[mips] [IAS] Do not generate redundant move when expanding lw/sw with symbol.

[tomatabacu]

Summary

Even though there is no 2nd register operand in the "lw/sw $8, symbol" case, we still try to find one,
and we end up with $0, which makes us generate an unnecessary "addu $8, $8, $0" (a.k.a. "move $8, $8").

We can avoid this by checking if the 2nd register operand is different from $0, before generating the addu.

[dsanders]

LGTM

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

if (ExprOffset->getKind() == MCExpr::SymbolRef) {
       TempInst.addOperand(MCOperand::CreateExpr(HiExpr));
     }
   }
   // Add the instruction to the list.
   Instructions.push_back(TempInst);
   // Prepare TempInst for next instruction.
   TempInst.clear();
   // Add temp register to base.
+  if (BaseRegNum != Mips::ZERO) {
     TempInst.setOpcode(Mips::ADDu);
     TempInst.addOperand(MCOperand::CreateReg(TmpRegNum));
     TempInst.addOperand(MCOperand::CreateReg(TmpRegNum));
     TempInst.addOperand(MCOperand::CreateReg(BaseRegNum));
     Instructions.push_back(TempInst);
     TempInst.clear();
+  }
   // And finally, create original instruction with low part
   // of offset and new base.
   TempInst.setOpcode(Inst.getOpcode());
   TempInst.addOperand(MCOperand::CreateReg(RegOpNum));
   TempInst.addOperand(MCOperand::CreateReg(TmpRegNum));
   if (isImmOpnd)
     TempInst.addOperand(MCOperand::CreateImm(LoOffset));
   else {

test/MC/Mips/mips-expansions.s

 # CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
 # CHECK: lui     $10, 10                 # encoding: [0x0a,0x00,0x0a,0x3c]
 # CHECK: addu    $10, $10, $4            # encoding: [0x21,0x50,0x44,0x01]
 # CHECK: lw      $10, 123($10)           # encoding: [0x7b,0x00,0x4a,0x8d]
 # CHECK: lui     $1, 2                   # encoding: [0x02,0x00,0x01,0x3c]
 # CHECK: addu    $1, $1, $9              # encoding: [0x21,0x08,0x29,0x00]
 # CHECK: sw      $10, 57920($1)          # encoding: [0x40,0xe2,0x2a,0xac]
 
+# CHECK:     lui     $8, %hi(symbol)     # encoding: [A,A,0x08,0x3c]
+# CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK-NOT: move    $8, $8              # encoding: [0x21,0x40,0x00,0x01]
+# CHECK:     lw      $8, %lo(symbol)($8) # encoding: [A,A,0x08,0x8d]
+# CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+# CHECK:     lui     $1, %hi(symbol)     # encoding: [A,A,0x01,0x3c]
+# CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_HI, kind: fixup_Mips_HI16
+# CHECK-NOT: move    $1, $1              # encoding: [0x21,0x08,0x20,0x00]
+# CHECK:     sw      $8, %lo(symbol)($1) # encoding: [A,A,0x28,0xac]
+# CHECK:                                 #   fixup A - offset: 0, value: symbol@ABS_LO, kind: fixup_Mips_LO16
+
 # CHECK: lui     $1, %hi(symbol)
 # CHECK: ldc1    $f0, %lo(symbol)($1)
 # CHECK: lui     $1, %hi(symbol)
 # CHECK: sdc1    $f0, %lo(symbol)($1)
 
     li $5,123
     li $6,-2345
     li $7,65538
# CHECK: sdc1    $f0, %lo(symbol)($1)
     .set noat
     lw  $t2, symbol($a0)
     .set at
     sw  $t2, symbol($t1)
 
     lw  $t2, 655483($a0)
     sw  $t2, 123456($t1)
 
+    lw  $8, symbol
+    sw  $8, symbol
+
     ldc1 $f0, symbol
     sdc1 $f0, symbol