This instruction is only available when the jsconv target_feature is available,
so on ARMv8.3 or higher.
It is used e.g. by Ruffle[0] to speed up its conversion from f64 to i32, or by
any JS engine probably.
I’ve picked the stdarch_aarch64_jscvt feature because it’s the name of the
FEAT_JSCVT, but hesitated with naming it stdarch_aarch64_jsconv (the name of
the target_feature) or stdarch_aarch64_jcvt (the name of the C intrinsic) or
stdarch_aarch64_fjcvtzs (the name of the instruction), this choice is purely
arbitrary and I guess it could be argued one way or another. I wouldn’t expect
it to stay unstable for too long, so ultimately this shouldn’t matter much.
This feature is now tracked in this issue[1].
[0] https://github.com/ruffle-rs/ruffle/pull/21780
[1] https://github.com/rust-lang/rust/issues/147555
They are defined in the aarch64 module, so this cfg is pointless.
Note that these instructions do exist for arm, but the aarch64 ones are
already stable, so this would need some additional work to implement
them for arm.
While many intrinsics use `.insn` to generate raw machine code from
numbers, all ratified instructions can be symbolic
using `.option` directives.
By saving the assembler environment with `.option push` then modifying
the architecture with `.option arch`, we can temporarily enable certain
extensions (as we use `.option pop` immediately after the target
instruction, surrounding environment is completely intact in this
commit; *almost* completely intact in general).
This commit modifies the `pause` *hint* intrinsic to use symbolic
*instruction* because we want to expose it even if the Zihintpause
extension is unavailable on the target.
This commit performs various improvements (better register allocation,
less register clobbering on the worst case and better readability) of
RISC-V inline assembly use cases.
Note that it does not change the `p` module (which defines the "P"
extension draft instructions but very likely to change).
1. Use `lateout` as possible.
Unlike `out(reg)` and `in(reg)` pair, `lateout(reg)` and `in(reg)`
can share the same register because they state that the late-output
register is written after all the reads are performed.
It can improve register allocation.
2. Add `preserves_flags` option as possible.
While RISC-V doesn't have _regular_ condition codes, RISC-V inline
assembly in the Rust language assumes that some registers
(mainly vector state registers) may be overwritten by default.
By adding `preserves_flags` to the intrinsics corresponding
instructions without overwriting them, it can minimize register
clobbering on the worst case.
3. Use trailing semicolon.
As `asm!` declares an action and it doesn't return a value by
itself, it would be better to have trailing semicolon to denote that
an `asm!` call is effectively a statement.
4. Make most of `asm!` calls multi-lined.
`rustfmt` makes some simple (yet long) `asm!` calls multi-lined but
it does not perform formatting of complex `asm!` calls with inputs
and/or outputs. To keep consistency, it makes most of the `asm!`
calls multi-lined.
