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    SubjectRe: Question about missing "cld" in x86 string assembly code


    On 2011年12月18日 01:08, Brian Gerst wrote:
    > On Sat, Dec 17, 2011 at 9:58 AM, Nai Xia<nai.xia@gmail.com> wrote:
    >> Hi Andi,
    >>
    >> Seems I used a stale email address of you from a related git commit log,
    >> so this is a resend, sorry.
    >>
    >> =======
    >> Hi,
    >>
    >> I notice that all x86 assembly code for string operations containing
    >> "scasb, lodsb", etc does not have "cld" at the beginning.
    >> Is this 100% safe?
    >> Or in other words, how could we be sure that
    >> there is no "std" generated by compiler somewhere just before
    >> the string operations?
    >>
    >>
    >> Thanks,
    >>
    >> Nai
    >> --
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    >
    > The i386 ELF ABI states "The direction flag must be set to the
    > ‘‘forward’’ (that is, zero) direction before entry and upon exit from
    > a function." Therefore it can be assumed to be clear, unless
    > explicitly set.

    Hmm, I get those lines now. Thanks for reply.
    Seems gcc started to strictly conform to this ABI since 4.3 . But glibc
    seems still have some leading "cld" string code, maybe just prepared for
    non-conforming kernels other than Linux.

    >
    > --
    > Brian Gerst
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