[ImplicitNullCheck] NFC isSuitableMemoryOp cleanup

Summary:
isSuitableMemoryOp method is repsonsible for verification
that instruction is a candidate to use in implicit null check.
Additionally it checks that base register is not re-defined before.
In case base has been re-defined it just returns false and lookup
is continued while any suitable instruction will not succeed this check
as well. This results in redundant further operations.

So when we found that base register has been re-defined we just
stop.

Patch by Serguei Katkov!

Reviewers: reames, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29119

llvm-svn: 293736

llvm/lib/CodeGen/ImplicitNullChecks.cpp

@@ -158,11 +158,16 @@ class ImplicitNullChecks : public MachineFunctionPass {
                                    MachineBasicBlock *HandlerMBB);
   void rewriteNullChecks(ArrayRef<NullCheck> NullCheckList);
 
-  /// Is \p MI a memory operation that can be used to implicitly null check the
-  /// value in \p PointerReg?  \p PrevInsts is the set of instruction seen since
+  enum SuitabilityResult { SR_Suitable, SR_Unsuitable, SR_Impossible };
+
+  /// Return SR_Suitable if \p MI a memory operation that can be used to
+  /// implicitly null check the value in \p PointerReg, SR_Unsuitable if
+  /// \p MI cannot be used to null check and SR_Impossible if there is
+  /// no sense to continue lookup due to any other instruction will not be able
+  /// to be used. \p PrevInsts is the set of instruction seen since
   /// the explicit null check on \p PointerReg.
-  bool isSuitableMemoryOp(MachineInstr &MI, unsigned PointerReg,
-                          ArrayRef<MachineInstr *> PrevInsts);
+  SuitabilityResult isSuitableMemoryOp(MachineInstr &MI, unsigned PointerReg,
+                                       ArrayRef<MachineInstr *> PrevInsts);
 
   /// Return true if \p FaultingMI can be hoisted from after the the
   /// instructions in \p InstsSeenSoFar to before them.  Set \p Dependence to a
@@ -283,30 +288,33 @@ static bool AnyAliasLiveIn(const TargetRegisterInfo *TRI,
   return false;
 }
 
-bool ImplicitNullChecks::isSuitableMemoryOp(
-    MachineInstr &MI, unsigned PointerReg, ArrayRef<MachineInstr *> PrevInsts) {
+ImplicitNullChecks::SuitabilityResult
+ImplicitNullChecks::isSuitableMemoryOp(MachineInstr &MI, unsigned PointerReg,
+                                       ArrayRef<MachineInstr *> PrevInsts) {
   int64_t Offset;
   unsigned BaseReg;
 
   if (!TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI) ||
       BaseReg != PointerReg)
-    return false;
+    return SR_Unsuitable;
 
   // We want the load to be issued at a sane offset from PointerReg, so that
   // if PointerReg is null then the load reliably page faults.
   if (!(MI.mayLoad() && !MI.isPredicable() && Offset < PageSize))
-    return false;
+    return SR_Unsuitable;
 
   // Finally, we need to make sure that the load instruction actually is
   // loading from PointerReg, and there isn't some re-definition of PointerReg
   // between the compare and the load.
+  // If PointerReg has been redefined before then there is no sense to continue
+  // lookup due to this condition will fail for any further instruction.
   for (auto *PrevMI : PrevInsts)
     for (auto &PrevMO : PrevMI->operands())
       if (PrevMO.isReg() && PrevMO.getReg() &&
           TRI->regsOverlap(PrevMO.getReg(), PointerReg))
-        return false;
+        return SR_Impossible;
 
-  return true;
+  return SR_Suitable;
 }
 
 bool ImplicitNullChecks::canHoistLoadInst(
@@ -481,9 +489,11 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
       return false;
 
     MachineInstr *Dependence;
-    if (isSuitableMemoryOp(MI, PointerReg, InstsSeenSoFar) &&
-        canHoistLoadInst(&MI, PointerReg, InstsSeenSoFar, NullSucc,
-                         Dependence)) {
+    SuitabilityResult SR = isSuitableMemoryOp(MI, PointerReg, InstsSeenSoFar);
+    if (SR == SR_Impossible)
+      return false;
+    if (SR == SR_Suitable && canHoistLoadInst(&MI, PointerReg, InstsSeenSoFar,
+                                              NullSucc, Dependence)) {
       NullCheckList.emplace_back(&MI, MBP.ConditionDef, &MBB, NotNullSucc,
                                  NullSucc, Dependence);
       return true;