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    SubjectRe: [PATCH -tip] x86: atomic64: inline atomic64_read()


    On Fri, 3 Jul 2009, Ingo Molnar wrote:
    > > +
    > > + asm volatile(
    > > + "mov %%ebx, %%eax\n\t"
    > > + "mov %%ecx, %%edx\n\t"
    > > + LOCK_PREFIX "cmpxchg8b %1\n"
    > > + : "=&A" (res)
    > > + : "m" (*ptr)
    > > + );
    >
    > That clobbers or affects eax, ebx, ecx, edx. The only registers it
    > does not clobber is esi, edi and rbp.

    No, it only clobbers eax/edx. Yes, it touches ebx/ecx, but that's kind of
    incidental, and read-only, and doesn't matter.

    > Linus, what's your preference in this case?

    I think it's kind of subtle to do that thing, but it _is_ very much a
    special case. It's odd to use a cmpxchg8b to just do a read in the first
    place, so the fact that we then play other games with it is kind of
    immaterial.

    Adding a comment about the magic "we don't care about the initial values
    in eax/edx and ebx/ecx, but they have to match in case we end up doing a
    writeback" might be a good idea.

    Linus


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