Jonathan Brouwer
65097900fd
Unalign `PackedFingerprint` on all hosts, not just x86 and x86-64 Back in https://github.com/rust-lang/rust/pull/78646, `DepNode` was modified to store an unaligned `PackedFingerprint` instead of an 8-byte-aligned `Fingerprint`. That reduced the size of DepNode from 24 bytes to 17 bytes (nowadays 18 bytes), resulting in considerable memory savings in incremental builds. See https://github.com/rust-lang/rust/pull/152695#issuecomment-3907091509 for a benchmark demonstrating the impact of *removing* that optimization. At the time (and today), the unaligning was only performed on x86 and x86-64 hosts, because those CPUs are known to generally have low overhead for unaligned memory accesses. Hosts with other CPU architectures would continue to use an 8-byte-aligned fingerprint and a 24-byte DepNode. Given the subsequent rise of aarch64 (especially on macOS) and other architectures, it's a shame that some commonly-used builds of rustc don't get those memory-size benefits, based on a decision made several years ago under different circumstances. We don't have benchmarks to show the actual effect of unaligning DepNode fingerprints on various non-x86 hosts, but it seems very likely to be a good idea on Apple chips, and I have no particular reason to think that it will be catastrophically bad on other hosts. And we don't typically perform this kind of speculative pessimization in other parts of the compiler.
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