Fix uninitialized read in ARM's PrintAsmOperand

Summary:
Fix read of uninitialized RC variable in ARM's PrintAsmOperand when
hasRegClassConstraint returns false. This was causing
inline-asm-operand-implicit-cast test to fail in r338206.

Reviewers: t.p.northover, weimingz, javed.absar, chill

Reviewed By: chill

Subscribers: chill, eraman, kristof.beyls, chrib, llvm-commits

Differential Revision: https://reviews.llvm.org/D49984

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@338268 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/ARM/ARMAsmPrinter.cpp

@@ -367,8 +367,9 @@ bool ARMAsmPrinter::PrintAsmOperand(const MachineInstr *MI, unsigned OpNum,
 
       unsigned NumVals = InlineAsm::getNumOperandRegisters(Flags);
       unsigned RC;
-      InlineAsm::hasRegClassConstraint(Flags, RC);
-      if (RC == ARM::GPRPairRegClassID) {
+      const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
+      if (InlineAsm::hasRegClassConstraint(Flags, RC) &&
+          ARM::GPRPairRegClass.hasSubClassEq(TRI->getRegClass(RC))) {
         if (NumVals != 1)
           return true;
         const MachineOperand &MO = MI->getOperand(OpNum);

test/CodeGen/ARM/inlineasm-64bit.ll

@@ -104,3 +104,11 @@ define i64 @tied_64bit_lookback_test(i64 %in) nounwind {
   %res = extractvalue {i64, i32, i64} %vars, 2
   ret i64 %res
 }
+
+; Check access to low and high part with a specific register pair constraint
+define i64 @low_high_specific_reg_pair(i64 %in) nounwind {
+; CHECK-LABEL: low_high_specific_reg_pair
+; CHECK: mov r3, r2
+  %res = call i64 asm "mov ${0:R}, ${1:Q}", "=&{r2},0"(i64 %in)
+  ret i64 %res
+}