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    Subjectsingle copy atomicity for double load/stores on 32-bit systems
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    Hi Peter,

    Had an interesting lunch time discussion with our hardware architects pertinent to
    "minimal guarantees expected of a CPU" section of memory-barriers.txt


    | (*) These guarantees apply only to properly aligned and sized scalar
    | variables. "Properly sized" currently means variables that are
    | the same size as "char", "short", "int" and "long". "Properly
    | aligned" means the natural alignment, thus no constraints for
    | "char", two-byte alignment for "short", four-byte alignment for
    | "int", and either four-byte or eight-byte alignment for "long",
    | on 32-bit and 64-bit systems, respectively.


    I'm not sure how to interpret "natural alignment" for the case of double
    load/stores on 32-bit systems where the hardware and ABI allow for 4 byte
    alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....)

    I presume (and the question) that lkmm doesn't expect such 8 byte load/stores to
    be atomic unless 8-byte aligned

    ARMv7 arch ref manual seems to confirm this. Quoting

    | LDM, LDC, LDC2, LDRD, STM, STC, STC2, STRD, PUSH, POP, RFE, SRS, VLDM, VLDR,
    | VSTM, and VSTR instructions are executed as a sequence of word-aligned word
    | accesses. Each 32-bit word access is guaranteed to be single-copy atomic. A
    | subsequence of two or more word accesses from the sequence might not exhibit
    | single-copy atomicity

    While it seems reasonable form hardware pov to not implement such atomicity by
    default it seems there's an additional burden on application writers. They could
    be happily using a lockless algorithm with just a shared flag between 2 threads
    w/o need for any explicit synchronization. But upgrade to a new compiler which
    aggressively "packs" struct rendering long long 32-bit aligned (vs. 64-bit before)
    causing the code to suddenly stop working. Is the onus on them to declare such
    memory as c11 atomic or some such.

    Thx,
    -Vineet

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