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-fno-semantic-interposition is currently the CC1 default. (The opposite disables some interprocedural optimizations.) However, it does not infer dso_local: on most targets accesses to ExternalLinkage functions/variables defined in the current module still need PLT/GOT. This patch makes explicit -fno-semantic-interposition infer dso_local, so that PLT/GOT can be eliminated if targets implement local aliases for AsmPrinter::getSymbolPreferLocal (currently only x86). Currently we check whether the module flag "SemanticInterposition" is 0. If yes, infer dso_local. In the future, we can infer dso_local unless "SemanticInterposition" is 1: frontends other than clang will also benefit from the optimization if they don't bother setting the flag. (There will be risks if they do want ELF interposition: they need to set "SemanticInterposition" to 1.)
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