[mips] Check the register class before replacing materializations of zero with $zero in microMIPS.

[vkalintiris]

Summary

The microMIPS register class GPRMM16 does not contain the $zero register.
However, MipsSEDAGToDAGISel::replaceUsesWithZeroReg() would replace uses
of the $dst register:

[d]addiu, $dst, $zero, 0

with the $zero register, without checking for membership in the register
class of the target machine operand.

[dsanders]

LGTM

llvm/trunk/lib/Target/Mips/MipsSEISelDAGToDAG.cpp

for (MachineRegisterInfo::use_iterator U = MRI->use_begin(DstReg),
     unsigned OpNo = U.getOperandNo();
     MachineInstr *MI = MO.getParent();
     ++U;
 
     // Do not replace if it is a phi's operand or is tied to def operand.
     if (MI->isPHI() || MI->isRegTiedToDefOperand(OpNo) || MI->isPseudo())
       continue;
 
+    // Also, we have to check that the register class of the operand
+    // contains the zero register.
+    if (!MRI->getRegClass(MO.getReg())->contains(ZeroReg))
+      continue;
+
     MO.setReg(ZeroReg);
   }
 
   return true;
 }
 
 void MipsSEDAGToDAGISel::initGlobalBaseReg(MachineFunction &MF) {
   MipsFunctionInfo *MipsFI = MF.getInfo<MipsFunctionInfo>();

llvm/trunk/test/CodeGen/Mips/micromips-zero-mat-uses.ll

+; RUN: llc -march=mips -mcpu=mips32r2 -mattr=+micromips,+nooddspreg -O0 < %s | FileCheck %s
+
+; CHECK: addiu    $[[R0:[0-9]+]], $zero, 0
+; CHECK: subu16   $2, $[[R0]], ${{[0-9]+}}
+define i32 @foo() {
+  %1 = sub i32 0, undef
+  ret i32 %1
+}