SIL: Use TinyPtrVector for the argument list in SILBasicBlock

A TinyPtrVector is the right choice, because ~98% of blocks have 0 or 1 arguments.

Author: eeckstein

include/swift/SIL/SILBasicBlock.h

@@ -22,6 +22,7 @@
 #include "swift/SIL/SILArgumentArrayRef.h"
 #include "swift/SIL/SILInstruction.h"
 #include "swift/SIL/SILArgument.h"
+#include "llvm/ADT/TinyPtrVector.h"
 
 namespace swift {
 
@@ -49,7 +50,9 @@ public llvm::ilist_node<SILBasicBlock>, public SILAllocated<SILBasicBlock> {
   SILSuccessor *PredList;
 
   /// This is the list of basic block arguments for this block.
-  std::vector<SILArgument *> ArgumentList;
+  /// A TinyPtrVector is the right choice, because ~98% of blocks have 0 or 1
+  /// arguments.
+  TinyPtrVector<SILArgument *> ArgumentList;
 
   /// The ordered set of instructions in the SILBasicBlock.
   InstListType InstList;
@@ -178,8 +181,8 @@ public llvm::ilist_node<SILBasicBlock>, public SILAllocated<SILBasicBlock> {
   // SILBasicBlock Argument List Inspection and Manipulation
   //===--------------------------------------------------------------------===//
 
-  using arg_iterator = std::vector<SILArgument *>::iterator;
-  using const_arg_iterator = std::vector<SILArgument *>::const_iterator;
+  using arg_iterator = TinyPtrVector<SILArgument *>::iterator;
+  using const_arg_iterator = TinyPtrVector<SILArgument *>::const_iterator;
 
   bool args_empty() const { return ArgumentList.empty(); }
   size_t args_size() const { return ArgumentList.size(); }

lib/SIL/IR/SILBasicBlock.cpp

@@ -193,7 +193,7 @@ SILFunctionArgument *SILBasicBlock::replaceFunctionArgument(
 
   // TODO: When we switch to malloc/free allocation we'll be leaking memory
   // here.
-  ArgumentList[i] = NewArg;
+  *(ArgumentList.begin() + i) = NewArg;
 
   return NewArg;
 }
@@ -219,7 +219,7 @@ SILPhiArgument *SILBasicBlock::replacePhiArgument(unsigned i, SILType Ty,
 
   // TODO: When we switch to malloc/free allocation we'll be leaking memory
   // here.
-  ArgumentList[i] = NewArg;
+  *(ArgumentList.begin() + i) = NewArg;
 
   return NewArg;
 }