[llvm-dev] When AVR backend generates mulsu instruction ?

[vivek pandya via llvm-dev]

Thanks Dylan,

I am working on a backend which has mulhsu instruction that performs
multiplication between signed and unsigned number and returns upper 32 bits
into result register.  I think I also need to write some code probably as
you indicated to check signedness of the operands and based on that lower
to mulhsu instruction.

-Vivek

On Mon, Feb 27, 2017 at 11:13 AM, Dylan McKay <me at dylanmckay.io> wrote:

> Hey Vivek,
>
> We don't directly emit the MULSURdRr instruction. On top of this, I don't
> believe any of the AVR multiplication instructions have patterns in
> TableGen.
>
> This is because the AVR mul instructions are quite strange. Almost all of
> the other instructions of the 'Rd, Rr' format take the values of Rd and Rr,
> perform a computation and place the result in the destination register Rd.
> The mul instructions implicitly use r0 and r1 to store the results. This
> makes it quite hard to fit into TableGen and so we perform custom lowering
> for these nodes.
>
> We expand the MUL DAG node into 'ISD::UMUL_LOHI' or 'ISD::SMUL_LOHI'. We
> see these in AVRISelDAGToDAG.cpp and custom lower them (see
> AVRISelDAGToDAG::selectMultiplication) into MULSRdRr or MULRdRr,
> depending on signedness. Later on we have a custom inserter which inserts
> instructions to clear the r1,r0 scratch registers after use.
>
> The MULSURdRr instruction you mentioned is not generated by LLVM unless
> you use it yourself as part of some inline assembly.
>
> On Mon, Feb 27, 2017 at 12:41 AM, vivek pandya via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello LLVMDevs,
>>
>> I am looking for an example for how to lower LLVM IR to mulsu kind of
>> instruction.  I found that AVR back end have such instruction but
>> AVRInstrInfo.td does not define any DAG pattern for which this
>> instruction gets emitted.
>> def MULSURdRr : FMUL2RdRr<1,
>> (outs),
>> (ins GPR8:$lhs, GPR8:$rhs),
>> "mulsu\t$lhs, $rhs",
>> []>,
>> Requires<[SupportsMultiplication]>;
>>  Also simple grep around related words does not show any other
>> information.
>>
>> Can some one explain me how this kind of instruction should be lowered ?
>>
>> Sincerely,
>> Vivek
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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