[GlobalDCE] Add !llvm.used.conditional support for conditionally used global variables #3396
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Looks like there's a failure on macOS which may be relevant:
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Addressed (on the Swift side), it was the IR Verifier not liking the slightly-different version of llvm.used.conditional that is used in this test :) The next run should be clean. |
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… global variables As part of the optimization work to emit dead-strippable symbols for Swift and enable GlobalDCE to remove those symbols when possible, we need to mark some Swift symbols as conditionally used based on a set of other symbols. Background info can be found here: https://bugs.swift.org/browse/SR-14509. This diff adds a module-level name metadata !llvm.used.conditional, which contains a list of metadata triplets, that describes the conditions under which we allow removal of globals even if they are mentioned in @llvm.used. See the included LangRef changes to details. The implementation is a strict addition, it does not change any semantics and does not affect IR or passes in any way, unless !llvm.used.conditional is present in the input IR.
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This PR lands a preview version of !llvm.used.conditional that is not yet merged in upstream LLVM and is being reviewed at https://reviews.llvm.org/D104496, but it's important to unblock progress in adopting these changes in Swift. Once upstream LLVM gets !llvm.used.metadata (or a differently-named or differently-implemented) support, which downstream change will be reverted.
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