• Leszek Swirski's avatar
    [interpreter] Fix block resurrection by LoopHeader · 18b63625
    Leszek Swirski authored
    Loop headers in the interpreter would start a new basic block, which
    among other things would reset the liveness of that block. This meant
    that a loop created after dead code, without a check for whether the
    code is currently dead or not, would "resurrect" that block's liveness,
    making the inside of the loop live even though the loop itself is
    unreachable.
    
    This works fine, since the loop is still unreachable, but can breaks
    DCHECKs in bytecode liveness analysis for cases where a register is
    supposed to be initialised before the loop, in the dead code, and is
    then used inside the loop, in the resurrected code.
    
    Normally this wouldn't be a problem, since blocks are normally killed on
    the statement level and we check for deadness during statement
    iteration, but `foo() = x` introduces an expression-level block killer
    (being re-written to `foo[throw ReferenceError] = x`) and we don't check
    for deadness after assignment Lhs preparation.
    
    This does mean that we have to fix the InterpreterJumps test, to not try
    to jump into the middle of a loop (since this could revive the loop).
    This can only happen when manually creating bytecode, bytecode generated
    from JavaScript is always reducible.
    
    Bug: chromium:1230597
    Change-Id: I8403ccdeae7e5450adf629026e2ca8a134c81877
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3275557
    Commit-Queue: Leszek Swirski <leszeks@chromium.org>
    Reviewed-by: 's avatarMarja Hölttä <marja@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#77846}
    18b63625
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