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

vivek pandya via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 26 22:40:34 PST 2017

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.


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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170227/2094a350/attachment.html>

More information about the llvm-dev mailing list