[TableGen] AsmMatcherEmitter.cpp: replace a sequence of "if" to "switch" in emitValidateOperandClass. NFC.

[vpykhtin]

Summary

Hi,

this NFC change replaces a sequence of generated

if (Kind == MCK_XXX) {

if (Operand.isXXX())
   return MCTargetAsmParser::Match_Success;

}

to

switch (Kind) {
default: break;
...
case MCK_xxx: ...

if (Operand.isXXX())
   return MCTargetAsmParser::Match_Success;

break;
...
}

Potentially can generate faster Kind check and significantly eases step debugging.

Example of output (from AMDGPU backend):

switch(Kind) {
default: break;
// 'Clamp' class
case MCK_Clamp:
  if (Operand.isClamp())
    return MCTargetAsmParser::Match_Success;
  break;
// 'DMask' class
case MCK_DMask:
  if (Operand.isDMask())
    return MCTargetAsmParser::Match_Success;
  break;

...

} // end switch(Kind)

[ab]

Nicer indeed! One question and one nit.

utils/TableGen/AsmMatcherEmitter.cpp

2156

space before '('

2157

Do we ever hit the default? Perhaps you could put an llvm_unreachable().

[vpykhtin]

In D18394#384788, @ab wrote:

Nicer indeed! One question and one nit.

Thanks for looking into this, default is a very common case - it handles registers below the switch. Will fix space on submit.

[ab]

SGTM

utils/TableGen/AsmMatcherEmitter.cpp

static void emitValidateOperandClass(AsmMatcherInfo &Info,
   // FIXME: Use a more specific diagnostic type.
   OS << "  if (Operand.isToken())\n";
   OS << "    return isSubclass(matchTokenString(Operand.getToken()), Kind) ?\n"
      << "             MCTargetAsmParser::Match_Success :\n"
      << "             MCTargetAsmParser::Match_InvalidOperand;\n\n";
 
   // Check the user classes. We don't care what order since we're only
   // actually matching against one of them.
+  OS << "  switch(Kind) {\n"
+        "  default: break;\n";
   for (const auto &CI : Info.Classes) {
     if (!CI.isUserClass())
       continue;
 
     OS << "  // '" << CI.ClassName << "' class\n";
-    OS << "  if (Kind == " << CI.Name << ") {\n";
+    OS << "  case " << CI.Name << ":\n";
     OS << "    if (Operand." << CI.PredicateMethod << "())\n";
     OS << "      return MCTargetAsmParser::Match_Success;\n";
     if (!CI.DiagnosticType.empty())
       OS << "    return " << Info.Target.getName() << "AsmParser::Match_"
          << CI.DiagnosticType << ";\n";
-    OS << "  }\n\n";
+    else
+      OS << "    break;\n";
   }
+  OS << "  } // end switch(Kind)\n\n";
 
   // Check for register operands, including sub-classes.
   OS << "  if (Operand.isReg()) {\n";
   OS << "    MatchClassKind OpKind;\n";
   OS << "    switch (Operand.getReg()) {\n";
   OS << "    default: OpKind = InvalidMatchClass; break;\n";
   for (const auto &RC : Info.RegisterClasses)
     OS << "    case " << Info.Target.getName() << "::"