[TableGen] AsmMatcher: allow repeated input operands.

[simon_tatham]

Summary

If an assembly instruction has to mention an input operand name twice,
for example the MVE VMOV instruction that accesses two lanes of the
same vector by writing 'vmov r1, r2, q0[3], q0[1]', then the obvious
way to write its AsmString is to include the same operand (here $Qd)
twice. But this causes the AsmMatcher generator to omit that
instruction completely from the match table, on the basis that the
generator isn't clever enough to deal with the duplication.

But you need to have _some_ way of dealing with an instruction like
this - and in this case, where the mnemonic is shared with many other
instructions that the AsmMatcher does handle, it would be very painful
to take it out of the AsmMatcher system completely.

A nicer way is to add a custom AsmMatchConverter routine, and let that
deal with the problem if the autogenerated converter can't. But that
doesn't work, because TableGen leaves the instruction out of its table
_even_ if you provide a custom converter.

Solution: this change, which makes TableGen relax the restriction on
duplicated operands in the case where there's a custom converter.

[t.p.northover]

Looks reasonable.

llvm/trunk/utils/TableGen/AsmMatcherEmitter.cpp

if (!CommentDelimiter.empty() &&
     PrintFatalError(TheDef->getLoc(),
                   "asmstring for instruction has comment character in it, "
                   "mark it isCodeGenOnly");
 
   // Reject matchables with operand modifiers, these aren't something we can
   // handle, the target should be refactored to use operands instead of
   // modifiers.
   //
-  // Also, check for instructions which reference the operand multiple times;
-  // this implies a constraint we would not honor.
+  // Also, check for instructions which reference the operand multiple times,
+  // if they don't define a custom AsmMatcher: this implies a constraint that
+  // the built-in matching code would not honor.
   std::set<std::string> OperandNames;
   for (const AsmOperand &Op : AsmOperands) {
     StringRef Tok = Op.Token;
     if (Tok[0] == '$' && Tok.find(':') != StringRef::npos)
       PrintFatalError(TheDef->getLoc(),
                       "matchable with operand modifier '" + Tok +
                       "' not supported by asm matcher.  Mark isCodeGenOnly!");
     // Verify that any operand is only mentioned once.
     // We reject aliases and ignore instructions for now.
-    if (!IsAlias && Tok[0] == '$' && !OperandNames.insert(Tok).second) {
+    if (!IsAlias && TheDef->getValueAsString("AsmMatchConverter").empty() &&
+        Tok[0] == '$' && !OperandNames.insert(Tok).second) {
       LLVM_DEBUG({
         errs() << "warning: '" << TheDef->getName() << "': "
                << "ignoring instruction with tied operand '"
                << Tok << "'\n";
       });
       return false;
     }
   }