[llvm] [NFC][TableGen] Minor code cleanup in RegisterInfoEmitter (PR #181479)

[Rahul Joshi via llvm-commits]

https://github.com/jurahul updated https://github.com/llvm/llvm-project/pull/181479

>From 1aed8ba115afaae2e2c09b1672c7f078b1ce6921 Mon Sep 17 00:00:00 2001
From: Rahul Joshi <rjoshi at nvidia.com>
Date: Sat, 14 Feb 2026 06:10:04 -0800
Subject: [PATCH] [NFC][TableGen] Minor code cleanup in RegisterInfoEmitter

Minor cleanup in codegen for `getSubClassWithSubReg` and
`getSubRegisterClass`:
- Cache `RegisterClasses.size()`. Its a list, so calling
  `RegisterClasses.size()` multiple times can be expensive.
- Use `getMinimalTypeForRange` to determine the entry type for table.
- Extract common code for table lookup into a lambda.
---
 llvm/utils/TableGen/RegisterInfoEmitter.cpp | 52 +++++++++------------
 1 file changed, 23 insertions(+), 29 deletions(-)

diff --git a/llvm/utils/TableGen/RegisterInfoEmitter.cpp b/llvm/utils/TableGen/RegisterInfoEmitter.cpp
index 3df439d681e8a..0e82e538829f1 100644
--- a/llvm/utils/TableGen/RegisterInfoEmitter.cpp
+++ b/llvm/utils/TableGen/RegisterInfoEmitter.cpp
@@ -29,6 +29,7 @@
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Format.h"
+#include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/CodeGenHelpers.h"
 #include "llvm/TableGen/Error.h"
@@ -1534,13 +1535,23 @@ void RegisterInfoEmitter::runTargetDesc(raw_ostream &OS, raw_ostream &MainOS,
        << " const {\n";
     // Use the smallest type that can hold a regclass ID with room for a
     // sentinel.
-    if (RegisterClasses.size() <= UINT8_MAX)
-      OS << "  static const uint8_t Table[";
-    else if (RegisterClasses.size() <= UINT16_MAX)
-      OS << "  static const uint16_t Table[";
-    else
-      PrintFatalError("Too many register classes.");
-    OS << RegisterClasses.size() << "][" << SubRegIndicesSize << "] = {\n";
+    const size_t NumRegClasses = RegisterClasses.size();
+    const char *RegClassTy = getMinimalTypeForRange(NumRegClasses + 1);
+    auto emitTableLookup = [&]() {
+      OS << formatv(R"(
+  };
+  assert(RC && \"Missing regclass\");
+  if (!Idx) return RC;
+  --Idx;
+  assert(Idx < {} && \"Bad subreg\");
+  unsigned TV = Table[RC->getID()][Idx];
+  return TV ? getRegClass(TV - 1) : nullptr;
+})",
+                    SubRegIndicesSize);
+    };
+
+    OS << formatv("  static constexpr {} Table[{}][{}] = {{\n", RegClassTy,
+                  NumRegClasses, SubRegIndicesSize);
     for (const auto &RC : RegisterClasses) {
       OS << "    {\t// " << RC.getName() << "\n";
       for (auto &Idx : SubRegIndices) {
@@ -1552,28 +1563,15 @@ void RegisterInfoEmitter::runTargetDesc(raw_ostream &OS, raw_ostream &MainOS,
       }
       OS << "    },\n";
     }
-    OS << "  };\n  assert(RC && \"Missing regclass\");\n"
-       << "  if (!Idx) return RC;\n  --Idx;\n"
-       << "  assert(Idx < " << SubRegIndicesSize << " && \"Bad subreg\");\n"
-       << "  unsigned TV = Table[RC->getID()][Idx];\n"
-       << "  return TV ? getRegClass(TV - 1) : nullptr;\n}\n\n";
+    emitTableLookup();
 
-    // Emit getSubRegisterClass
+    // Emit getSubRegisterClass.
     OS << "const TargetRegisterClass *" << ClassName
        << "::getSubRegisterClass(const TargetRegisterClass *RC, unsigned Idx)"
        << " const {\n";
 
-    // Use the smallest type that can hold a regclass ID with room for a
-    // sentinel.
-    if (RegisterClasses.size() <= UINT8_MAX)
-      OS << "  static const uint8_t Table[";
-    else if (RegisterClasses.size() <= UINT16_MAX)
-      OS << "  static const uint16_t Table[";
-    else
-      PrintFatalError("Too many register classes.");
-
-    OS << RegisterClasses.size() << "][" << SubRegIndicesSize << "] = {\n";
-
+    OS << formatv("  static constxpr {} Table[{}][{}] = {{\n", RegClassTy,
+                  NumRegClasses, SubRegIndicesSize);
     for (const auto &RC : RegisterClasses) {
       OS << "    {\t// " << RC.getName() << '\n';
       for (auto &Idx : SubRegIndices) {
@@ -1599,11 +1597,7 @@ void RegisterInfoEmitter::runTargetDesc(raw_ostream &OS, raw_ostream &MainOS,
 
       OS << "    },\n";
     }
-    OS << "  };\n  assert(RC && \"Missing regclass\");\n"
-       << "  if (!Idx) return RC;\n  --Idx;\n"
-       << "  assert(Idx < " << SubRegIndicesSize << " && \"Bad subreg\");\n"
-       << "  unsigned TV = Table[RC->getID()][Idx];\n"
-       << "  return TV ? getRegClass(TV - 1) : nullptr;\n}\n\n";
+    emitTableLookup();
   }
 
   EmitRegUnitPressure(OS, ClassName);