• Benedikt Meurer's avatar
    [turbofan] Unify handling of zeros. · 8ead5698
    Benedikt Meurer authored
    Following up on the earlier work regarding redundant Smi checks in
    https://chromium-review.googlesource.com/c/v8/v8/+/1246181, it was
    noticed that the handling of the 0 and -0 and how some operations
    identify these is not really consistent, but was still rather ad-hoc.
    This change tries to unify the handling a bit by making sure that all
    number comparisons generally pass truncations that identify zeros, since
    for the number comparisons in JavaScript there's no difference between
    0 and -0. In the same spirit NumberAbs and NumberToBoolean should also
    pass these truncations, since they also don't care about the differences
    between 0 and -0.
    
    Adjust NumberCeil, NumberFloor, NumberTrunc, NumberMin and NumberMax
    to pass along any incoming kIdentifiesZeros truncation, since these
    operations also don't really care whether the inputs can be -0 if the
    use nodes don't care.
    
    Also utilize the kIdentifiesZeros truncation for NumberModulus with
    Signed32 inputs, because it's kind of common to do something like
    `x % 2 === 0`, where it doesn't really matter whether `x % 2` would
    eventually produce a negative zero (since that would still be considered
    true for the sake of the comparison).
    
    This also adds a whole lot of tests to ensure that not only are these
    optimizations correct, but also that we do indeed perform them.
    
    Drive-by-fix: The `NumberAbs(x)` would incorrectly lower to just `x` for
    PositiveIntegerOrMinusZeroOrNaN inputs, which was obviously wrong in
    case of -0. This was fixed as well, and an appropriate test was added.
    
    The reason for the unification is that with the introduction of Word64
    for CheckBounds (which is necessary to support large TypedArrays and
    DataViews) we can no longer safely pass Word32 truncations for the
    interesting cases, since the index might be outside the Signed32 or
    Unsigned32 ranges, but we still identify 0 and -0 for the sake of the
    bounds check, and so it's important that this is handled consistently
    to not regress performance on TypedArrays and DataViews accesses.
    
    Bug: v8:8015, v8:8178
    Change-Id: Ia1d32f1b726754cea1e5793105d9423d84a6393a
    Reviewed-on: https://chromium-review.googlesource.com/1246172Reviewed-by: 's avatarSigurd Schneider <sigurds@chromium.org>
    Reviewed-by: 's avatarBenedikt Meurer <bmeurer@chromium.org>
    Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
    Cr-Commit-Position: refs/heads/master@{#56325}
    8ead5698
number-comparison-truncations.js 4.02 KB