[LLVMdev] Adding address registers to back-end

[Evan Cheng]

On Oct 19, 2007, at 8:15 AM, Boris Boesler wrote:

> Hi!
>
>   I'm writing a new back-end for a new architecture. First, I'll do
> some "tests" with an existing back-end (I chose the Sparc back-end).
> My architecture has special address-registers and I want to add such
> new address-registers to my Sparc back-end.
>
> 1) I defined a new register call AddrRegs
> 2) I registered the class AddrRegs (addRegisterClass(MVT::iPTR, .. ))
> 3) I added method addPointerRegClass() to my InstrInfo class
>
>   The compiler generates the some code as before, but that seems to
> be ok, because I haven't used ptr_rc yet.
>
> 4) I changed the address mode MEMri:
>
> def MEMri : Operand<iPTR> {
>    let PrintMethod = "printMemOperand";
>    // was: let MIOperandInfo = (ops IntRegs, i32imm);
>    let MIOperandInfo = (ops ptr_rc, i32imm);
> }
>
> for the C code int c; void f(void) { c = 4711; } I get the error
> message:
>
> Register class of operand and regclass of use don't agree!
> Operand = 0
> Op->Val = 0x42b08d60: i32 = SETHIi 0x42b08d00
> MI = STri       %reg1026
> VReg = 1026
> VReg RegClass     size = 4, align = 4
> Expected RegClass size = 4, align = 4
>
>   The GlobalAddress for variable c is replaced by an ADD(HI(c), LO
> (c)) during lowering. I assume the code-generator cant place values
> in the address registers? All address-registers are elements in the
> register sets IntRegs and AddrRegs.

/// F3_12 multiclass - Define a normal F3_1/F3_2 pattern in one shot.
multiclass F3_12<string OpcStr, bits<6> Op3Val, SDNode OpNode> {
   def rr  : F3_1<2, Op3Val,
                  (outs IntRegs:$dst), (ins IntRegs:$b, IntRegs:$c),
                  !strconcat(OpcStr, " $b, $c, $dst"),
                  [(set IntRegs:$dst, (OpNode IntRegs:$b, IntRegs: 
$c))]>;
   def ri  : F3_2<2, Op3Val,
                  (outs IntRegs:$dst), (ins IntRegs:$b, i32imm:$c),
                  !strconcat(OpcStr, " $b, $c, $dst"),
                  [(set IntRegs:$dst, (OpNode IntRegs:$b, simm13:$c))]>;
}

defm ADD   : F3_12<"add", 0b000000, add>;

Instruction ADD output register class is IntRegs. It does not match  
AddrRegs. That's why you are getting the assertion.

What you need is to define a parallel set of instructions that target  
the address register class. Then you can tell the instruction  
selector to select to these instructions instead of the normal ADD  
instruction. Perhaps the solution is to custom lower load / store  
addresses to be "casted". Then write patterns that fold the cast and  
select to the address register class variant of ADD, etc. There  
hasn't been any publicly committed targets that require address  
register class so I haven't spent anytime thinking of a clean  
solution. For a hackish solution, please read the thread called "Q  
about instruction pattern matching" between Andreas Fredriksson and  
myself.

Evan


>
>   Using address registers is part of the ppc code-generator and I'm
> checking that one, but I can't see the differences. Can someone guide
> me?
>
> Thanks,
> Boris
>
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