[LLVMdev] initialize 'dag' variable and interpret asmstring in tablegen .td file

[Chris Lattner]

On Fri, 6 May 2005, Tzu-Chien Chiu wrote:

> The macro $src, $dest used in Instruction::AsmString must be
> "declared" in Instruction::OperandList, right?

Yes, these refer to the arguments of the machine instruction.

> $$ has special meaning?

$$ = $ in the output file.

-Chris

> On 5/6/05, Chris Lattner <sabre at nondot.org> wrote:
>> On Fri, 6 May 2005, Tzu-Chien Chiu wrote:
>>> llvm/lib/Target/X86/X86InstrInfo.td:
>>> class X86Inst<bits<8> opcod, Format f, ImmType i, dag ops, string
>>> AsmStr>  : Instruction {
>>>    ....
>>>    dag OperandList = ops;
>>>    string AsmString = AsmStr;
>>> }
>>>
>>> def MOV32mi : Ii32<0xC7, MRM0m, (ops i32mem:$dst, i32imm:$src),
>>>                   "mov{l} {$src, $dst|$dst, $src}">;
>>>
>>> I cannot find any document on initializing the 'dag' type variable,
>>> and I cannot understand the syntax of "asmstring" either.
>>
>> The DAG operator is just a nested set of parens, e.g. (X (Y Z), Q), where
>> X,Y,Z,Q are tblgen defs.  This is described here:
>> http://llvm.cs.uiuc.edu/docs/TableGenFundamentals.html#values
>>
>>> how does the x86 asmwrite interpret the "AsmString" ?
>>
>> The X86 has two asm formats: Intel and AT&T.  Things in {}'s get split up
>> and treated as one or the other.  The 0 part is AT&T, the 1th part is
>> Intel.  It might make it more clear to just say that this string:
>>
>>       "mov{l} {$src, $dst|$dst, $src}"
>>
>> is fully equivalent to this one:
>>
>>       "{movl $src, $dst|mov $dst, $src}"
>>
>> No other targets support multiple output formats, so they don't have to
>> worry about this.
>>
>> -Chris
>>
>> --
>> http://nondot.org/sabre/
>> http://llvm.cs.uiuc.edu/
>>
>

-Chris

-- 
http://nondot.org/sabre/
http://llvm.cs.uiuc.edu/