[llvm-dev] How to define subreg relationship in td file?

[Diogo Sampaio via llvm-dev]

We also had quad registers defined in function of 4 single registers, then we couldn't extract paired register from it easily. 
So we changed and made them defined in function of paired registers. We also named the sub indices as to know what size of sub-register we want. Is something alike this: 
  
// Defining sub-register indexes 
def sub_p0 : SubRegIndex<128,  0>; 
def sub_p1 : SubRegIndex<128,128>; 
  
def sub_s0 : SubRegIndex<64,  0>; 
def sub_s1 : SubRegIndex<64, 64>; 
def sub_s2 : ComposedSubRegIndex<sub_p1, sub_s0>; 
def sub_s3 : ComposedSubRegIndex<sub_p1, sub_s1>; 
  
// Composed register classes 
 
// Paired Registers 
class PGR<string n, list<Register> subregs> 
      : : RegisterWithSubRegs<n, subregs> { 
  let SubRegIndices = [sub_s0, sub_s1]; 
} 
  
// Quad Register 
 
class QGR<string n, list<Register> subregs> 
      : RegisterWithSubRegs<n, subregs> { 
  let SubRegIndices = [sub_p0, sub_p1]; 
} 
  
// Defining registers 
def P0  : PGR< 0,   "$r0r1", [ R0,  R1]>; 
def P2  : PGR< 2,   "$r2r3", [ R2,  R3]>; 
.... 
    
def Q0  : QGR< 0,     "$r0r1r2r3", [ P0,  P2]>; 
  
// 
Now can use any of those 6 sub-index to reference 2 sub-pair registers or any of the 4 single registers. 
  
 
  
  Diogo Sampaio
Senior Compiler Engineer • Kalray
Phone: 
dsampaio at kalrayinc.com • www.kalrayinc.com 
     
    Intelligent Data Processing
>From Cloud to Edge      

   

-----Original Message-----

From: Fraser <llvm-dev at lists.llvm.org>
To: jackie <jackie_linzz at 126.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Date: Tuesday, 10 August 2021 2:15 PM CEST
Subject: Re: [llvm-dev] How to define subreg relationship in td file?

Hi,

You can (I'm hesitant to say you "must") model this with extra "fake"
(in some sense) physical registers which represent the paired and
quadrupled registers. This will also tell the register allocator that
allocating VRPair0 will clobber both VR0 *and VR1*, for example.

So you have half the number of VRPair registers which each have two VRs
as subregisters, then half that again of VRQuad registers which have
two VRPairs as subregisters. The subregister properties will apply
transitively so your VRQuads will have four VR subregisters using
composed subregister indices which LLVM will either automatically infer
or you can specify yourself with ComposedSubRegIndex.

Take a look at RISCVRegisterInfo.td for inspriation. Each vector V
register is paired into VM2 which are in turn paried into VM4s and then
into VM8s.

Cheers,
Fraser

On Tue, 2021-08-10 at 19:54 +0800, 林政宗 via llvm-dev wrote:
> Hi, there.
> 
> I met a problem about defining SubReg relationship when defining
> Registers in td file.
> The target has a kind of vector register named VR which is of type
> v4f32.  
> 2 VR makes 1 DVR. 4 VR makes 1 QVR.
> I have some code like this:
> -------------------------------------------------------------------
> -----------------------------------
> def VRPairs : RegisterTuples<[vsub_0, vsub_1],
>                              [(decimate (rotl VRegs, 0), 2),
>                               (decimate (rotl VRegs, 1), 2)],
>                              []>;
> 
> def VRQuads : RegisterTuples<[vsub_0, vsub_1, vsub_2, vsub_3],
>                              [(decimate (rotl VRegs, 0), 4),
>                               (decimate (rotl VRegs, 1), 4),
>                               (decimate (rotl VRegs, 2), 4),
>                               (decimate (rotl VRegs, 3), 4)],
>                              []>;
> def DVRRegs : RegisterClass<"xxx", [v8f32], 256, (add VRPairs)>;
>  
> def QVRRegs : RegisterClass<"xxx", [v16f32], 512, (add VRQuads)>;
> -------------------------------------------------------------------
> -----------------------------------
> But also, 2 DVR makes 1 QVR. QVR has two subregs, each of which is 1
> DVR.
> How can I describe the SubReg relationship between DVR and QVR? 
> Could anyone help? Thanks in advance!
> 
> 
> BR,
> Jerry
> 
> 
> 
>  
> 
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> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> 
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 

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