[llvm-dev] Questions about load/store incrementing address modes

[Martin J. O'Riordan via llvm-dev]

Thanks again for your help Steve,

 

I’m thinking perhaps my “SelectADDRrr” pattern is inadequate.  The sign-extension is at the hardware level, the code generator sees (should see) it as a 16-bit signed register value.  My implementation is just:

 

bool SHAVEISelDAGtoDAG::SelectADDRrr(SDValue &Addr, SDValue &Base, SDValue &Offset) {

  if ((Addr.getOpcode() == ISD::ADD) {

    Base = Addr.getOperand(0);

    Offset = Addr.getOperand(1);

    return true;

  }

 

  return false;

}

 

I don’t have any special checks on the offset (or the base for that matter) on the naive assumption that it would not have been invoked if the constraints were not already met.  But don’t worry about it, you’ve given me a fresh avenue to investigate - a few DEBUG dumps should show me the error of my ways :)  I’m guessing that I need to check that the offset operand is truly a 16-bit register and return false if it isn’t.  A nice simple fix if that is all that is needed - thanks again for shedding light on this for me.

 

            MartinO

 

From: Steve Montgomery [mailto:stephen.montgomery3 at btinternet.com] 
Sent: 02 November 2015 20:25
To: Martin J. O'Riordan <martin.oriordan at movidius.com>
Cc: LLVM Developers <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Questions about load/store incrementing address modes

 

 

On 2 Nov 2015, at 10:27, Martin J. O'Riordan <martin.oriordan at movidius.com <mailto:martin.oriordan at movidius.com> > wrote:

 

Thanks Steve, I will try this out.  I hadn’t realised that TableGen was restricted to matching instructions with more than one output operand.  I’m assuming that this is only a limitation for inferring an instruction from the patterns, because it does seem to manage schedules okay.

 

I’m basing my statement on the material at the end of the “Selection DAG Select Phase” in “The LLVM Target-Independent Code Generator”, http://llvm.org/docs/CodeGenerator.html#selectiondag-select-phase. I’ve not actually checked TableGen though so can’t be 100% sure that the documentation is still in date.

 



Curiously, my memory Reg32+Reg16 pattern is very similar to yours (the 16-bit offset is sign-extended though):

 

// Memory address: 32-bit base register + 16-bit offset register

def ADDRrr : ComplexPattern<iPTR, 2, "SelectADDRrr", []>;

def MEMrr : Operand<iPTR> {

  let PrintMethod = "printMemOffsetOperand";

  let MIOperandInfo = (ops RC32, RC16_l);

}

 

but it is still happy to select for offset’s > 16-bits.  There is something I am just not yet getting right, but it looks like I am on the right track.

 

I believe that the MIOperandInfo will constrain the register class for your 16-bit offset operand to RC16_1 but in itself it won’t affect the matching of the operand. Your SelectADDRrr will need to contain code to match an i32 added to a sign-extended i16. If you’ve already done that, then I’m out of ideas, sorry.

 

Steve

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