MachineSink: make shouldSink a TII target hook

Some targets may disagree on what they want sunk or not sunk,
so make this a target hook instead of hardcoded.

llvm-svn: 264799

llvm/include/llvm/Target/TargetInstrInfo.h

@@ -256,6 +256,18 @@ public:
     return MI->isAsCheapAsAMove();
   }
 
+  /// Return true if the instruction should be sunk by MachineSink.
+  ///
+  /// MachineSink determines on its own whether the instruction is safe to sink;
+  /// this gives the target a hook to override the default behavior with regards
+  /// to which instructions should be sunk.
+  /// The default behavior is to not sink insert_subreg, subreg_to_reg, and
+  /// reg_sequence. These are meant to be close to the source to make it easier
+  /// to coalesce.
+  virtual bool shouldSink(const MachineInstr &MI) const {
+    return !MI.isInsertSubreg() && !MI.isSubregToReg() && !MI.isRegSequence();
+  }
+
   /// Re-issue the specified 'original' instruction at the
   /// specific location targeting a new destination register.
   /// The register in Orig->getOperand(0).getReg() will be substituted by

llvm/lib/CodeGen/MachineSink.cpp

@@ -472,10 +472,6 @@ bool MachineSinking::PostponeSplitCriticalEdge(MachineInstr *MI,
   return true;
 }
 
-static bool AvoidsSinking(MachineInstr *MI, MachineRegisterInfo *MRI) {
-  return MI->isInsertSubreg() || MI->isSubregToReg() || MI->isRegSequence();
-}
-
 /// collectDebgValues - Scan instructions following MI and collect any
 /// matching DBG_VALUEs.
 static void collectDebugValues(MachineInstr *MI,
@@ -724,9 +720,8 @@ static bool SinkingPreventsImplicitNullCheck(MachineInstr *MI,
 /// instruction out of its current block into a successor.
 bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore,
                                      AllSuccsCache &AllSuccessors) {
-  // Don't sink insert_subreg, subreg_to_reg, reg_sequence. These are meant to
-  // be close to the source to make it easier to coalesce.
-  if (AvoidsSinking(MI, MRI))
+  // Don't sink instructions that the target prefers not to sink.
+  if (!TII->shouldSink(*MI))
     return false;
 
   // Check if it's safe to move the instruction.