[MachineCombine] add a hook for resource length limit

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -1093,6 +1093,9 @@ public:
                       SmallVectorImpl<MachineInstr *> &DelInstrs,
                       DenseMap<unsigned, unsigned> &InstrIdxForVirtReg) const;
 
+  /// The limit on resource length extension we accept in MachineCombiner Pass.
+  virtual int getExtendResourceLenLimit() const { return 0; }
+
   /// This is an architecture-specific helper function of reassociateOps.
   /// Set special operand attributes for new instructions after reassociation.
   virtual void setSpecialOperandAttr(MachineInstr &OldMI1, MachineInstr &OldMI2,

llvm/lib/CodeGen/MachineCombiner.cpp

@@ -406,12 +406,14 @@ bool MachineCombiner::preservesResourceLen(
                     << ResLenBeforeCombine
                     << " and after: " << ResLenAfterCombine << "\n";);
   LLVM_DEBUG(
-      ResLenAfterCombine <= ResLenBeforeCombine
+      ResLenAfterCombine <=
+      ResLenBeforeCombine + TII->getExtendResourceLenLimit()
           ? dbgs() << "\t\t  As result it IMPROVES/PRESERVES Resource Length\n"
           : dbgs() << "\t\t  As result it DOES NOT improve/preserve Resource "
                       "Length\n");
 
-  return ResLenAfterCombine <= ResLenBeforeCombine;
+  return ResLenAfterCombine <=
+         ResLenBeforeCombine + TII->getExtendResourceLenLimit();
 }
 
 /// \returns true when new instruction sequence should be generated