diff --git a/include/swift/SIL/SILBasicBlock.h b/include/swift/SIL/SILBasicBlock.h
index de4332fc4cf..d479c74100d 100644
--- a/include/swift/SIL/SILBasicBlock.h
+++ b/include/swift/SIL/SILBasicBlock.h
@@ -135,7 +135,6 @@ public:
   const SILArgument *getBBArg(unsigned i) const { return BBArgList[i]; }
   SILArgument *getBBArg(unsigned i) { return BBArgList[i]; }
   SILArgument *replaceBBArg(unsigned i, SILType Ty, ValueDecl *D =nullptr);
-  size_t getBBArgIndex(SILArgument *Arg);
 
   /// \brief Remove all block arguments.
   void dropAllArgs() { BBArgList.clear(); }
diff --git a/include/swift/SILPasses/Utils/SCCVisitor.h b/include/swift/SILPasses/Utils/SCCVisitor.h
index 87f5d7887cc..c71a3d2dc3a 100644
--- a/include/swift/SILPasses/Utils/SCCVisitor.h
+++ b/include/swift/SILPasses/Utils/SCCVisitor.h
@@ -134,7 +134,7 @@ private:
 
     if (auto *A = dyn_cast<SILArgument>(User)) {
       auto *BB = A->getParent();
-      auto Index = BB->getBBArgIndex(A);
+      auto Index = A->getIndex();
 
       for (auto *Pred : BB->getPreds()) {
         auto *V = getArgForTerminator(Pred->getTerminator(), BB, Index);
diff --git a/lib/SIL/SILBasicBlock.cpp b/lib/SIL/SILBasicBlock.cpp
index 4a50e38d0e2..fdd329593e4 100644
--- a/lib/SIL/SILBasicBlock.cpp
+++ b/lib/SIL/SILBasicBlock.cpp
@@ -89,15 +89,6 @@ SILArgument *SILBasicBlock::replaceBBArg(unsigned i, SILType Ty, ValueDecl *D) {
   return NewArg;
 }
 
-size_t SILBasicBlock::getBBArgIndex(SILArgument *Target) {
-  auto *Block = Target->getParent();
-  for (size_t i = 0, e = Block->getNumBBArg(); i != e; ++i)
-    if (Block->getBBArg(i) == Target)
-      return i;
-
-  llvm_unreachable("Expected to find argument in block's argument list!");
-}
-
 SILArgument *SILBasicBlock::createArgument(SILType Ty) {
   return new (getModule()) SILArgument(Ty, this);
 }
diff --git a/lib/SILPasses/ArrayBoundsCheckOpts.cpp b/lib/SILPasses/ArrayBoundsCheckOpts.cpp
index d5a6eb6433d..74c027d6cb5 100644
--- a/lib/SILPasses/ArrayBoundsCheckOpts.cpp
+++ b/lib/SILPasses/ArrayBoundsCheckOpts.cpp
@@ -860,7 +860,7 @@ private:
     auto *PreheaderTerm = dyn_cast<BranchInst>(Preheader->getTerminator());
     if (!PreheaderTerm)
       return nullptr;
-    auto Start = PreheaderTerm->getArg(Header->getBBArgIndex(HeaderVal));
+    auto Start = PreheaderTerm->getArg(HeaderVal->getIndex());
 
     // Find the exit condition.
     auto CondBr = dyn_cast<CondBranchInst>(ExitingBlk->getTerminator());
diff --git a/lib/SILPasses/DeadCodeElimination.cpp b/lib/SILPasses/DeadCodeElimination.cpp
index 3c79c0f7b1e..d641bb77dcb 100644
--- a/lib/SILPasses/DeadCodeElimination.cpp
+++ b/lib/SILPasses/DeadCodeElimination.cpp
@@ -236,7 +236,7 @@ void DCE::propagateLiveBlockArgument(SILArgument *Arg) {
     return;
 
   auto *Block = Arg->getParent();
-  auto ArgIndex = Block->getBBArgIndex(Arg);
+  auto ArgIndex = Arg->getIndex();
 
   for (auto Pred : Block->getPreds())
     markTerminatorArgsLive(Pred, Block, ArgIndex);
diff --git a/lib/SILPasses/SimplifyCFG.cpp b/lib/SILPasses/SimplifyCFG.cpp
index 2b88a4fdc13..aa75d94b9e9 100644
--- a/lib/SILPasses/SimplifyCFG.cpp
+++ b/lib/SILPasses/SimplifyCFG.cpp
@@ -534,7 +534,7 @@ static bool isThreadableSwitchEnumInst(SwitchEnumInst *SEI,
   if (containsAllocStack(SEIBB))
       return false;
 
-  auto Idx = SEIBB->getBBArgIndex(Arg);
+  auto Idx = Arg->getIndex();
   auto IncomingBr0 = dyn_cast<BranchInst>(((*PIt))->getTerminator());
   ++PIt;
   auto IncomingBr1 = dyn_cast<BranchInst>((*PIt)->getTerminator());
diff --git a/lib/SILPasses/Utils/SILSSAUpdater.cpp b/lib/SILPasses/Utils/SILSSAUpdater.cpp
index 7c4636f8747..94e0b2a6273 100644
--- a/lib/SILPasses/Utils/SILSSAUpdater.cpp
+++ b/lib/SILPasses/Utils/SILSSAUpdater.cpp
@@ -129,7 +129,7 @@ static bool
 isEquivalentPHI(SILArgument *PHI,
                 llvm::SmallDenseMap<SILBasicBlock *, SILValue, 8> &ValueMap) {
   SILBasicBlock *PhiBB = PHI->getParent();
-  size_t Idx = PhiBB->getBBArgIndex(PHI);
+  size_t Idx = PHI->getIndex();
   for (auto *PredBB : PhiBB->getPreds()) {
     auto DesiredVal = ValueMap[PredBB];
     OperandValueArrayRef EdgeValues =
@@ -236,11 +236,11 @@ public:
     explicit PHI_iterator(SILArgument *P) // begin iterator
         : PredIt(P->getParent()->pred_begin()),
           BB(P->getParent()),
-          Idx(P->getParent()->getBBArgIndex(P)) {}
+          Idx(P->getIndex()) {}
     PHI_iterator(SILArgument *P, bool) // end iterator
         : PredIt(P->getParent()->pred_end()),
           BB(P->getParent()),
-          Idx(P->getParent()->getBBArgIndex(P)) {}
+          Idx(P->getIndex()) {}
 
     PHI_iterator &operator++() { ++PredIt; return *this; }
     bool operator==(const PHI_iterator& x) const { return PredIt == x.PredIt; }
@@ -298,7 +298,7 @@ public:
   static void AddPHIOperand(SILArgument *PHI, SILValue Val,
                             SILBasicBlock *Pred) {
     auto *PHIBB = PHI->getParent();
-    size_t PhiIdx = PHIBB->getBBArgIndex(PHI);
+    size_t PhiIdx = PHI->getIndex();
     auto *TI = Pred->getTerminator();
     changeEdgeValue(TI, PHIBB, PhiIdx, Val);
   }
@@ -322,7 +322,7 @@ public:
     SILArgument *PHI = ValueIsPHI(Val, Updater);
     if (PHI) {
       auto *PhiBB = PHI->getParent();
-      size_t PhiIdx = PhiBB->getBBArgIndex(PHI);
+      size_t PhiIdx = PHI->getIndex();
 
       // If all predecessor edges are 'not set' this is a new phi.
       for (auto *PredBB : PhiBB->getPreds()) {