[mips] Access $at only if necessary.

Summary:
This patch doesn't really change the logic behind expandMemInst but it allows
us to assemble .S files that use .set noat with some macros. For example:

.set noat
lw $k0, offset($k1)

Can expand to:
lui	$k0, %hi(offset)
addu	$k0, $k0, $k1
lw	$k0, %lo(offset)($k0)

with no need to access $at.

Reviewers: dsanders, vmedic

Reviewed By: dsanders, vmedic

Differential Revision: http://reviews.llvm.org/D4159

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211165 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

@@ -1125,8 +1125,6 @@ void MipsAsmParser::expandMemInst(MCInst &Inst, SMLoc IDLoc,
   unsigned ImmOffset, HiOffset, LoOffset;
   const MCExpr *ExprOffset;
   unsigned TmpRegNum;
-  unsigned AtRegNum = getReg(
-      (isGP64()) ? Mips::GPR64RegClassID : Mips::GPR32RegClassID, getATReg());
   // 1st operand is either the source or destination register.
   assert(Inst.getOperand(0).isReg() && "expected register operand kind");
   unsigned RegOpNum = Inst.getOperand(0).getReg();
@@ -1149,7 +1147,12 @@ void MipsAsmParser::expandMemInst(MCInst &Inst, SMLoc IDLoc,
   // 1st instruction in expansion is LUi. For load instruction we can use
   // the dst register as a temporary if base and dst are different,
   // but for stores we must use $at.
-  TmpRegNum = (isLoad && (BaseRegNum != RegOpNum)) ? RegOpNum : AtRegNum;
+  if (isLoad && (BaseRegNum != RegOpNum))
+    TmpRegNum = RegOpNum;
+  else
+    TmpRegNum = getReg(
+      (isGP64()) ? Mips::GPR64RegClassID : Mips::GPR32RegClassID, getATReg());
+
   TempInst.setOpcode(Mips::LUi);
   TempInst.addOperand(MCOperand::CreateReg(TmpRegNum));
   if (isImmOpnd)

test/MC/Mips/mips-expansions.s

@@ -44,7 +44,9 @@
     la $a0, 20($a1)
     la $7,65538($8)
 
+    .set noat
     lw  $t2, symbol($a0)
+    .set at
     sw  $t2, symbol($t1)
 
     lw  $t2, 655483($a0)