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

[Nikita Popov via llvm-commits]

https://github.com/nikic created https://github.com/llvm/llvm-project/pull/150773

Reverts llvm/llvm-project#139829.

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

>From fbee92502f784da3e1be8c56b9031ea52f2d8fc0 Mon Sep 17 00:00:00 2001
From: Nikita Popov <github at npopov.com>
Date: Sat, 26 Jul 2025 17:34:51 +0200
Subject: [PATCH] Revert "[MIPS]Fix QNaNs in the MIPS legacy NaN encodings
 (#139829)"

This reverts commit 019d7694df91d75322e3c19ebc2e1aa7d3120e02.
---
 llvm/lib/Target/Mips/MipsISelLowering.cpp | 29 -----------------------
 llvm/lib/Target/Mips/MipsISelLowering.h   |  1 -
 llvm/test/CodeGen/Mips/qnan.ll            | 14 -----------
 3 files changed, 44 deletions(-)
 delete mode 100644 llvm/test/CodeGen/Mips/qnan.ll

diff --git a/llvm/lib/Target/Mips/MipsISelLowering.cpp b/llvm/lib/Target/Mips/MipsISelLowering.cpp
index 0e581a7a16503..ec6b382151660 100644
--- a/llvm/lib/Target/Mips/MipsISelLowering.cpp
+++ b/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
 //===----------------------------------------------------------------------===//
diff --git a/llvm/lib/Target/Mips/MipsISelLowering.h b/llvm/lib/Target/Mips/MipsISelLowering.h
index 31ac5d4c185bc..c65c76ccffc75 100644
--- a/llvm/lib/Target/Mips/MipsISelLowering.h
+++ b/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.
diff --git a/llvm/test/CodeGen/Mips/qnan.ll b/llvm/test/CodeGen/Mips/qnan.ll
deleted file mode 100644
index e5b4aa1b42ee7..0000000000000
--- a/llvm/test/CodeGen/Mips/qnan.ll
+++ /dev/null
@@ -1,14 +0,0 @@
-; RUN: llc -O3 -mcpu=mips32r2 -mtriple=mips-linux-gnu < %s -o - | FileCheck %s -check-prefixes=MIPS_Legacy
-; RUN: llc -O3 -mcpu=mips32r2 -mtriple=mips-linux-gnu -mattr=+nan2008 < %s -o - | FileCheck %s -check-prefixes=MIPS_NaN2008
-
-define dso_local float @nan(float noundef %a, float noundef %b) local_unnamed_addr #0 {
-; MIPS_Legacy: $CPI0_0:
-; MIPS_Legacy-NEXT: .4byte  0x7fa00000 # float NaN
-
-; MIPS_NaN2008: $CPI0_0:
-; MIPS_NaN2008-NEXT: .4byte  0x7fc00000 # float NaN
-
-entry:
-  %0 = tail call float @llvm.minimum.f32(float %a, float %b)
-  ret float %0
-}