• Austin Clements's avatar
    runtime: fix heap bitmap repeating with large scalar tails · a8ae93fd
    Austin Clements authored
    When heapBitsSetType repeats a source bitmap with a scalar tail
    (typ.ptrdata < typ.size), it lays out the tail upon reaching the end
    of the source bitmap by simply increasing the number of bits claimed
    to be in the incoming bit buffer. This causes later iterations to read
    the appropriate number of zeros out of the bit buffer before starting
    on the next repeat of the source bitmap.
    
    Currently, however, later iterations of the loop continue to read bits
    from the source bitmap *regardless of the number of bits currently in
    the bit buffer*. The bit buffer can only hold 32 or 64 bits, so if the
    scalar tail is large and the padding bits exceed the size of the bit
    buffer, the read from the source bitmap on the next iteration will
    shift the incoming bits into oblivion when it attempts to put them in
    the bit buffer. When the buffer does eventually shift down to where
    these bits were supposed to be, it will contain zeros. As a result,
    words that should be marked as pointers on later repetitions are
    marked as scalars, so the garbage collector does not trace them. If
    this is the only reference to an object, it will be incorrectly freed.
    
    Fix this by adding logic to drain the bit buffer down if it is large
    instead of reading more bits from the source bitmap.
    
    Fixes #11286.
    
    Change-Id: I964432c4b9f1cec334fc8c3da0ff16460203feb6
    Reviewed-on: https://go-review.googlesource.com/11360Reviewed-by: 's avatarRuss Cox <rsc@golang.org>
    a8ae93fd
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