llvm-project

9ade4e26 - [NFCI][LTO][lld] Optimize away symbol copies within LTO global resolution in ELF (#106193)

[NFCI][LTO][lld] Optimize away symbol copies within LTO global resolution in ELF (#106193) `StringMap<T>` creates a [copy of the string](https://github.com/llvm/llvm-project/blob/d4c519e7b2ac21350ec08b23eda44bf4a2d3c974/llvm/include/llvm/ADT/StringMapEntry.h#L55-L58) for entry insertions and intentionally keep copies [since the implementation optimizes string memory usage](https://github.com/llvm/llvm-project/blob/d4c519e7b2ac21350ec08b23eda44bf4a2d3c974/llvm/include/llvm/ADT/StringMap.h#L124). On the other hand, linker keeps copies of symbol names [1] in `lld::elf::parseFiles` [2] before invoking `compileBitcodeFiles` [3]. This change proposes to optimize away string copies inside [LTO::GlobalResolutions](https://github.com/llvm/llvm-project/blob/24e791b4164986a1ca7776e3ae0292ef20d20c47/llvm/include/llvm/LTO/LTO.h#L409), which will make LTO indexing more memory efficient for ELF. There are similar opportunities for other (COFF, wasm, MachO) formats. The optimization takes place for lld (ELF) only. For the rest of use cases (gold plugin, `llvm-lto2`, etc), LTO owns a string saver to keep copies and use global resolution key for de-duplication. Together with @kazutakahirata's work to make `ComputeCrossModuleImport` more memory efficient, we see a ~20% peak memory usage reduction in a binary where peak memory usage needs to go down. Thanks to the optimization in https://github.com/llvm/llvm-project/commit/329ba523ccbbe68a12434926c92fd9a86494d958, the max (as opposed to the sum) of `ComputeCrossModuleImport` or `GlobalResolution` shows up in peak memory usage. * Regarding correctness, the set of [resolved](https://github.com/llvm/llvm-project/blob/80c47ad3aec9d7f22e1b1bdc88960a91b66f89f1/llvm/lib/LTO/LTO.cpp#L739) [per-module symbols](https://github.com/llvm/llvm-project/blob/80c47ad3aec9d7f22e1b1bdc88960a91b66f89f1/llvm/include/llvm/LTO/LTO.h#L188-L191) is a subset of [llvm::lto::InputFile::Symbols](https://github.com/llvm/llvm-project/blob/80c47ad3aec9d7f22e1b1bdc88960a91b66f89f1/llvm/include/llvm/LTO/LTO.h#L120). And bitcode symbol parsing saves symbol name when iterating `obj->symbols` in `BitcodeFile::parse` already. This change updates `BitcodeFile::parseLazy` to keep copies of per-module undefined symbols. * Presumably the undefined symbols in a LTO unit (copied in this patch in linker unique saver) is a small set compared with the set of symbols in global-resolution (copied before this patch), making this a worthwhile trade-off. Benchmarking this change alone shows measurable memory savings across various benchmarks. [1] ELF https://github.com/llvm/llvm-project/blob/1cea5c2138bef3d8fec75508df6dbb858e6e3560/lld/ELF/InputFiles.cpp#L1748 [2] https://github.com/llvm/llvm-project/blob/ef7b18a53c0d186dcda1e322be6035407fdedb55/lld/ELF/Driver.cpp#L2863 [3] https://github.com/llvm/llvm-project/blob/ef7b18a53c0d186dcda1e322be6035407fdedb55/lld/ELF/Driver.cpp#L2995