[RegisterBankInfo] Avoid heap allocation in most cases.

The OperandsMapper class is used heavy in RegBankSelect and each
instantiation triggered a heap allocation for the array of operands.
Instead, use a SmallVector with a big enough size such that most of the
cases do not have to use dynamically allocated memory.

This improves the compile time of the RegBankSelect pass.

llvm-svn: 281916

include/llvm/CodeGen/GlobalISel/RegisterBankInfo.h

@@ -193,7 +193,8 @@ public:
   class OperandsMapper {
     /// The OpIdx-th cell contains the index in NewVRegs where the VRegs of the
     /// OpIdx-th operand starts. -1 means we do not have such mapping yet.
-    std::unique_ptr<int[]> OpToNewVRegIdx;
+    /// Note: We use a SmallVector to avoid heap allocation for most cases.
+    SmallVector<int, 8> OpToNewVRegIdx;
     /// Hold the registers that will be used to map MI with InstrMapping.
     SmallVector<unsigned, 8> NewVRegs;
     /// Current MachineRegisterInfo, used to create new virtual registers.

lib/CodeGen/GlobalISel/RegisterBankInfo.cpp

@@ -513,7 +513,7 @@ RegisterBankInfo::OperandsMapper::OperandsMapper(
     MachineRegisterInfo &MRI)
     : MRI(MRI), MI(MI), InstrMapping(InstrMapping) {
   unsigned NumOpds = MI.getNumOperands();
-  OpToNewVRegIdx.reset(new int[NumOpds]);
+  OpToNewVRegIdx.resize(NumOpds);
   std::fill(&OpToNewVRegIdx[0], &OpToNewVRegIdx[NumOpds],
             OperandsMapper::DontKnowIdx);
   assert(InstrMapping.verify(MI) && "Invalid mapping for MI");