[GlobalISel] Add default action for G_FNEG

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.

Reviewers: qcolombet, ab, kristof.beyls, t.p.northover, aditya_nandakumar, dsanders

Reviewed By: qcolombet

Subscribers: dberris, rovka, llvm-commits

Differential Revision: https://reviews.llvm.org/D30721

llvm-svn: 297301

llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp

@@ -539,6 +539,38 @@ LegalizerHelper::lower(MachineInstr &MI, unsigned TypeIdx, LLT Ty) {
     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;
+  }
   }
 }
 

llvm/lib/CodeGen/GlobalISel/LegalizerInfo.cpp

@@ -42,6 +42,7 @@ LegalizerInfo::LegalizerInfo() : TablesInitialized(false) {
 
   DefaultActions[TargetOpcode::G_BRCOND] = WidenScalar;
   DefaultActions[TargetOpcode::G_INSERT] = NarrowScalar;
+  DefaultActions[TargetOpcode::G_FNEG] = Lower;
 }
 
 void LegalizerInfo::computeTables() {
@@ -96,6 +97,9 @@ LegalizerInfo::getAction(const InstrAspect &Aspect) const {
     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());

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

@@ -0,0 +1,48 @@
+# 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)
+...