[lld] [LLD][COFF] Prevent merging .bss into shared sections (PR #202817)

[Evgenii Kudriashov via llvm-commits]

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@@ -1716,8 +1724,17 @@ void Writer::mergeSections() {
   // whatever section it is being merged into (usually .data) so that the image
   // need not actually contain all of the zeros.
   auto it = ctx.config.merge.find(".bss");
-  if (it != ctx.config.merge.end())
-    mergeSection(*it);
+  if (it != ctx.config.merge.end()) {
+    // Resolve the final merge target name following the chain.
+    StringRef toSection = getMergeDestination(it->first, it->second);
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e-kud wrote:

So the idea is to get the final section and check whether it is shared. But we do the same already in `mergeSections`. So I extracted `getMergeDestination` to call it beforehand. Inside there should be only one iteration of `while` loop.

The problem in removing `getMergeDestination` from `mergeSection` is that we have another site calling `mergeSection` (a few lines above). To avoid this double checking what do you think if I put `getMergeDestination` before another call site and add an assert to `mergeSection` if the section, we are merging in, is not final. Or is the assert too strong?

https://github.com/llvm/llvm-project/pull/202817