• Russ Cox's avatar
    cmd/gc: fix liveness vs regopt mismatch for input variables · 26ad5d4f
    Russ Cox authored
    The inputs to a function are marked live at all times in the
    liveness bitmaps, so that the garbage collector will not free
    the things they point at and reuse the pointers, so that the
    pointers shown in stack traces are guaranteed not to have
    been recycled.
    
    Unfortunately, no one told the register optimizer that the
    inputs need to be preserved at all call sites. If a function
    is done with a particular input value, the optimizer will stop
    preserving it across calls. For single-word values this just
    means that the value recorded might be stale. For multi-word
    values like slices, the value recorded could be only partially stale:
    it can happen that, say, the cap was updated but not the len,
    or that the len was updated but not the base pointer.
    Either of these possibilities (and others) would make the
    garbage collector misinterpret memory, leading to memory
    corruption.
    
    This came up in a real program, in which the garbage collector's
    'slice len ≤ slice cap' check caught the inconsistency.
    
    Fixes #7944.
    
    LGTM=iant
    R=golang-codereviews, iant
    CC=golang-codereviews, khr
    https://golang.org/cl/100370045
    26ad5d4f
opt.h 5.16 KB