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

Dylan McKay via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 26 22:43:40 PST 2017

Is this code an IR or a machine pass?

If it is IR, you might have trouble checking the signedness of the types
because LLVM IR doesn't make a distinction between unsigned and signed
types, and the [mul instruction](
http://llvm.org/docs/LangRef.html#mul-instruction) is general enough to be
used with either one.

On Mon, Feb 27, 2017 at 7:40 PM, vivek pandya <vivekvpandya at gmail.com>

> 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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