Clobbers: use a more efficient bitmask representation in API.#58
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Currently, the `Function` trait requires a `&[PReg]` for the clobber-list for a given instruction. In most cases where clobbers are used, the list may be long: e.g., ABIs specify a fixed set of registers that are clobbered and there may be ~half of all registers in this list. What's more, the list can't be shared for e.g. all calls of a given ABI, because actual return-values (defs) can't be clobbers. So we need to allocate space for long, sometimes-slightly-different lists; this is inefficient for the embedder and for us. It's much more efficient to use a bitmask to represent a set of physical registers. With current data structure bitpacking limitations, we can support at most 128 physical registers; this means we can use a `u128` bitmask. This also allows e.g. an embedder to start with a constant for a given ABI, and mask out bits for actual return-value registers on call instructions. This PR makes that change, for minor but positive performance impact.
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- Handle call instructions' clobbers with the clobbers API, using RA2's clobbers bitmask (bytecodealliance/regalloc2#58) rather than clobbers list; - Pull in changes from bytecodealliance/regalloc2#59 for much more sane edge-case behavior w.r.t. liverange splitting. Currently refers to local path for RA2 so will fail CI; I will update this once RA2 PRs 58 and 59 are merged, version-bumped and released. Fixes bytecodealliance#4291.
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Nice! Small suggestions below.
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Includes improvements in splitting performance (bytecodealliance#59) and more efficient handling of clobber lists (bytecodealliance#58). Semver break because of API change in bytecodealliance#58.
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cfallin
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- Handle call instructions' clobbers with the clobbers API, using RA2's clobbers bitmask (bytecodealliance/regalloc2#58) rather than clobbers list; - Pull in changes from bytecodealliance/regalloc2#59 for much more sane edge-case behavior w.r.t. liverange splitting.
cfallin
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- Handle call instructions' clobbers with the clobbers API, using RA2's clobbers bitmask (bytecodealliance/regalloc2#58) rather than clobbers list; - Pull in changes from bytecodealliance/regalloc2#59 for much more sane edge-case behavior w.r.t. liverange splitting.
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- Handle call instructions' clobbers with the clobbers API, using RA2's clobbers bitmask (bytecodealliance/regalloc2#58) rather than clobbers list; - Pull in changes from bytecodealliance/regalloc2#59 for much more sane edge-case behavior w.r.t. liverange splitting.
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- Handle call instructions' clobbers with the clobbers API, using RA2's clobbers bitmask (bytecodealliance/regalloc2#58) rather than clobbers list; - Pull in changes from bytecodealliance/regalloc2#59 for much more sane edge-case behavior w.r.t. liverange splitting.
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Currently, the
Functiontrait requires a&[PReg]for theclobber-list for a given instruction. In most cases where clobbers are
used, the list may be long: e.g., ABIs specify a fixed set of registers
that are clobbered and there may be ~half of all registers in this list.
What's more, the list can't be shared for e.g. all calls of a given ABI,
because actual return-values (defs) can't be clobbers. So we need to
allocate space for long, sometimes-slightly-different lists; this is
inefficient for the embedder and for us.
It's much more efficient to use a bitmask to represent a set of physical
registers. With current data structure bitpacking limitations, we can
support at most 128 physical registers; this means we can use a
u128bitmask. This also allows e.g. an embedder to start with a constant for
a given ABI, and mask out bits for actual return-value registers on call
instructions.
This PR makes that change, for minor but positive performance impact.