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Avoid copying some undef memory in MIR #62655

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Commits on Dec 2, 2019

  1. Avoid copying memory representation of undef data

    During MIR interpretation it may happen that a place containing
    uninitialized bytes is copied. This would read the current
    representation of these bytes and write it to the destination even
    though they must, by definition, not matter to the execution.
    
    This elides that representation change when no bytes are defined in such
    a copy, saving some cpu cycles. In such a case, the memory of the target
    allocation is not touched at all which also means that sometimes no
    physical page backing the memory allocation of the representation needs
    to be provided by the OS at all, reducing memory pressure on the system.
    HeroicKatora committed Dec 2, 2019
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