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    SubjectRe: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning
    Linus Torvalds wrote:
    >
    > On Mon, 12 Jan 2009, Bernd Schmidt wrote:
    >> Something at the back of my mind said "aliasing".
    >>
    >> $ gcc linus.c -O2 -S ; grep subl linus.s
    >> subl $1624, %esp
    >> $ gcc linus.c -O2 -S -fno-strict-aliasing; grep subl linus.s
    >> subl $824, %esp
    >>
    >> That's with 4.3.2.
    >
    > Interesting.
    >
    > Nonsensical, but interesting.
    >
    > Since they have no overlap in lifetime, confusing this with aliasing is
    > really really broken (if the functions _hadn't_ been inlined, you'd have
    > gotten the same address for the two variables anyway! So anybody who
    > thinks that they need different addresses because they are different types
    > is really really fundmantally confused!).

    I've never really looked at the stack slot sharing code. But I think
    it's not hard to see what's going on: "no overlap in lifetime" may be a
    temporary state. Let's say you have

    {
    variable_of_some_type a;
    writes to a;
    other stuff;
    reads from a;
    }
    {
    variable_of_some_other_type b;
    writes to b;
    other stuff;
    reads from b;
    }

    At the point where the compiler generates RTL, stack space has to be
    allocated for variables A and B. At this point the lifetimes are
    non-overlapping. However, if the compiler chooses to put them into the
    same stack location, the RTL-based alias analysis will happily conclude
    (based on the differing types) that the reads from A and the writes to B
    can't possibly conflict, and some passes may end up reordering them.
    End result: overlapping lifetimes and overlapping stack slots. Oops.


    Bernd
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