[NFC][TableGen] Code cleanup in Wasm disassember emitter (#135992)

- Use range for loop to iterate over instructions.
- Emit generated code in anonymous namespace instead of `llvm` and
reduce the scope of this to just the type declarations.
- Emit generated tables as static constexpr
- Replace code to search in operand table with `std::search`.
- Skip the last "null" entry in PrefixTable and use range for loop to
search PrefixTable in the .cpp code.
- Do not generate `WebAssemblyInstructionTableSize` definition as its
already defined in the .cpp file.
- Remove {} for single statement loop/if/else bodies.

Author: jurahul

llvm/lib/Target/WebAssembly/Disassembler/WebAssemblyDisassembler.cpp

@@ -38,9 +38,9 @@ using DecodeStatus = MCDisassembler::DecodeStatus;
 
 #include "WebAssemblyGenDisassemblerTables.inc"
 
-namespace {
 static constexpr int WebAssemblyInstructionTableSize = 256;
 
+namespace {
 class WebAssemblyDisassembler final : public MCDisassembler {
   std::unique_ptr<const MCInstrInfo> MCII;
 
@@ -171,10 +171,10 @@ MCDisassembler::DecodeStatus WebAssemblyDisassembler::getInstruction(
   // If this is a prefix byte, indirect to another table.
   if (WasmInst->ET == ET_Prefix) {
     WasmInst = nullptr;
-    // Linear search, so far only 2 entries.
-    for (auto PT = PrefixTable; PT->Table; PT++) {
-      if (PT->Prefix == Opc) {
-        WasmInst = PT->Table;
+    // Linear search, so far only 4 entries.
+    for (const auto &[Prefix, Table] : PrefixTable) {
+      if (Prefix == Opc) {
+        WasmInst = Table;
         break;
       }
     }

llvm/utils/TableGen/WebAssemblyDisassemblerEmitter.cpp

@@ -1,4 +1,4 @@
-//===- WebAssemblyDisassemblerEmitter.cpp - Disassembler tables -*- C++ -*-===//
+//===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
@@ -16,10 +16,11 @@
 #include "WebAssemblyDisassemblerEmitter.h"
 #include "Common/CodeGenInstruction.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/iterator.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/TableGen/Record.h"
 
-static constexpr int WebAssemblyInstructionTableSize = 256;
+constexpr int WebAssemblyInstructionTableSize = 256;
 
