llvm dev - Apr 2014 - [LLVMdev] how to interpret MRMDestReg in X86InstrFormat.td?

hi,

i am struggling to understand how MRMDestReg is used in X86.

in X86InstrFormat.td, we have this:


class Format<bits<7> val> {
  bits<7> Value val;

}
def MRMDestReg : Format<3>


i think eventually, MRMDestReg is mapped back to ModMRM byte. but this
still doesnt make sense to me why MRMDestReg is defined this way, and how
it is mapped back to ModRM byte.

any hint please?

thanks!
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140424/2568625f/attachment.html>

MRMDestReg means that 2 register operands are encoded in the modrm byte of
the instruction. Operand 0 will be encoded in the r/m field. The other
operand will be encoded in the reg field. And the mod field will be 0b11.

The format<3> just represent a enum value that represents this encoding
form.

MRMSrcReg is similar but operand 0 is in reg and operand 1 is in r/m.

MRMDestMem means that operand 0 is memory and is encoded in mod and r/m.
And operand 1 is in reg.

MRMSrcMem means that operand 0 is a register in reg. Operand 1 is memory in
mod and r/m.

MRM0r means that a single register operand is encoded in mod(0b11) and r/m.
Reg will be explicitly 0.

MRM0m means that a single memory operand is encoded in mod(not 0b11) and
r/m. Reg will be explicitly 0.

There are additional formats for specifying nearly the entire modrm byte.
Plus some other variations.

The translation from the encoding enum into mod rm byte is in
X86MCCodeEmitter.cpp.


On Wed, Apr 23, 2014 at 8:30 PM, Jun Koi <junkoi2004 at gmail.com> wrote:
> hi,
>
> i am struggling to understand how MRMDestReg is used in X86.
>
> in X86InstrFormat.td, we have this:
>
>
> class Format<bits<7> val> {
>   bits<7> Value > val;
>
> }
> def MRMDestReg : Format<3>
>
>
> i think eventually, MRMDestReg is mapped back to ModMRM byte. but this
> still doesnt make sense to me why MRMDestReg is defined this way, and how
> it is mapped back to ModRM byte.
>
> any hint please?
>
> thanks!
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

-- 
~Craig
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140423/0a7e2260/attachment.html>

On Thu, Apr 24, 2014 at 1:46 PM, Craig Topper <craig.topper at
gmail.com>wrote:
> MRMDestReg means that 2 register operands are encoded in the modrm byte of
> the instruction. Operand 0 will be encoded in the r/m field. The other
> operand will be encoded in the reg field. And the mod field will be 0b11.
>
> The format<3> just represent a enum value that represents this
encoding
> form.
>
> MRMSrcReg is similar but operand 0 is in reg and operand 1 is in r/m.
>
> MRMDestMem means that operand 0 is memory and is encoded in mod and r/m.
> And operand 1 is in reg.
>
> MRMSrcMem means that operand 0 is a register in reg. Operand 1 is memory
> in mod and r/m.
>
> MRM0r means that a single register operand is encoded in mod(0b11) and
> r/m. Reg will be explicitly 0.
>
> MRM0m means that a single memory operand is encoded in mod(not 0b11) and
> r/m. Reg will be explicitly 0.
>
> There are additional formats for specifying nearly the entire modrm byte.
> Plus some other variations.
>
> The translation from the encoding enum into mod rm byte is in
> X86MCCodeEmitter.cpp.
>
>
awesome! but why there are 2 X86MCCodeEmitter.cp?

    lib/Target/X86/X86CodeEmitter.cpp
    lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

this is confused, as a quick glance can tell me that there are a lot of
overlap between these 2 files.


thanks.
Jun

> On Wed, Apr 23, 2014 at 8:30 PM, Jun Koi <junkoi2004 at gmail.com>
wrote:
>
>> hi,
>>
>> i am struggling to understand how MRMDestReg is used in X86.
>>
>> in X86InstrFormat.td, we have this:
>>
>>
>> class Format<bits<7> val> {
>>   bits<7> Value >> val;
>>
>> }
>> def MRMDestReg : Format<3>
>>
>>
>> i think eventually, MRMDestReg is mapped back to ModMRM byte. but this
>> still doesnt make sense to me why MRMDestReg is defined this way, and
how
>> it is mapped back to ModRM byte.
>>
>> any hint please?
>>
>> thanks!
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>
>
> --
> ~Craig
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140424/99d179ef/attachment.html>