Cranelift: box a few fields in MachBuffer/MachBufferFinalized to avoid memmoves. (#14300)

• Cranelift: box a few fields in MachBuffer/MachBufferFinalized to avoid memmoves.

Issue initially raised by alexcrichton at last week’s Cranelift meeting: we have SmallVecs that are (ironically) quite large in MachBuffer, and the “finalized” split causes significant memmoves when the MachBufferFinalized is put together with pieces of the MachBuffer during finalization.

The initial design intent (six years ago!) was to avoid allocations for the common case of a small function compilation: a MachBuffer would contain enough buffer space for the machine code + metadata for anything up to, say, a kilobyte of machine code. But that benefit was lost when we moved to the “finalization” design.

The split itself is necessary: it captures an algorithmic finalization step, and that data that we have before and after differs. (Specifically, we translate label indices into buffer offsets inside of several record types; those are different types, we shouldn’t type-pun them, and we have to pass through all the data anyway so those vecs are not a simple bulk data move.)

However many of the metadata arrays are simple passthroughs: for example, the machine-code buffer itself.

This is a simple/mechanical refactor that puts fields that don’t change type during finalization in a MachBufferInner, puts that in a Box, and then holds it from either the MachBuffer or MachBufferFinalized. Thus the finalization can move one pointer over instead of kilobytes of buffers.

On a quick Sightglass run with default.suite, I see one benchmark’s compilation time move:

compilation :: cycles :: spidermonkey-markdown

    Δ = 16405327.75 ± 16285800.71 (confidence = 99%)

    modified.so is 1.00x to 1.02x faster than base.so!

    ┌────────────┬────────────┬───────────────┬────────────┬─────────────┐
    │ Min        │ Max        │ Mean          │ Median     │ Engine      │
    ├────────────┼────────────┼───────────────┼────────────┼─────────────┤
    │ 1258771422 │ 1408382397 │ 1333799348.93 │ 1343472392 │ base.so     │
    ├────────────┼────────────┼───────────────┼────────────┼─────────────┤
    │ 1254241200 │ 1405715279 │ 1317394021.18 │ 1313587478 │ modified.so │
    └────────────┴────────────┴───────────────┴────────────┴─────────────┘

so a mean ~1% speedup.

There is definitely further work to do to try to reuse allocations across compilations by holding something in the Context, as we do with e.g. regalloc2::Ctx. The complication there is that MachBuffer is monomorphized on I because it holds I::LabelUses and those can differ between architectures; and cranelift_codegen::Context can be used to recompile for different ISAs with each compile invocation, so the actual type can necessarily differ between invocations. I suppose we could collapse all individual LabelUse enums into one shared one that has ISA-prefixed names for each arm, then remove the monomorphization in MachBuffer; but I’ll leave that for future work.

• Refactor: remove most typestate and T/FinalizedT distinctions in MachBuffer.

Instead, create in-place “enums” (bitpacked sum-types) and patch in place as we finalize. Allows most fields of MachBuffer to remain in-situ from initial emission through finalization and stencil-to-absolute conversion.

• Fix comment.

• doc-comment formatting fix

• Fix no-default-features build