[RISCV] Custom lowering of FLT_ROUNDS_

Differential Revision: https://reviews.llvm.org/D90854

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -372,6 +372,10 @@ RISCVTargetLowering::RISCVTargetLowering(const TargetMachine &TM,
     setOperationAction(ISD::STRICT_FP_TO_SINT, MVT::i32, Custom);
   }
 
+  if (Subtarget.hasStdExtF()) {
+    setOperationAction(ISD::FLT_ROUNDS_, XLenVT, Custom);
+  }
+
   setOperationAction(ISD::GlobalAddress, XLenVT, Custom);
   setOperationAction(ISD::BlockAddress, XLenVT, Custom);
   setOperationAction(ISD::ConstantPool, XLenVT, Custom);
@@ -2161,6 +2165,8 @@ SDValue RISCVTargetLowering::LowerOperation(SDValue Op,
     return lowerMGATHER(Op, DAG);
   case ISD::MSCATTER:
     return lowerMSCATTER(Op, DAG);
+  case ISD::FLT_ROUNDS_:
+    return lowerGET_ROUNDING(Op, DAG);
   }
 }
 
@@ -4107,6 +4113,37 @@ SDValue RISCVTargetLowering::lowerMSCATTER(SDValue Op,
                                  MSN->getMemoryVT(), MSN->getMemOperand());
 }
 
+SDValue RISCVTargetLowering::lowerGET_ROUNDING(SDValue Op,
+                                               SelectionDAG &DAG) const {
+  const MVT XLenVT = Subtarget.getXLenVT();
+  SDLoc DL(Op);
+  SDValue Chain = Op->getOperand(0);
+  SDValue SysRegNo = DAG.getConstant(
+      RISCVSysReg::lookupSysRegByName("FRM")->Encoding, DL, XLenVT);
+  SDVTList VTs = DAG.getVTList(XLenVT, MVT::Other);
+  SDValue RM = DAG.getNode(RISCVISD::READ_CSR, DL, VTs, Chain, SysRegNo);
+
+  // Encoding used for rounding mode in RISCV differs from that used in
+  // FLT_ROUNDS. To convert it the RISCV rounding mode is used as an index in a
+  // table, which consists of a sequence of 4-bit fields, each representing
+  // corresponding FLT_ROUNDS mode.
+  static const int Table =
+      (int(RoundingMode::NearestTiesToEven) << 4 * RISCVFPRndMode::RNE) |
+      (int(RoundingMode::TowardZero) << 4 * RISCVFPRndMode::RTZ) |
+      (int(RoundingMode::TowardNegative) << 4 * RISCVFPRndMode::RDN) |
+      (int(RoundingMode::TowardPositive) << 4 * RISCVFPRndMode::RUP) |
+      (int(RoundingMode::NearestTiesToAway) << 4 * RISCVFPRndMode::RMM);
+
+  SDValue Shift =
+      DAG.getNode(ISD::SHL, DL, XLenVT, RM, DAG.getConstant(2, DL, XLenVT));
+  SDValue Shifted = DAG.getNode(ISD::SRL, DL, XLenVT,
+                                DAG.getConstant(Table, DL, XLenVT), Shift);
+  SDValue Masked = DAG.getNode(ISD::AND, DL, XLenVT, Shifted,
+                               DAG.getConstant(7, DL, XLenVT));
+
+  return DAG.getMergeValues({Masked, Chain}, DL);
+}
+
 // Returns the opcode of the target-specific SDNode that implements the 32-bit
 // form of the given Opcode.
 static RISCVISD::NodeType getRISCVWOpcode(unsigned Opcode) {
@@ -4584,6 +4621,13 @@ void RISCVTargetLowering::ReplaceNodeResults(SDNode *N,
     if (SDValue V = lowerVECREDUCE(SDValue(N, 0), DAG))
       Results.push_back(V);
     break;
+  case ISD::FLT_ROUNDS_: {
+    SDVTList VTs = DAG.getVTList(Subtarget.getXLenVT(), MVT::Other);
+    SDValue Res = DAG.getNode(ISD::FLT_ROUNDS_, DL, VTs, N->getOperand(0));
+    Results.push_back(Res.getValue(0));
+    Results.push_back(Res.getValue(1));
+    break;
+  }
   }
 }
 

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -533,6 +533,7 @@ private:
                             bool HasMask = true) const;
   SDValue lowerFixedLengthVectorExtendToRVV(SDValue Op, SelectionDAG &DAG,
                                             unsigned ExtendOpc) const;
+  SDValue lowerGET_ROUNDING(SDValue Op, SelectionDAG &DAG) const;
 
   bool isEligibleForTailCallOptimization(
       CCState &CCInfo, CallLoweringInfo &CLI, MachineFunction &MF,

llvm/test/CodeGen/RISCV/fpenv.ll

@@ -0,0 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=riscv32 -mattr=+f -verify-machineinstrs < %s | FileCheck -check-prefix=RV32IF %s
+; RUN: llc -mtriple=riscv64 -mattr=+f -verify-machineinstrs < %s | FileCheck -check-prefix=RV64IF %s
+
+define i32 @func_01() {
+; RV32IF-LABEL: func_01:
+; RV32IF:       # %bb.0:
+; RV32IF-NEXT:    frrm a0
+; RV32IF-NEXT:    slli a0, a0, 2
+; RV32IF-NEXT:    lui a1, 66
+; RV32IF-NEXT:    addi a1, a1, 769
+; RV32IF-NEXT:    srl a0, a1, a0
+; RV32IF-NEXT:    andi a0, a0, 7
+; RV32IF-NEXT:    ret
+;
+; RV64IF-LABEL: func_01:
+; RV64IF:       # %bb.0:
+; RV64IF-NEXT:    frrm a0
+; RV64IF-NEXT:    slli a0, a0, 2
+; RV64IF-NEXT:    lui a1, 66
+; RV64IF-NEXT:    addiw a1, a1, 769
+; RV64IF-NEXT:    srl a0, a1, a0
+; RV64IF-NEXT:    andi a0, a0, 7
+; RV64IF-NEXT:    ret
+  %rm = call i32 @llvm.flt.rounds()
+  ret i32 %rm
+}
+
+declare i32 @llvm.flt.rounds()