[LLVMdev] X86 disassembler is quite broken on handling REX

[Craig Topper]

I believe this particular error is caused by this. That seems easy enough
to just drop the bit. Do you have other non-mmx examples?

    case TYPE_MM:                                         \
      if (index > 7)                                      \
        *valid = 0;                                       \
      return prefix##_MM0 + index;

On Tue, Dec 23, 2014 at 10:17 PM, Jun Koi <junkoi2004 at gmail.com> wrote:
>
> hi,
>
> i think the current X86 disassembler is quite broken and fails badly on
> handling REX for x86_64 code.
>
> below are some examples:
>
> $ echo "0x0f,0xeb,0xc3"|./Release+Asserts/bin/llvm-mc -disassemble
> -triple=x86_64
>     .text
>     por    %mm3, %mm0
>
> $ echo "0x40,0x0f,0xeb,0xc3"|./Release+Asserts/bin/llvm-mc -disassemble
> -triple=x86_64
>     .text
>     por    %mm3, %mm0
>
> $ echo "0x41,0x0f,0xeb,0xc3"|./Release+Asserts/bin/llvm-mc -disassemble
> -triple=x86_64
>     .text
> <stdin>:1:1: warning: invalid instruction encoding
> 0x41,0x0f,0xeb,0xc3
> ^
>
>
> the last example should also return "por %mm3, %mm0", but it fails to
> understand the input.
>
> the reason stays with this line in X86DisassemblerDecoder.cpp:
>
>     rm  |= bFromREX(insn->rexPrefix) << 3;
>
> we can see that we take into account REX.B, but for "por" (0F EB), this
> should be ignored.
>
> there are quite a lot of other instructions taking into account REX like
> this, while according to the manual, REX should be ignored.
>
> i dont see any clean solution for this issue without some significant
> changes into the way we decode ModRM & providing more information to .td
> files.
>
> any idea?
>
> thanks.
> Jun
>


-- 
~Craig
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