Revert "[MIPS]Fix QNaNs in the MIPS legacy NaN encodings" (#150773)

Reverts llvm/llvm-project#139829.

We can't just randomly change the value of constants during lowering.

Fixes llvm/llvm-project#149295.

Author: nikic

llvm/lib/Target/Mips/MipsISelLowering.cpp

@@ -522,9 +522,6 @@ MipsTargetLowering::MipsTargetLowering(const MipsTargetMachine &TM,
 
   setOperationAction(ISD::TRAP, MVT::Other, Legal);
 
-  setOperationAction(ISD::ConstantFP, MVT::f32, Custom);
-  setOperationAction(ISD::ConstantFP, MVT::f64, Custom);
-
   setTargetDAGCombine({ISD::SDIVREM, ISD::UDIVREM, ISD::SELECT, ISD::AND,
                        ISD::OR, ISD::ADD, ISD::SUB, ISD::AssertZext, ISD::SHL,
                        ISD::SIGN_EXTEND});
@@ -1360,8 +1357,6 @@ LowerOperation(SDValue Op, SelectionDAG &DAG) const
   case ISD::FP_TO_SINT:         return lowerFP_TO_SINT(Op, DAG);
   case ISD::READCYCLECOUNTER:
     return lowerREADCYCLECOUNTER(Op, DAG);
-  case ISD::ConstantFP:
-    return lowerConstantFP(Op, DAG);
   }
   return SDValue();
 }
@@ -3019,30 +3014,6 @@ SDValue MipsTargetLowering::lowerFP_TO_SINT(SDValue Op,
   return DAG.getNode(ISD::BITCAST, SDLoc(Op), Op.getValueType(), Trunc);
 }
 
-SDValue MipsTargetLowering::lowerConstantFP(SDValue Op,
-                                            SelectionDAG &DAG) const {
-  SDLoc DL(Op);
-  EVT VT = Op.getSimpleValueType();
-  SDNode *N = Op.getNode();
-  ConstantFPSDNode *CFP = cast<ConstantFPSDNode>(N);
-
-  if (!CFP->isNaN() || Subtarget.isNaN2008()) {
-    return SDValue();
-  }
-
-  APFloat NaNValue = CFP->getValueAPF();
-  auto &Sem = NaNValue.getSemantics();
-
-  // The MSB of the mantissa should be zero for QNaNs in the MIPS legacy NaN
-  // encodings, and one for sNaNs. Check every NaN constants and make sure
-  // they are correctly encoded for legacy encodings.
-  if (!NaNValue.isSignaling()) {
-    APFloat RealQNaN = NaNValue.getSNaN(Sem);
-    return DAG.getConstantFP(RealQNaN, DL, VT);
-  }
-  return SDValue();
-}
-
 //===----------------------------------------------------------------------===//
 //                      Calling Convention Implementation
 //===----------------------------------------------------------------------===//

llvm/lib/Target/Mips/MipsISelLowering.h

@@ -592,7 +592,6 @@ class TargetRegisterClass;
     SDValue lowerEH_DWARF_CFA(SDValue Op, SelectionDAG &DAG) const;
     SDValue lowerFP_TO_SINT(SDValue Op, SelectionDAG &DAG) const;
     SDValue lowerREADCYCLECOUNTER(SDValue Op, SelectionDAG &DAG) const;
-    SDValue lowerConstantFP(SDValue Op, SelectionDAG &DAG) const;
 
     /// isEligibleForTailCallOptimization - Check whether the call is eligible
     /// for tail call optimization.

llvm/test/CodeGen/Mips/nan_lowering.ll

@@ -0,0 +1,25 @@
+; RUN: llc -mtriple=mips-linux-gnu -mattr=-nan2008 < %s | FileCheck %s
+; RUN: llc -mtriple=mips-linux-gnu -mattr=+nan2008 < %s | FileCheck %s
+
+; Make sure that lowering does not corrupt the value of NaN values,
+; regardless of what the NaN mode is.
+
+define float @test1() {
+; CHECK: .4byte 0x7fc00000
+  ret float bitcast (i32 u0x7fc00000 to float)
+}
+
+define float @test2() {
+; CHECK: .4byte 0x7fc00001
+  ret float bitcast (i32 u0x7fc00001 to float)
+}
+
+define float @test3() {
+; CHECK: .4byte 0x7f800000
+  ret float bitcast (i32 u0x7f800000 to float)
+}
+
+define float @test4() {
+; CHECK: .4byte 0x7f800001
+  ret float bitcast (i32 u0x7f800001 to float)
+}