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

[nai.xia]

On 2011å12æ18æ 01:08, Brian Gerst wrote:
> On Sat, Dec 17, 2011 at 9:58 AM, Nai Xia<nai.xia@xxxxxxxxx>  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
> ââforwardââ (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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