[llvm-dev] Help understanding and lowering LLVM IDS conditional codes correctly

[Hal Finkel via llvm-dev]

On 02/25/2017 03:06 AM, vivek pandya via llvm-dev wrote:
> Note: Question is written after describing what I have coded.
>
> Hello LLVMDevs,
>
> I am trying to impliment floating point comparsion for an architecture 
> which
> supports following type of floating point comparision if FPU is available:
> fcmp.un --> true if one of the operand is NaN
> fcmp.lt <http://fcmp.lt> --> ordered less than, if any input NaN then 
> return false
> fcmp.eq --> ordered equal, if any input NaN then return false
> fcmp.le --> ordered less equal, if any input NaN then return false
> fcmp.gt <http://fcmp.gt> --> ordered grater than, if any input NaN 
> then return false
> fcmp.ne <http://fcmp.ne> --> ordered not equal, if any input NaN then 
> return true
> fcmp.ge <http://fcmp.ge> --> ordered grater equal, if any input NaN 
> then return false
>
> When FPU is not present I need to generate a library call,
>
> so I have added following code in LowerBR_CC function in 
> XXXISelLowering.cpp
>
> const XXXSubtarget &STI = static_cast<const XXXSubtarget&>
>  (DAG.getSubtarget());
>  XXXCC::CondCodes TCC;
>  getFPCCtoXXCC(CC,TCC);
>  TargetCC = DAG.getConstant(TCC, dl, MVT::i8);
>  if (STI.useHardFloat()) {
>     // if fcmp instruction is available use it
>    SDValue Flag = DAG.getNode(XXXISD::FCMP, dl, MVT::Glue, LHS, RHS,
>                       TargetCC);
>    return DAG.getNode(XXXISD::BR_CC, dl, Op.getValueType(),
>                   Chain, Dest, TargetCC, Flag);
>  }
>  else {
>     // else generate library call
>    DAG.getTargetLoweringInfo().softenSetCCOperands(DAG, MVT::f32, LHS, 
> RHS,
>                                                    CC, dl);
>
>    SDValue Flag = DAG.getNode(XXXISD::CMP, dl, MVT::Glue, LHS, RHS);
>
>    if (!RHS.getNode()) {
>      RHS = DAG.getConstant(0, dl, LHS.getValueType());
>      TargetCC = DAG.getConstant(XXXCC::COND_NE, dl, MVT::i8);
>    }
>    return DAG.getNode(XXXISD::BR_CC, dl, MVT::Other,
>                   Chain, Dest, TargetCC, Flag);
>  }
>
>  and code for getFPCCtoXXCC() is as following:
>
>  static void getFPCCtoXXCC(ISD::CondCode CC, XXXCC::CondCodes &CondCode) {
>   switch (CC) {
>     default:
>       llvm_unreachable("Unknown FP condition!");
>     case ISD::SETEQ:
>     case ISD::SETOEQ:
>       CondCode = XXXCC::COND_E;
>       break;
>     case ISD::SETGT:
>     case ISD::SETOGT:
>       CondCode = XXXCC::COND_GT;
>       break;
>     case ISD::SETGE:
>     case ISD::SETOGE:
>       CondCode = XXXCC::COND_GE;
>       break;
>     case ISD::SETOLT:
>     case ISD::SETLT:
>       CondCode = XXXCC::COND_LT;
>       break;
>     case ISD::SETOLE:
>     case ISD::SETLE:
>       CondCode = XXXCC::COND_LE;
>       break;
>     case ISD::SETONE:
>     case ISD::SETNE:
>       CondCode = XXXCC::COND_NE;
>       break;
>     case ISD::SETUO:
>       CondCode = XXXCC::COND_UN;
>       break;
>     case ISD::SETO:
>     case ISD::SETUEQ:
>     case ISD::SETUGT:
>     case ISD::SETUGE:
>     case ISD::SETULT:
>     case ISD::SETULE:
>     case ISD::SETUNE:
>       CC = getSetCCInverse(CC,false);
>       getFPCCtoMBCC(CC,CondCode);
>       break;
>   }
> }
>
>  I am generating wrong code when using floating point library call for
> comparions. For the following simple case:
> float branchTest(float a, float b) {
> float retVal;
> if (a == b) {
> retVal = a / b + 22.34;
> }
> return retVal;
> }
> I am getting:
> brlidr15,__nesf2
> nop
> beqir3,.LBB0_2 ; r3 is return regsiter
>
> Now I want to understand difference between three different version of 
> Condition
> Codes for same operation and how according to my target I should 
> handle them.
> For example let's consider SETNE, SETONE and SETUNE so for my 
> architecture
> I think for floating point all three are same

No, they're not the same. Please see:

   http://llvm.org/docs/LangRef.html#id290

which explains the difference between SETONE (one) and SETUNE (une). 
Regarding how SETNE is interpreted for FP, see the comment in the 
definition of CondCode in include/llvm/CodeGen/ISDOpcodes.h which 
explains, "// Don't care operations: undefined if the input is a nan.".

To support the unordered comparisons, if your FPU has only ordered 
comparisons, then you might need to do the underlying comparison, and a 
NaN check, and then OR the results. I think that using setCondCodeAction 
will cause the expansions for the hardware-unsupported variants to 
happen for you.

  -Hal

> so do I need to use
> getSetCCInverse() ? Also when I look at the code of
> TargetLowering::softenSetCCOperands I see that for some condition code 
> it uses
> getSetCCInverse() and also I am not able to understand the way it groups
> condition code in switch case for example :
>   case ISD::SETEQ:
>   case ISD::SETOEQ:
>     LC1 = (VT == MVT::f32) ? RTLIB::OEQ_F32 :
>           (VT == MVT::f64) ? RTLIB::OEQ_F64 : RTLIB::OEQ_F128;
>     break;
>   case ISD::SETNE:
>   case ISD::SETUNE:
>     LC1 = (VT == MVT::f32) ? RTLIB::UNE_F32 :
>           (VT == MVT::f64) ? RTLIB::UNE_F64 : RTLIB::UNE_F128;
>     break;
> here why SETNE and SETUNE is considered same, why SETONE is considered
> differently. Is there any guideline to lower conditional code properly?
>
> Sincerely,
> Vivek
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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