[ARM] Fix data race on RegisterBank initialization.

[huihuiz]

Summary

The initialization of RegisterBank needs to be done only once. The
logic of AlreadyInit has data race, use llvm::call_once instead.

This is continuing work of D73587.

llvm/lib/Target/ARM/ARMRegisterBankInfo.cpp

assert(checkValueMapping(ValueMappings[DPR3OpsIdx + 2],
          "Wrong value mapping for 3 DPR ops instruction");
 }
 #endif
 } // end namespace arm
 } // end namespace llvm
 
 ARMRegisterBankInfo::ARMRegisterBankInfo(const TargetRegisterInfo &TRI)
     : ARMGenRegisterBankInfo() {
-  static bool AlreadyInit = false;
   // We have only one set of register banks, whatever the subtarget
   // is. Therefore, the initialization of the RegBanks table should be
   // done only once. Indeed the table of all register banks
   // (ARM::RegBanks) is unique in the compiler. At some point, it
   // will get tablegen'ed and the whole constructor becomes empty.
-  if (AlreadyInit)
-    return;
-  AlreadyInit = true;
+  static llvm::once_flag InitializeRegisterBankFlag;
 
+  static auto InitializeRegisterBankOnce = [this](const auto &TRI) {
     const RegisterBank &RBGPR = getRegBank(ARM::GPRRegBankID);
     (void)RBGPR;
     assert(&ARM::GPRRegBank == &RBGPR && "The order in RegBanks is messed up");
 
     // Initialize the GPR bank.
     assert(RBGPR.covers(*TRI.getRegClass(ARM::GPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::GPRwithAPSRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::GPRnopcRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::rGPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::tGPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::tcGPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::tGPR_and_tcGPRRegClassID)) &&
            "Subclass not added?");
-  assert(RBGPR.covers(*TRI.getRegClass(ARM::tGPREven_and_tGPR_and_tcGPRRegClassID)) &&
+    assert(RBGPR.covers(
+               *TRI.getRegClass(ARM::tGPREven_and_tGPR_and_tcGPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.covers(*TRI.getRegClass(ARM::tGPROdd_and_tcGPRRegClassID)) &&
            "Subclass not added?");
     assert(RBGPR.getSize() == 32 && "GPRs should hold up to 32-bit");
 
 #ifndef NDEBUG
     ARM::checkPartialMappings();
     ARM::checkValueMappings();
 #endif
+  };
+
+  llvm::call_once(InitializeRegisterBankFlag, InitializeRegisterBankOnce, TRI);
 }
 
 const RegisterBank &
 ARMRegisterBankInfo::getRegBankFromRegClass(const TargetRegisterClass &RC,
                                             LLT) const {
   using namespace ARM;
 
   switch (RC.getID()) {