[llvm] [GISel][TableGen] Generate getRegBankFromRegClass (PR #99896)

[Matt Arsenault via llvm-commits]

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@@ -287,8 +291,88 @@ void RegisterBankEmitter::emitBaseClassImplementation(
      << "  for (auto RB : enumerate(RegBanks))\n"
      << "    assert(RB.index() == RB.value()->getID() && \"Index != ID\");\n"
      << "#endif // NDEBUG\n"
-     << "}\n"
-     << "} // end namespace llvm\n";
+     << "}\n";
+
+  uint32_t NoRegBanks = Banks.size();
+  uint32_t BitSize = NextPowerOf2(Log2_32(NoRegBanks));
+  uint32_t ElemsPerWord = 32 / BitSize;
+  uint32_t BitMask = (1 << BitSize) - 1;
+  bool HasAmbigousOrMissingEntry = false;
+  struct Entry {
+    std::string RCIdName;
+    std::string RBIdName;
+  };
+  std::vector<Entry> Entries;
+  for (const auto &Bank : Banks)
+    for (const auto *RC : Bank.register_classes()) {
+      if (RC->EnumValue >= Entries.size())
+        Entries.resize(RC->EnumValue + 1);
+      Entry &E = Entries[RC->EnumValue];
+      E.RCIdName = RC->getIdName();
+      if (!E.RBIdName.empty()) {
+        HasAmbigousOrMissingEntry = true;
+        E.RBIdName = "InvalidRegBankID";
+      } else {
+        E.RBIdName = (TargetName + "::" + Bank.getEnumeratorName()).str();
+      }
+    }
+  for (auto &E : Entries) {
+    if (E.RBIdName.empty()) {
+      HasAmbigousOrMissingEntry = true;
+      E.RBIdName = "InvalidRegBankID";
+    }
+  }
+  OS << "const RegisterBank &\n"
+     << TargetName << "GenRegisterBankInfo::getRegBankFromRegClass"
+     << "(const TargetRegisterClass &RC, LLT) const {\n";
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arsenm wrote:

Don't need the <<, can just rely on the string concat 

https://github.com/llvm/llvm-project/pull/99896