[LLVMdev] ARM backend playing with alternative jump table implementations

[Evan Cheng]

On Feb 17, 2009, at 2:04 PM, robert muth wrote:

> Hi list:
>
> I have been trying to get my feet wet with the ARM backend.

Welcome.

>
> As a warmup exercise I wanted to be able to move
> jumptables especially large ones out of the code section.
> Currently the idiom for jump tables loooks like this
>
> //          .set PCRELV0, (.LJTI9_0_0-(.LPCRELL0+8))
> //  .LPCRELL0:
> //          add r3, pc, #PCRELV0
> //          ldr pc, [r3, +r0, lsl #2]
> //  .LJTI9_0_0:
> //          .long    .LBB9_2
> //          .long    .LBB9_5
> //          .long    .LBB9_7
> //          .long    .LBB9_4
> //          .long    .LBB9_8
>
> I would like to be able to change this to something like:
>
>             ldr r3, .POOL_ADDR
>             ldr pc, [r3, +r0, lsl #2
>
> .POOL_ADDR:
>            .text .LJTI9_0_0:
>
>            .data
>   .LJTI9_0_0:
>           .long    .LBB9_2
>           .long    .LBB9_5
>           .long    .LBB9_7
>           .long    .LBB9_4
>           .long    .LBB9_8
>            .text

Ok. I think it's a worthwhile alternative jumptable codegen scheme.

>
>
> The code for the lowering lives mostly in SDValue  
> ARMTargetLowering::LowerBR_JT
> with some more heavy lifting done  by ARMISD::WrapperJT
> My attempts at this are marked in the code below.
> My problem is to come up with the right item/value to put into the  
> constant pool.

Off the top of my head, I think you need to enhance  
ARMConstantPoolValue. Perhaps add a new ARMCPKind for jumptable base  
address. You also have to encode the jumptable id in  
ARMConstantPoolValue.

Evan

>
> SDValue ARMTargetLowering::LowerBR_JT(SDValue Op, SelectionDAG &DAG) {
>   SDValue Chain = Op.getOperand(0);
>   SDValue Table = Op.getOperand(1);
>   SDValue Index = Op.getOperand(2);
>   DebugLoc dl = Op.getDebugLoc();
>
>   MVT PTy = getPointerTy();
>   JumpTableSDNode *JT = cast<JumpTableSDNode>(Table);
>   ARMFunctionInfo *AFI =  
> DAG.getMachineFunction().getInfo<ARMFunctionInfo>();
>   SDValue UId =  DAG.getConstant(AFI->createJumpTableUId(), PTy);
>   SDValue JTI = DAG.getTargetJumpTable(JT->getIndex(), PTy);
>
> #if 1
> // @@ GET TABLE BASE: current code
>   Table = DAG.getNode(ARMISD::WrapperJT, MVT::i32, JTI, UId);
> #else
>   // @ MY ATTEMPT AT MOVING THIS OUT
>   ARMConstantPoolValue *CPV = new  
> ARMConstantPoolValue("a_jump_table", 666);
>   SDValue TableValue = DAG.getTargetConstantPool(CPV, PTy, 2);
>   SDValue CPAddr = DAG.getNode(ARMISD::Wrapper, MVT::i32, TableValue);
>   Table = DAG.getLoad(PTy, dl, DAG.getEntryNode(), CPAddr, NULL, 0);
> #endif
>
>   Index = DAG.getNode(ISD::MUL, dl, PTy, Index, DAG.getConstant(4,  
> PTy));
>   //Index = DAG.getNode(ISD::MUL, dl, PTy, TableAddress,  
> DAG.getConstant(4, PTy));
>   SDValue Addr = DAG.getNode(ISD::ADD, dl, PTy, Index,  
> Table);SDValue APTy, Index, Table);
>
>
>
> Any help would be greatly appreciated.
>
> Robert
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