[llvm-dev] Difference between Predicates and Requires in td file

Krzysztof Parzyszek via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 12 08:26:54 PDT 2017

They are the same. There is a member "Predicates" in class Pat, and both 
forms assign a value to that member.

You can use whichever one you prefer. The "let" form allows you to 
specify it once for multiple defs.


On 9/12/2017 9:28 AM, 陳韋任 via llvm-dev wrote:
> Hi All,
>    I am looking for how to distinguish instructions between subtargets,
> and it seems both
> Predicates and Requires be able to do that. Take Hexagon as example, I see
>    let Predicates = [HasV5T] in {
>      def: Pat<(f32 (fminnum F32:$Rs, F32:$Rt)), (F2_sfmin F32:$Rs, F32:$Rt)>;
>      def: Pat<(f32 (fmaxnum F32:$Rs, F32:$Rt)), (F2_sfmax F32:$Rs, F32:$Rt)>;
>    }
> and this
>    def: Pat<(sra (add (sra I64:$src, u6_0ImmPred:$u6), 1), (i32 1)),
>                  (S2_asr_i_p_rnd DoubleRegs:$src, imm:$u6)>, Requires<[HasV5T]>;
> Are they really the same? If so, I guess if I have multiple
> instructions belong to one
> specific subtarget, I should use Predicates; otherwise, just use
> Requires. Am I right
> or wrong?
>    Thanks.
> Regards,
> chenwj

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