llvm dev - Nov 2008 - [LLVMdev] Custom lowering binary operations on one register machines.

Ours is an accumulator based architecture.

So one operand of ADD/SUB operations is in REG (accumulator) and the
other one is in Memory. The result can be left either in REG or memory.

The LLVM DAG for such operations expect both operands in REG.

for example:  
char a, b, c, d, e;
a = (b - c) + (d - e);

generates

addc:i8 (subc:i8(b,c), subc:i8(d,e))

Looks like we need to custom lower addc here.  LegalizeOp needs
modification to allow custom lowering of ADDC, etc.I will post a patch
to do the same.

Any suggestions on how do we custom lower such things?
is using MFI::CreateFixedObject () to generate a FI and then using that
FI to Store one REG operand to memory and then generate a Load from that
FI is a good idea?


sth like

store (subc:i8(d, e), FI)
addc:i8(subc:i8(b,c) - load (FI))

- Sanjiv

On Nov 10, 2008, at 9:51 AM, sanjiv gupta wrote:
> Ours is an accumulator based architecture.
>
> So one operand of ADD/SUB operations is in REG (accumulator) and the
> other one is in Memory. The result can be left either in REG or  
> memory.
>
> The LLVM DAG for such operations expect both operands in REG.
>
> for example:
> char a, b, c, d, e;
> a = (b - c) + (d - e);
>
> generates
>
> addc:i8 (subc:i8(b,c), subc:i8(d,e))
>
> Looks like we need to custom lower addc here.  LegalizeOp needs
> modification to allow custom lowering of ADDC, etc.I will post a patch
> to do the same.
>
> Any suggestions on how do we custom lower such things?
> is using MFI::CreateFixedObject () to generate a FI and then using  
> that
> FI to Store one REG operand to memory and then generate a Load from  
> that
> FI is a good idea?
That should work. I don't see an alternative. I don't think it's
legal
to re-associate the expression, right?

Evan
>
>
>
> sth like
>
> store (subc:i8(d, e), FI)
> addc:i8(subc:i8(b,c) - load (FI))
>
> - Sanjiv
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

>> is using MFI::CreateFixedObject () to generate a FI and then using  
>> that
>> FI to Store one REG operand to memory and then generate a Load from  
>> that
>> FI is a good idea?
>That should work. I don't see an alternative. I don't think it's
legal
>to re-associate the expression, right?
>Evan
Right. I too feel that there isn't an alternative.
Thanks Evan. You've been a great help to our efforts of making LLVM work for
PIC16.

- Sanjiv
>
>
>
> sth like
>
> store (subc:i8(d, e), FI)
> addc:i8(subc:i8(b,c) - load (FI))
>
> - Sanjiv
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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