[llvm-dev] X86 disassembler fails to handle 0x66 prefix?

[Adam Nowacki via llvm-dev]

Intel manual states that:
Instruction prefixes are divided into four groups, each with a set of allowable prefix codes. For each instruction, it
is only useful to include up to one prefix code from each of the four groups (Groups 1, 2, 3, 4). Groups 1 through 4
may be placed in any order relative to each other.

Group 1
- REP or REPE/REPZ is encoded using F3H
Group 3
- Operand-size override prefix is encoded using 66H

So according to the Intel manual the ordering is arbitrary.

On 2017-05-07 10:25, Flamedoge via llvm-dev wrote:
>> 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 <mailto:llvm-dev at lists.llvm.org>> wrote:
> 
>     On Sun, May 7, 2017 at 3:22 PM, Jun Koi <junkoi2004 at gmail.com <mailto: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.
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