[EDS] LLVM support for new conditional runtime records #8588
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Teach the dead stripping in LLD for MachO how to parse conditionally live records, and consider them when dead stripping. This is a demonstrative patch of a change that could made for LLD for ELF/COFF or other linkers to support effective EDS without LTO.
During thin-LTO thin-link, a global liveness analysis is performed to guide importing and exporting decisions between modules. EDS is far more effective when it can operate on the entire LTO unit, otherwise dependencies defined in other modules have to be conservatively assumed live. It is also far simpler to safely run EDS on the entire LTO unit, since otherwise it can be difficult to determine if the liveness of a copy of a dependency is representative of the liveness of the entity in the program as a whole. Lastly, just like regular DCE, EDS is more effective run multiple times. In this patch, we serialize EDS information (stripping conditions) into each module's summary, then collect this information during thin-link, making liveness decisions accordingly. We support both the old and new LTO backend.
NuriAmari
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Apr 17, 2024
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| ; RUN: opt -S -passes=globaldce < %s | FileCheck %s | |||
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This test tested what happens when the metadata contains null dependencies, which is now impossible.
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Corresponding forum post: https://forums.swift.org/t/rfc-conditional-runtime-records-revamped/71326 |
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In this patch, we replace the old EDS logic in GlobalDCE, with new logic that knows how to interpret the new conditionally live records. We prevent this stripping logic from running before full-LTO merge, as it's unclear if it is safe to evaluate the liveness of a dependency without considering the full LTO unit. The primary concern here is when a dependency has multiple defintions, each internalized into different modules. The liveness of one may not reflect the liveness of another.
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This adds the required LLVM support for the new form of conditional runtime records proposed in apple/swift#73060.