[LLVMdev] Lowering operations to 8-bit!

[Evan Cheng]

On Oct 1, 2007, at 11:33 AM, Alireza.Moshtaghi at microchip.com wrote:

> So does that mean that LLVM can't lower automatically to 8-bit values?

There is no inherent reason. LLVM should be able to lower to 8-bit  
values. It's probably a bug somewhere.

In TargetLowering.h:

bool isTypeLegal(MVT::ValueType VT) const {
     return !MVT::isExtendedVT(VT) && RegClassForVT[VT] != 0;
   }

Is there a 16-bit register class?

> I tried defining 8-bit pointers in the subtarget using "p:8:8:8"  
> but it
> asserts at line 566 of TargetData.cpp in the default case of
> TargetData::getIntPtrType()

Dunno why it's like this. Can you add case 1: return Type::Int8Ty?  
Does it work / help?

Evan

>
> Is it difficult to add 8-bit support?
>
> A.
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Chris Lattner
> Sent: Friday, September 28, 2007 5:01 PM
> To: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Lowering operations to 8-bit!
>
>
> On Sep 28, 2007, at 4:53 PM, <Alireza.Moshtaghi at microchip.com>
> <Alireza.Moshtaghi at microchip.com> wrote:
>
>> ExpandOp is not called at all.
>> In SelectionDAGLegalize::HandleOp() only the ValueType is
>> considered in
>> the switch statement to decide if it is legal or promote or expand.
>> As I trace back (correct me if I'm wrong) these values are set in
>> TargetLowering::computeRegisterProperties() and it is based on the
>> largest register class (in my case the smallest possible pointer  
>> size,
>> 16-bit)
>> So it reduces everything down to 16-bit and pretty much ignores the
>> fact
>> that ADD of i16 is supposed to be expanded.
>>
>> Am I doing the right analysis?
>
> Yes.  It sounds like the codegen is assuming the pointer type is
> valid in computeRegisterProperties or something.  Somehow i16 is
> getting marked as legal.
>
> -Chris
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