[GlobalISel] Add default action for G_FNEG

[volkan]

Summary

rL297171 introduced G_FNEG for floating-point negation instruction and IRTranslator started to translate FSUB -0.0, X to FNEG X. This patch adds a default action for G_FNEG to avoid breaking existing targets.

[qcolombet]

lib/CodeGen/GlobalISel/LegalizerHelper.cpp

556

Can't we simply reuse Ty?

[volkan]

lib/CodeGen/GlobalISel/LegalizerHelper.cpp

556

No, Ty is an instance of LLT.

[qcolombet]

lib/CodeGen/GlobalISel/LegalizerHelper.cpp

556

Ah right, I forgot we had to go through LLVM IR to build floating-point constant.

lib/CodeGen/GlobalISel/LegalizerHelper.cpp

MIRBuilder.buildInstr(Opcode)
       .addUse(RHS);
 
     unsigned Zero = MRI.createGenericVirtualRegister(Ty);
     MIRBuilder.buildConstant(Zero, 0);
     MIRBuilder.buildICmp(CmpInst::ICMP_NE, Overflow, HiPart, Zero);
     MI.eraseFromParent();
     return Legalized;
   }
+  case TargetOpcode::G_FNEG: {
+    // TODO: Handle vector types once we are able to
+    // represent them.
+    if (Ty.isVector())
+      return UnableToLegalize;
+    unsigned Res = MI.getOperand(0).getReg();
+    Type *ZeroTy;
+    LLVMContext &Ctx = MIRBuilder.getMF().getFunction()->getContext();
+    switch (Ty.getSizeInBits()) {
+    case 16:
+      ZeroTy = Type::getHalfTy(Ctx);
+      break;
+    case 32:
+      ZeroTy = Type::getFloatTy(Ctx);
+      break;
+    case 64:
+      ZeroTy = Type::getDoubleTy(Ctx);
+      break;
+    default:
+      llvm_unreachable("unexpected floating-point type");
+    }
+    ConstantFP &ZeroForNegation =
+        *cast<ConstantFP>(ConstantFP::getZeroValueForNegation(ZeroTy));
+    unsigned Zero = MRI.createGenericVirtualRegister(Ty);
+    MIRBuilder.buildFConstant(Zero, ZeroForNegation);
+    MIRBuilder.buildInstr(TargetOpcode::G_FSUB)
+        .addDef(Res)
+        .addUse(Zero)
+        .addUse(MI.getOperand(1).getReg());
+    MI.eraseFromParent();
+    return Legalized;
+  }
   }
 }
 
 LegalizerHelper::LegalizeResult
 LegalizerHelper::fewerElementsVector(MachineInstr &MI, unsigned TypeIdx,
                                      LLT NarrowTy) {
   // FIXME: Don't know how to handle secondary types yet.
   if (TypeIdx != 0)

lib/CodeGen/GlobalISel/LegalizerInfo.cpp

LegalizerInfo::LegalizerInfo() : TablesInitialized(false) {
   DefaultActions[TargetOpcode::G_INTRINSIC_W_SIDE_EFFECTS] = Legal;
 
   DefaultActions[TargetOpcode::G_ADD] = NarrowScalar;
   DefaultActions[TargetOpcode::G_LOAD] = NarrowScalar;
   DefaultActions[TargetOpcode::G_STORE] = NarrowScalar;
 
   DefaultActions[TargetOpcode::G_BRCOND] = WidenScalar;
   DefaultActions[TargetOpcode::G_INSERT] = NarrowScalar;
+  DefaultActions[TargetOpcode::G_FNEG] = Lower;
 }
 
 void LegalizerInfo::computeTables() {
   for (unsigned Opcode = 0; Opcode <= LastOp - FirstOp; ++Opcode) {
     for (unsigned Idx = 0; Idx != Actions[Opcode].size(); ++Idx) {
       for (auto &Action : Actions[Opcode][Idx]) {
         LLT Ty = Action.first;
         if (!Ty.isVector())
LegalizerInfo::getAction(const InstrAspect &Aspect) const {
 
   unsigned Opcode = Aspect.Opcode;
   LLT Ty = Aspect.Type;
   if (!Ty.isVector()) {
     auto DefaultAction = DefaultActions.find(Aspect.Opcode);
     if (DefaultAction != DefaultActions.end() && DefaultAction->second == Legal)
       return std::make_pair(Legal, Ty);
 
+    if (DefaultAction != DefaultActions.end() && DefaultAction->second == Lower)
+      return std::make_pair(Lower, Ty);
+
     if (DefaultAction == DefaultActions.end() ||
         DefaultAction->second != NarrowScalar)
       return std::make_pair(Unsupported, LLT());
     return findLegalAction(Aspect, NarrowScalar);
   }
 
   LLT EltTy = Ty.getElementType();
   int NumElts = Ty.getNumElements();

test/CodeGen/AArch64/GlobalISel/legalize-fneg.mir

+# RUN: llc -O0 -run-pass=legalizer -global-isel %s -o - | FileCheck %s
+
+--- |
+  target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
+  target triple = "aarch64--"
+  define void @test_fneg_f32() {
+  entry:
+    ret void
+  }
+  define void @test_fneg_f64() {
+  entry:
+    ret void
+  }
+...
+---
+name:            test_fneg_f32
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+body:             |
+  bb.1:
+    liveins: %s0
+    ; CHECK-LABEL: name: test_fneg_f32
+    ; CHECK: [[VAR:%[0-9]+]](s32) = COPY %s0
+    ; CHECK: [[ZERO:%[0-9]+]](s32) = G_FCONSTANT float -0.000000e+00
+    ; CHECK: [[RES:%[0-9]+]](s32) = G_FSUB [[ZERO]], [[VAR]]
+    ; CHECK: %s0 = COPY [[RES]](s32)
+    %0(s32) = COPY %s0
+    %1(s32) = G_FNEG %0
+    %s0 = COPY %1(s32)
+...
+---
+name:            test_fneg_f64
+registers:
+  - { id: 0, class: _ }
+  - { id: 1, class: _ }
+body:             |
+  bb.1:
+    liveins: %d0
+    ; CHECK-LABEL: name: test_fneg_f64
+    ; CHECK: [[VAR:%[0-9]+]](s64) = COPY %d0
+    ; CHECK: [[ZERO:%[0-9]+]](s64) = G_FCONSTANT double -0.000000e+00
+    ; CHECK: [[RES:%[0-9]+]](s64) = G_FSUB [[ZERO]], [[VAR]]
+    ; CHECK: %d0 = COPY [[RES]](s64)
+    %0(s64) = COPY %d0
+    %1(s64) = G_FNEG %0
+    %d0 = COPY %1(s64)
+...