[LLVMdev] how to interpret MRMDestReg in X86InstrFormat.td?

[Jun Koi]

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!
>>
>> _______________________________________________
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>>
>>
>
>
> --
> ~Craig
>
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