[AArch64][GlobalISel] Make G_UBFX/G_SBFX legalization check for constants

[paquette]

Summary

The original rule just checked the type, but this is actually only legal if it has a constant.

Since the legalizer isn't supposed to be conditional, we have to check this.

(I'll probably have to make the bitfield extract combines I've been putting up AArch64-specific for now and come up with a way to check for custom legalization rules in the combiner.)

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.h

private:
   bool legalizeShlAshrLshr(MachineInstr &MI, MachineRegisterInfo &MRI,
                            MachineIRBuilder &MIRBuilder,
                            GISelChangeObserver &Observer) const;
 
   bool legalizeSmallCMGlobalValue(MachineInstr &MI, MachineRegisterInfo &MRI,
                                   MachineIRBuilder &MIRBuilder,
                                   GISelChangeObserver &Observer) const;
   bool legalizeVectorTrunc(MachineInstr &MI, LegalizerHelper &Helper) const;
+  bool legalizeBitfieldExtract(MachineInstr &MI, MachineRegisterInfo &MRI,
+                               LegalizerHelper &Helper) const;
   const AArch64Subtarget *ST;
 };
 } // End llvm namespace.
 #endif

llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp

getActionDefinitionsBuilder(G_VECREDUCE_ADD)
           {{s8, v16s8}, {s16, v8s16}, {s32, v4s32}, {s32, v2s32}, {s64, v2s64}})
       .lower();
 
   getActionDefinitionsBuilder({G_UADDSAT, G_USUBSAT})
       .lowerIf([=](const LegalityQuery &Q) { return Q.Types[0].isScalar(); });
 
   getActionDefinitionsBuilder({G_FSHL, G_FSHR}).lower();
 
-  getActionDefinitionsBuilder({G_SBFX, G_UBFX}).legalFor({s32, s64});
+  getActionDefinitionsBuilder({G_SBFX, G_UBFX}).customFor({s32, s64});
+
   computeTables();
   verify(*ST.getInstrInfo());
 }
 
 bool AArch64LegalizerInfo::legalizeCustom(LegalizerHelper &Helper,
                                           MachineInstr &MI) const {
   MachineIRBuilder &MIRBuilder = Helper.MIRBuilder;
   MachineRegisterInfo &MRI = *MIRBuilder.getMRI();
bool AArch64LegalizerInfo::legalizeCustom(LegalizerHelper &Helper,
   case TargetOpcode::G_SHL:
   case TargetOpcode::G_ASHR:
   case TargetOpcode::G_LSHR:
     return legalizeShlAshrLshr(MI, MRI, MIRBuilder, Observer);
   case TargetOpcode::G_GLOBAL_VALUE:
     return legalizeSmallCMGlobalValue(MI, MRI, MIRBuilder, Observer);
   case TargetOpcode::G_TRUNC:
     return legalizeVectorTrunc(MI, Helper);
+  case TargetOpcode::G_SBFX:
+  case TargetOpcode::G_UBFX:
+    return legalizeBitfieldExtract(MI, MRI, Helper);
   }
 
   llvm_unreachable("expected switch to return");
 }
 
 static void extractParts(Register Reg, MachineRegisterInfo &MRI,
                          MachineIRBuilder &MIRBuilder, LLT Ty, int NumParts,
                          SmallVectorImpl<Register> &VRegs) {
bool AArch64LegalizerInfo::legalizeVaArg(MachineInstr &MI,
   MIRBuilder.buildStore(NewList, ListPtr,
                         *MF.getMachineMemOperand(MachinePointerInfo(),
                                                  MachineMemOperand::MOStore,
                                                  PtrSize, PtrAlign));
 
   MI.eraseFromParent();
   return true;
 }
+
+bool AArch64LegalizerInfo::legalizeBitfieldExtract(
+    MachineInstr &MI, MachineRegisterInfo &MRI, LegalizerHelper &Helper) const {
+  // Only legal if we can select immediate forms.
+  // TODO: Lower this otherwise.
+  return getConstantVRegValWithLookThrough(MI.getOperand(2).getReg(), MRI) &&
+         getConstantVRegValWithLookThrough(MI.getOperand(3).getReg(), MRI);
+}