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65e57bbed0
The import/export maps, and the ModuleToDefinedGVSummaries map, are all indexed by module paths, which are StringRef obtained from the module summary index, which already has a data structure than owns these strings (the ModulePathStringTable). Because these other maps are also StringMap, which makes a copy of the string key, we were keeping multiple extra copies of the module paths, leading to memory overhead. Change these to DenseMap keyed by StringRef, and document that the strings are owned by the index. The only exception is the llvm-link tool which synthesizes an import list from command line options, and I have added a string cache to maintain ownership there. I measured around 5% memory reduction in the thin link of a large binary. Differential Revision: https://reviews.llvm.org/D156580