Cranelift: update ValueDataPacked to support full Value range.#12613
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cfallin wants to merge 1 commit intobytecodealliance:mainfrom
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Cranelift: update ValueDataPacked to support full Value range.#12613cfallin wants to merge 1 commit intobytecodealliance:mainfrom
cfallin wants to merge 1 commit intobytecodealliance:mainfrom
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This updates the `ValueDataPacked` scheme from the old
```
(enum tag) (CLIF type) (value 1) (value 2)
/// | tag:2 | type:14 | x:24 | y:24
```
encoding in a `u64` to a new
```
/// | tag:2 | type:14 | x:32 | y:32
```
encoding, with a `packed` tag attribute to ensure the struct fits in
10 bytes. This permits the full range of `Value` (a `u32` entity
index) to be encoded, removing the remaining major limit on function
body size after the work in bytecodealliance#12611 to address bytecodealliance#12229.
Curiously, this appears to be a *speedup* in compile time of 3-5% on
bz2 and 3% on spidermonkey-json (Sightglass, 50 data points each). My
best guess as to why is that putting the value fields in their own
`u32`s allows for quick access without shifts/masks, which is actually
better than the unaligned accesses (caused by 10-byte size) -- which
have no penalty on modern mainstream CPUs -- and 25% size inflation of
the value-definitions array.
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This updates the
ValueDataPackedscheme from the oldencoding in a
u64to a newencoding, with a
packedtag attribute to ensure the struct fits in 10 bytes. This permits the full range ofValue(au32entity index) to be encoded, removing the remaining major limit on function body size after the work in #12611 to address #12229.Curiously, this appears to be a speedup in compile time of 3-5% on bz2 and 3% on spidermonkey-json (Sightglass, 50 data points each). My best guess as to why is that putting the value fields in their own
u32s allows for quick access without shifts/masks, which is actually better than the unaligned accesses (caused by 10-byte size) -- which have no penalty on modern mainstream CPUs -- and 25% size inflation of the value-definitions array.