Fix 1226 and document associated methods #1230
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Araq
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May 28, 2014
Fix 1226 and document associated methods
Clyybber
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Mar 13, 2024
## Summary * use the goto-using CGIR for JavaScript code generation * as a side-effect, multiple bugs with tracking of the current exception are fixed * exceptions thrown from `asm` or `.emit` code cannot be expected to be caught anymore (this was already undefined behaviour) ## Details The CGIR for a body is first passed to the new `jsflow` module, where the shape of the necessary JavaScript is computed, with the focus on the control-flow constructs (`if`, `switch`, `try`, etc.). Constructs are translated as follows: * `cnkGotoStmt` and `cnkJoinStmt` are translated to `break` and labeled blocks (JavaScript doesn't support goto directly) * `cnkCaseStmt` dispatchers are translated to `switch`/`case` statements * `cnkFinally` and `cnkExcept` are translated to `try`/`finally` and `try`/`catch`, respectively It's first computed which statements `try`s and labeled blocks *must* enclose. Afterwards, their starts are reordered such that valid JavaScript is formed. Since `finally` and `catch` clauses can then intercept `break`s or `throw`s that they shouldn't, `finally` and `catch` clauses are guarded by a boolean flag. Immediately before an explicit (`break` and `throw`) or implicit (procedure call) jump that would enter a clause it shouldn't, the boolean locals for the problematic clauses are set to false. Upon exit of a disabled clause, the local is set back to true. #### Case Statements Direct translation of `cnkCaseStmt`s into `switch`/`case` statement where each branch is a `break` would result in very deep block nesting for `case` statements with many branches. The resulting JavaScript code can cause the V8 JavaScript engine to fail/crash when compiling the code! Therefore, the jump targets need to be inlined whenever possible. `jsflow` computes the code regions eligible for inlining, and `jsgen` handles the actual inlining. #### Current Exception Tracking When control-flow leaves an `except` handler, the `prevJSError` global (whose content is returned by `getCurrentException`) needs to be reset to the exception caught by the enclosing handler, something that was previously missing for the most part. The exception is restored: * at the end of a block when the block is broken out of from within a `catch` * at the start of a `finally` clause intercepting a `break` from within a `catch` * at the start of a `finally` clause when it's entered due to an exception If there's no enclosing `catch` within a procedure when the current exception has to be restored, the value of `lastJSError` is captured at procedure entry. Tracking of the current exception still doesn't include in-flight exceptions, so `tfinally6.nim` still fails. #### Unhandled Exception Reporting Instead of incrementing, at run-time, a global counter (`excHandler`) before entering a `try` and decrementing it again on exit, and then reporting unhandled exceptions when throwing, the module-init procedures (i.e., top-level code) are wrapped in a `try`/`catch` that reports the escaping exceptions. This is simpler to implement and should also have less run-time overhead. #### Throwing with `asm` `asm` and `emit` statements are considered not to raise, and thus they don't prevent `except` and `finally` sections from being eliminated by unreachable code elimination. The `tnativeexc` test is updated to not rely on undefined behaviour. ### Unreachable Code Elimination Labels that mark the exit of case statements are now eliminated too, if structured control-flow doesn't leave the case statement (i.e., each branch ends in a noreturn-like statement). --------- Co-authored-by: Clay Sweetser <Varriount@users.noreply.github.com>
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