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compiler: update RATIONALE.md for native check (#1432)

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Signed-off-by: Edoardo Vacchi <evacchi@users.noreply.github.com>
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Edoardo Vacchi
2023-05-04 01:37:13 +02:00
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commit a912ea844e
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internal/engine/compiler/RATIONALE.md
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@@ -75,3 +75,19 @@ and as of Go 1.20, these assembler functions are considered as _unsafe_ for asyn
From the Go runtime point of view, the execution of runtime-generated machine codes is considered as a part of
that trampoline function. Therefore, runtime-generated machine code is also correctly considered unsafe for async preemption.
## Why context cancellation is handled in Go code rather than native code
Since [wazero v1.0.0-pre.9](https://github.com/tetratelabs/wazero/releases/tag/v1.0.0-pre.9), the runtime
supports integration with Go contexts to interrupt execution after a timeout, or in response to explicit cancellation.
This support is internally implemented as a special opcode `builtinFunctionCheckExitCode` that triggers the execution of
a Go function (`ModuleInstance.FailIfClosed`) that atomically checks a sentinel value at strategic points in the code
(e.g. [within loops][checkexitcode_loop]).
[It _is indeed_ possible to check the sentinel value directly, without leaving the native world][native_check], thus sparing some cycles;
however, because native code never preempts (see section above), this may lead to a state where the other goroutines
never get the chance to run, and thus never get the chance to set the sentinel value; effectively preventing
cancellation from taking place.
[checkexitcode_loop]: https://github.com/tetratelabs/wazero/blob/86444c67a37dbf9e693ae5b365901f64968d9025/internal/wazeroir/compiler.go#L467-L476
[native_check]: https://github.com/tetratelabs/wazero/issues/1409