[X86][Disassembler] Shrink X86GenDisassemblerTables.inc from 36M to 6.1M

In x86Disassembler{OneByte,TwoByte,...}Codes,
"/* EmptyTable */" is very common. Omitting it saves lots of space.
Also, there is no need to display a table entry in multiple lines.

It is also common that the whole OpcodeDecision is { MODRM_ONEENTRY, 0}.
Make use of zero-initialization.

llvm/utils/TableGen/X86DisassemblerTables.cpp

@@ -667,16 +667,9 @@ void DisassemblerTables::emitModRMDecision(raw_ostream &o1, raw_ostream &o2,
   static uint32_t sEntryNumber = 1;
   ModRMDecisionType dt = getDecisionType(decision);
 
-  if (dt == MODRM_ONEENTRY && decision.instructionIDs[0] == 0)
-  {
-    o2.indent(i2) << "{ /* ModRMDecision */" << "\n";
-    i2++;
-
-    o2.indent(i2) << stringForDecisionType(dt) << "," << "\n";
-    o2.indent(i2) << 0 << " /* EmptyTable */\n";
-
-    i2--;
-    o2.indent(i2) << "}";
+  if (dt == MODRM_ONEENTRY && decision.instructionIDs[0] == 0) {
+    // Empty table.
+    o2 << "{ " << stringForDecisionType(dt) << ", 0 }";
     return;
   }
 
@@ -725,14 +718,8 @@ void DisassemblerTables::emitModRMDecision(raw_ostream &o1, raw_ostream &o2,
     i1--;
   }
 
-  o2.indent(i2) << "{ /* struct ModRMDecision */" << "\n";
-  i2++;
-
-  o2.indent(i2) << stringForDecisionType(dt) << "," << "\n";
-  o2.indent(i2) << EntryNumber << " /* Table" << EntryNumber << " */\n";
-
-  i2--;
-  o2.indent(i2) << "}";
+  o2 << "{ " << stringForDecisionType(dt) << ", " << EntryNumber << " /* Table"
+     << EntryNumber << " */ }";
 
   switch (dt) {
     default:
@@ -764,30 +751,43 @@ void DisassemblerTables::emitModRMDecision(raw_ostream &o1, raw_ostream &o2,
 void DisassemblerTables::emitOpcodeDecision(raw_ostream &o1, raw_ostream &o2,
                                             unsigned &i1, unsigned &i2,
                                             unsigned &ModRMTableNum,
-                                            OpcodeDecision &decision) const {
-  o2.indent(i2) << "{ /* struct OpcodeDecision */" << "\n";
-  i2++;
-  o2.indent(i2) << "{" << "\n";
-  i2++;
-
-  for (unsigned index = 0; index < 256; ++index) {
-    o2.indent(i2);
-
-    o2 << "/* 0x" << format("%02hhx", index) << " */" << "\n";
-
-    emitModRMDecision(o1, o2, i1, i2, ModRMTableNum,
-                      decision.modRMDecisions[index]);
-
-    if (index <  255)
-      o2 << ",";
-
-    o2 << "\n";
+                                            OpcodeDecision &opDecision) const {
+  o2 << "{";
+  ++i2;
+
+  unsigned index;
+  for (index = 0; index < 256; ++index) {
+    auto &decision = opDecision.modRMDecisions[index];
+    ModRMDecisionType dt = getDecisionType(decision);
+    if (!(dt == MODRM_ONEENTRY && decision.instructionIDs[0] == 0))
+      break;
+  }
+  if (index == 256) {
+    // If all 256 entries are MODRM_ONEENTRY, omit output.
+    assert(MODRM_ONEENTRY == 0);
+    --i2;
+    o2 << "},\n";
+  } else {
+    o2 << " /* struct OpcodeDecision */ {\n";
+    ++i2;
+    for (index = 0; index < 256; ++index) {
+      o2.indent(i2);
+
+      o2 << "/* 0x" << format("%02hhx", index) << " */ ";
+
+      emitModRMDecision(o1, o2, i1, i2, ModRMTableNum,
+                        opDecision.modRMDecisions[index]);
+
+      if (index < 255)
+        o2 << ",";
+
+      o2 << "\n";
+    }
+    --i2;
+    o2.indent(i2) << "}\n";
+    --i2;
+    o2.indent(i2) << "},\n";
   }
-
-  i2--;
-  o2.indent(i2) << "}" << "\n";
-  i2--;
-  o2.indent(i2) << "}" << "\n";
 }
 
 void DisassemblerTables::emitContextDecision(raw_ostream &o1, raw_ostream &o2,
@@ -803,14 +803,10 @@ void DisassemblerTables::emitContextDecision(raw_ostream &o1, raw_ostream &o2,
   for (unsigned index = 0; index < IC_max; ++index) {
     o2.indent(i2) << "/* ";
     o2 << stringForContext((InstructionContext)index);
-    o2 << " */";
-    o2 << "\n";
+    o2 << " */ ";
 
     emitOpcodeDecision(o1, o2, i1, i2, ModRMTableNum,
                        decision.opcodeDecisions[index]);
-
-    if (index + 1 < IC_max)
-      o2 << ", ";
   }
 
   i2--;