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Implements clock ID support for getting time/res in wasi (#605)

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* Implements clock ID support for getting time/res in wasi

Signed-off-by: Anuraag Agrawal <anuraaga@gmail.com>
This commit is contained in:
Anuraag Agrawal
2022-05-31 12:11:52 +09:00
committed by GitHub
parent 062bee4845
commit d54a1348c4
3 changed files with 281 additions and 50 deletions
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RATIONALE.md
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@@ -351,6 +351,26 @@ See https://github.com/bytecodealliance/wasmtime/blob/2ca01ae9478f199337cf743a6a
Their semantics match when `pathLen` == the length of `path`, so in practice this difference won't matter match.
### ClockResGet
A clock's resolution is hardware and OS dependent so requires a system call to retrieve an accurate value.
Go does not provide a function for getting resolution, so without CGO we don't have an easy way to get an actual
value. For now, we return fixed values of 1us for realtime and 1ns for monotonic, assuming that realtime clocks are
often lower precision than monotonic clocks. In the future, this could be improved by having OS+arch specific assembly
to make syscalls.
For example, Go implements time.Now for linux-amd64 with this [assembly](https://github.com/golang/go/blob/f19e4001808863d2ebfe9d1975476513d030c381/src/runtime/time_linux_amd64.s).
Because retrieving resolution is not generally called often, unlike getting time, it could be appropriate to only
implement the fallback logic that does not use VDSO (executing syscalls in user mode). The syscall for clock_getres
is 229 and should be usable. https://pkg.go.dev/syscall#pkg-constants.
If implementing similar for Windows, [mingw](https://github.com/mirror/mingw-w64/blob/6a0e9165008f731bccadfc41a59719cf7c8efc02/mingw-w64-libraries/winpthreads/src/clock.c#L77
) is often a good source to find the Windows API calls that correspond
to a POSIX method.
Writing assembly would allow making syscalls without CGO, but comes with the cost that it will require implementations
across many combinations of OS and architecture.
## Signed encoding of integer global constant initializers
wazero treats integer global constant initializers signed as their interpretation is not known at declaration time. For
example, there is no signed integer [value type](https://www.w3.org/TR/2019/REC-wasm-core-1-20191205/#value-types%E2%91%A0).