 void llvm::emitWebAssemblyDisassemblerTables(
     raw_ostream &OS,
@@ -29,8 +30,8 @@ void llvm::emitWebAssemblyDisassemblerTables(
   std::map<unsigned,
            std::map<unsigned, std::pair<unsigned, const CodeGenInstruction *>>>
       OpcodeTable;
-  for (unsigned I = 0; I != NumberedInstructions.size(); ++I) {
-    const CodeGenInstruction &CGI = *NumberedInstructions[I];
+  for (const auto &[Idx, CGI] :
+       enumerate(make_pointee_range(NumberedInstructions))) {
     const Record &Def = *CGI.TheDef;
     if (!Def.getValue("Inst"))
       continue;
@@ -75,27 +76,31 @@ void llvm::emitWebAssemblyDisassemblerTables(
       }
     }
     // Set this instruction as the one to use.
-    CGIP = {I, &CGI};
+    CGIP = {Idx, &CGI};
   }
-  OS << "#include \"MCTargetDesc/WebAssemblyMCTargetDesc.h\"\n";
-  OS << "\n";
-  OS << "namespace llvm {\n\n";
-  OS << "static constexpr int WebAssemblyInstructionTableSize = ";
-  OS << WebAssemblyInstructionTableSize << ";\n\n";
-  OS << "enum EntryType : uint8_t { ";
-  OS << "ET_Unused, ET_Prefix, ET_Instruction };\n\n";
-  OS << "struct WebAssemblyInstruction {\n";
-  OS << "  uint16_t Opcode;\n";
-  OS << "  EntryType ET;\n";
-  OS << "  uint8_t NumOperands;\n";
-  OS << "  uint16_t OperandStart;\n";
-  OS << "};\n\n";
+
+  OS << R"(
+#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
+
+namespace {
+enum EntryType : uint8_t { ET_Unused, ET_Prefix, ET_Instruction };
+
+struct WebAssemblyInstruction {
+  uint16_t Opcode;
+  EntryType ET;
+  uint8_t NumOperands;
+  uint16_t OperandStart;
+};
+} // end anonymous namespace
+
+)";
+
   std::vector<std::string> OperandTable, CurOperandList;
   // Output one table per prefix.
   for (const auto &[Prefix, Table] : OpcodeTable) {
     if (Table.empty())
       continue;
-    OS << "WebAssemblyInstruction InstructionTable" << Prefix;
+    OS << "constexpr WebAssemblyInstruction InstructionTable" << Prefix;
     OS << "[] = {\n";
     for (unsigned I = 0; I < WebAssemblyInstructionTableSize; I++) {
       auto InstIt = Table.find(I);
@@ -116,53 +121,43 @@ void llvm::emitWebAssemblyDisassemblerTables(
         }
         // See if we already have stored this sequence before. This is not
         // strictly necessary but makes the table really small.
-        size_t OperandStart = OperandTable.size();
-        if (CurOperandList.size() <= OperandTable.size()) {
-          for (size_t J = 0; J <= OperandTable.size() - CurOperandList.size();
-               ++J) {
-            size_t K = 0;
-            for (; K < CurOperandList.size(); ++K) {
-              if (OperandTable[J + K] != CurOperandList[K])
-                break;
-            }
-            if (K == CurOperandList.size()) {
-              OperandStart = J;
-              break;
-            }
-          }
-        }
+        auto SearchI =
+            std::search(OperandTable.begin(), OperandTable.end(),
+                        CurOperandList.begin(), CurOperandList.end());
+        OS << std::distance(OperandTable.begin(), SearchI);
         // Store operands if no prior occurrence.
-        if (OperandStart == OperandTable.size()) {
+        if (SearchI == OperandTable.end())
           llvm::append_range(OperandTable, CurOperandList);
-        }
-        OS << OperandStart;
       } else {
         auto PrefixIt = OpcodeTable.find(I);
         // If we have a non-empty table for it that's not 0, this is a prefix.
-        if (PrefixIt != OpcodeTable.end() && I && !Prefix) {
+        if (PrefixIt != OpcodeTable.end() && I && !Prefix)
           OS << "  { 0, ET_Prefix, 0, 0";
-        } else {
+        else
           OS << "  { 0, ET_Unused, 0, 0";
-        }
       }
       OS << "  },\n";
     }
     OS << "};\n\n";
   }
   // Create a table of all operands:
-  OS << "const uint8_t OperandTable[] = {\n";
-  for (auto &Op : OperandTable) {
+  OS << "constexpr uint8_t OperandTable[] = {\n";
+  for (const auto &Op : OperandTable)
     OS << "  " << Op << ",\n";
-  }
   OS << "};\n\n";
+
   // Create a table of all extension tables:
-  OS << "struct { uint8_t Prefix; const WebAssemblyInstruction *Table; }\n";
-  OS << "PrefixTable[] = {\n";
+  OS << R"(
+constexpr struct {
+  uint8_t Prefix;
+  const WebAssemblyInstruction *Table;
+} PrefixTable[] = {
+)";
+
   for (const auto &[Prefix, Table] : OpcodeTable) {
     if (Table.empty() || !Prefix)
       continue;
     OS << "  { " << Prefix << ", InstructionTable" << Prefix << " },\n";
   }
-  OS << "  { 0, nullptr }\n};\n\n";
-  OS << "} // end namespace llvm\n";
+  OS << "};\n";
 }