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

[Chris Lattner]

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

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