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

[Steve Montgomery via llvm-dev]

HI Martin,

Since your addresses are i32, I’d imagine that the ADD is i32 and therefore both its operands are also i32. Matching the i32 Offset operand with an i16 register is likely to lead to problems.

Essentially, you need to write code that matches:

	(add $base (sext (i16 $offset))

so something like:

	if (Addr.getOpcode() == ISD::ADD) {
	  if (isSextFromi16(Addr.getOperand(1)) {
	    Base = Addr.getOperand(0);
	    Offset = Addr.getOperand(1).getOperand(0);	// unextended i16
	    return true;
	  }
	  if (isSextFromi16(Addr.getOperand(0)) {
	    Base = Addr.getOperand(1);
	    Offset = Addr.getOperand(0).getOperand(0);
	    return true;
	  }
	  return false;
	}

You’ll need to write isSextFromi16 which could be as simple as checking that its parameter is ISD::SIGN_EXTEND whose operand is i16. You could extend it match other operands, such as:
- zext for which bit 15 of the operand is known to be 0 (use SelectionDAG::MaskedValueIsZero)
- sextload of an i16 (if your target supports it)
- any i32 operand for which bits 15-31 are the same (use SelectionDAG::ComputeNumSignBits)

In the last two cases you’d need to turn the sextload into an i16 load and truncate the i32 respectively so that they are i16 operands but I think that’s possible during instruction selection.

Hope that helps.

Steve

> On 2 Nov 2015, at 21:16, Martin J. O'Riordan <martin.oriordan at movidius.com <mailto:martin.oriordan at movidius.com>> wrote:
> 
> 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 J  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 <mailto:stephen.montgomery3 at btinternet.com>] 
> Sent: 02 November 2015 20:25
> To: Martin J. O'Riordan <martin.oriordan at movidius.com <mailto:martin.oriordan at movidius.com>>
> Cc: LLVM Developers <llvm-dev at lists.llvm.org <mailto: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 <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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