[MachineLICM] Add shouldHoist method to TargetInstrInfo

[critson]

Summary

Add a shouldHoist method to TargetInstrInfo which is queried by
MachineLICM to override hoisting decisions for a given target.
This mirrors functionality provided by shouldSink.

[foad]

llvm/include/llvm/CodeGen/TargetInstrInfo.h

385

Since this hook only allows targets to prevent things from being hoisted, I wonder if "Return false if the instruction should not be hoisted by MachineLICM" would describe it better?

Similarly "override the default behavior" fails to mention that it can only be overridden in one direction.

[critson]

• Address reviewer comments.

[foad]

Seems fine to me.

llvm/include/llvm/CodeGen/TargetInstrInfo.h

public:
 
   /// 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.
   virtual bool shouldSink(const MachineInstr &MI) const { return true; }
 
+  /// Return false if the instruction should not be hoisted by MachineLICM.
+  ///
+  /// MachineLICM determines on its own whether the instruction is safe to
+  /// hoist; this gives the target a hook to extend this assessment and prevent
+  /// an instruction being hoisted from a given loop for target specific
+  /// reasons.
+  virtual bool shouldHoist(const MachineInstr &MI,
+                           const MachineLoop *FromLoop) const {
+    return true;
+  }
+
   /// 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
   /// DestReg:SubIdx. Any existing subreg index is preserved or composed with
   /// SubIdx.
   virtual void reMaterialize(MachineBasicBlock &MBB,
                              MachineBasicBlock::iterator MI, Register DestReg,
                              unsigned SubIdx, const MachineInstr &Orig,

llvm/lib/CodeGen/MachineLICM.cpp

bool MachineLICMBase::IsLICMCandidate(MachineInstr &I) {
 
   // Convergent attribute has been used on operations that involve inter-thread
   // communication which results are implicitly affected by the enclosing
   // control flows. It is not safe to hoist or sink such operations across
   // control flow.
   if (I.isConvergent())
     return false;
 
+  if (!TII->shouldHoist(I, CurLoop))
+    return false;
+
   return true;
 }
 
 /// Returns true if the instruction is loop invariant.
 bool MachineLICMBase::IsLoopInvariantInst(MachineInstr &I) {
   if (!IsLICMCandidate(I)) {
     LLVM_DEBUG(dbgs() << "LICM: Instruction not a LICM candidate\n");
     return false;