include the alias symbol's own flags so N_WEAK_DEF is preserved in the Mach-O n_desc field.

https://github.com/llvm/llvm-project/pull/198148