llvm dev - Apr 2021 - Implicit register move using TableGen

Dear all,

I need to implement the following behavior for all instructions that 
require two source operands:

%1 = add i16 %a, %b

Should match the ADD instruction which has 1 outs and 1 ins. The second 
operand %b should be moved into the implicit register R14 (fixed).
For now I implemented the PseudoADD instruction which gets expanded into 
a move plus the mentioned ADD instruction.
This obviously creates a low of unnecessary moves and prevents any 
optimizations of the register R14 (e.g. R14 could be used as destination 
register in previous operations without the need for a move).

I'd like to transform the dag:

(set GPR:$rd, (add GPR:$rs1, GPR:$rs2))

to something like:

(set R14, GPR:$rs2), (set GPR:$rd, (add GPR:$rs1))

Is it possible to specify this transformation using TableGen? If now how 
could I achieve this?


Thanks.

Best Regards,
Marco Speziali

In tablegen you can write

(set GPR:$rd, (add GPR:$rs1, R14))

On Sat, Apr 10, 2021 at 9:19 AM Marco Speziali via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Dear all,
>
> I need to implement the following behavior for all instructions that
> require two source operands:
>
> %1 = add i16 %a, %b
>
> Should match the ADD instruction which has 1 outs and 1 ins. The second
> operand %b should be moved into the implicit register R14 (fixed).
> For now I implemented the PseudoADD instruction which gets expanded into
> a move plus the mentioned ADD instruction.
> This obviously creates a low of unnecessary moves and prevents any
> optimizations of the register R14 (e.g. R14 could be used as destination
> register in previous operations without the need for a move).
>
> I'd like to transform the dag:
>
> (set GPR:$rd, (add GPR:$rs1, GPR:$rs2))
>
> to something like:
>
> (set R14, GPR:$rs2), (set GPR:$rd, (add GPR:$rs1))
>
> Is it possible to specify this transformation using TableGen? If now how
> could I achieve this?
>
>
> Thanks.
>
> Best Regards,
> Marco Speziali
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-- 
~Craig
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