llvm dev - May 2017 - X86 disassembler fails to handle 0x66 prefix?

Hello,

I have seen this bug for quite a while, and even in the latest code it
is still there: that is X86 disassembler does not handle 0x66 prefix
properly, if it is put behind 0xF3 prefix.

The below commands should return the same output, but not.

$ echo "0xf3 0x66 0xa5"|./build/bin/llvm-mc --disassemble
-triple=x86_64 -output-asm-variant=1
.text
rep
movsw word ptr es:[rdi], word ptr [rsi]

$ echo "0x66 0xf3 0xa6"|./build/bin/llvm-mc --disassemble
-triple=x86_64 -output-asm-variant=1
.text
cmpsb byte ptr [rsi], byte ptr es:[rdi]


You can see just by exchanging the order of 0xf3 & 0x66, we get
different result. F3 in this case is not really a prefix for REP I
think.

Is there any solution to fix this?


Thanks.
Jun

On Sun, May 7, 2017 at 3:22 PM, Jun Koi <junkoi2004 at gmail.com>
wrote:
> Hello,
>
> I have seen this bug for quite a while, and even in the latest code it
> is still there: that is X86 disassembler does not handle 0x66 prefix
> properly, if it is put behind 0xF3 prefix.
>
> The below commands should return the same output, but not.
>
> $ echo "0xf3 0x66 0xa5"|./build/bin/llvm-mc --disassemble
> -triple=x86_64 -output-asm-variant=1
> .text
> rep
> movsw word ptr es:[rdi], word ptr [rsi]
>
> $ echo "0x66 0xf3 0xa6"|./build/bin/llvm-mc --disassemble
> -triple=x86_64 -output-asm-variant=1
> .text
> cmpsb byte ptr [rsi], byte ptr es:[rdi]
>
>
> You can see just by exchanging the order of 0xf3 & 0x66, we get
> different result. F3 in this case is not really a prefix for REP I
> think.
Just to clarify, the correct output for both cases would be: "rep
movsw word ptr es:[rdi], word ptr [rsi]"


Thanks.

> F3 in this case is not really a prefix for REP I think.
http://x86.renejeschke.de/html/file_module_x86_id_279.html

Intel arch. manual shows REP uses F3 and F2 as starting prefixes.

Kevin

On Sun, May 7, 2017 at 12:40 AM, Jun Koi via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On Sun, May 7, 2017 at 3:22 PM, Jun Koi <junkoi2004 at gmail.com>
wrote:
> > Hello,
> >
> > I have seen this bug for quite a while, and even in the latest code it
> > is still there: that is X86 disassembler does not handle 0x66 prefix
> > properly, if it is put behind 0xF3 prefix.
> >
> > The below commands should return the same output, but not.
> >
> > $ echo "0xf3 0x66 0xa5"|./build/bin/llvm-mc --disassemble
> > -triple=x86_64 -output-asm-variant=1
> > .text
> > rep
> > movsw word ptr es:[rdi], word ptr [rsi]
> >
> > $ echo "0x66 0xf3 0xa6"|./build/bin/llvm-mc --disassemble
> > -triple=x86_64 -output-asm-variant=1
> > .text
> > cmpsb byte ptr [rsi], byte ptr es:[rdi]
> >
> >
> > You can see just by exchanging the order of 0xf3 & 0x66, we get
> > different result. F3 in this case is not really a prefix for REP I
> > think.
>
> Just to clarify, the correct output for both cases would be: "rep
> movsw word ptr es:[rdi], word ptr [rsi]"
>
>
> Thanks.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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