Question about missing "cld" in x86 string assembly code

Question about missing "cld" in x86 string assembly code

[Nai Xia]

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

Question about missing "cld" in x86 string assembly code

[Nai Xia]

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

[Brian Gerst]

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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

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.

--
Brian Gerst

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

[Brian Gerst]

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
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

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.

--
Brian Gerst

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

[Linus Torvalds]

On Sat, Dec 17, 2011 at 9:08 AM, Brian Gerst <brgerst@gmail.com> wrote:
>
> 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.

The exception, of course, being bootup, fault and interrupt handlers,
and after we've called out to foreign code (ie BIOS).

So there *are* a few cld's sprinkled around, they are just fairly rare.

                    Linus

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

[Linus Torvalds]

On Sat, Dec 17, 2011 at 9:08 AM, Brian Gerst <brgerst@gmail.com> wrote:
>
> 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.

The exception, of course, being bootup, fault and interrupt handlers,
and after we've called out to foreign code (ie BIOS).

So there *are* a few cld's sprinkled around, they are just fairly rare.

                    Linus

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

[nai.xia]

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
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>
> 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

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

[nai.xia]

On 2011a1'12ae??18ae?JPY 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
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>
> The i386 ELF ABI states "The direction flag must be set to the
> a??a??forwarda??a?? (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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Re: Question about missing "cld" in x86 string assembly code

[nai.xia]

On 2011年12月18日 02:39, Linus Torvalds wrote:
> On Sat, Dec 17, 2011 at 9:08 AM, Brian Gerst<brgerst@gmail.com>  wrote:
>>
>> 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.
>
> The exception, of course, being bootup, fault and interrupt handlers,
> and after we've called out to foreign code (ie BIOS).

Yeah, I think I see these cld's now. Thanks for the answers!


Nai
>
> So there *are* a few cld's sprinkled around, they are just fairly rare.
>
>                      Linus

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

[nai.xia]

On 2011a1'12ae??18ae?JPY 02:39, Linus Torvalds wrote:
> On Sat, Dec 17, 2011 at 9:08 AM, Brian Gerst<brgerst@gmail.com>  wrote:
>>
>> The i386 ELF ABI states "The direction flag must be set to the
>> a??a??forwarda??a?? (that is, zero) direction before entry and upon exit from
>> a function."  Therefore it can be assumed to be clear, unless
>> explicitly set.
>
> The exception, of course, being bootup, fault and interrupt handlers,
> and after we've called out to foreign code (ie BIOS).

Yeah, I think I see these cld's now. Thanks for the answers!


Nai
>
> So there *are* a few cld's sprinkled around, they are just fairly rare.
>
>                      Linus

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Question about missing "cld" in x86 string assembly code

[Nai Xia]

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

Question about missing "cld" in x86 string assembly code

[Nai Xia]

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

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
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