[ELF] Set up initial live symbol(s) to avoid incorrect reclaim of atoms

[davide]

Summary

If no initial live symbols are set up, and deadStrip() == true, the Resolver ends up reclaiming all the symbols that aren't absolute. This is wrong.
This patch fixes the issue by setting entrySymbolName() as live, and this allows us to self-host lld when --gc-sections is enabled.
There are still quite a few problems with --gc-sections (quite a few tests failing), so the option can't be enabled by default.

P.S. This particular change depends on --gc-section so can't be tested easily. I would like to commit it as is and start looking at the test failures. I'll eventually add an unit test for this when --gc-sections will be enabled.

[ruiu]

I think this is okay for now, but we should treat entry symbol name as a dead strip root unconditionally regardless of architecture or configuration. Curentlly, I believe if you call addDeadStripRoot and dead stripping is not enabled, it will fail because of an assertion error. That's a silly assertion -- that prevented us from add atoms to the root set "just in case". We should remove that assert and copy an entry symbol to the root set unconditionally.

Protip: run "svn diff --diff-cmd=diff -x -u100000" or "git show -U100000" to include an entire file to a patch, so that we can see all code on Phab.

lib/ReaderWriter/ELF/ELFLinkingContext.cpp

112

Remove this comment.

[davide]

Thank you for the info re patch context. I think we should remove the assertion as well, and I'll do in a subsequent change. Do you think it makes sense to remove the assertion also in the other methods, i.e. deadStripRoots() and globalsAreDeadStripRoots() ?

[ruiu]

Yes, I think we should remove them as well.

[ruiu]

Anyways, let's go ahead with this patch. LGTM.

lib/ReaderWriter/ELF/ELFLinkingContext.cpp

     _writer = createWriterELF(this->targetHandler());
     break;
   }
+
+  // If -dead_strip, set up initial live symbols.
+  if (deadStrip())
+    // Entry point is live.
+    addDeadStripRoot(entrySymbolName());
   return true;
 }