[mips] Fix expansion of memory operation if destination register is not a GPR.

[matheusalmeida]

Summary

The assembler tries to reuse the destination register for memory operations whenever
it can but it's not possible to do so if the destination register is not a GPR.

Example:

ldc1 $f0, sym

should expand to:

lui $at, %hi(sym)
ldc1 $f0, %lo(sym)($at)

It's entirely wrong to expand to:

lui $f0, %hi(sym)
ldc1 $f0, %lo(sym)($f0)

[dsanders]

LGTM with some minor changes to the comments

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

1125–1153

'These are the some types' is strange english. I think you intended 'These are some of the types'

1128–1130

Does %hi/%lo take an immediate?

If not, then '((offset >> 16) & 0xffff)' or 'offset_hi16' might be better

1146

True since there is no destination register on a store but 'must not clobber the source register' would be a bit clearer.

[matheusalmeida]

Fix grammar.

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

if (isImmOpnd) {
     HiOffset = (ImmOffset & 0xffff0000) >> 16;
     // If msb of LoOffset is 1(negative number) we must increment HiOffset.
     if (LoOffset & 0x8000)
       HiOffset++;
   } else
     ExprOffset = Inst.getOperand(2).getExpr();
   // All instructions will have the same location.
   TempInst.setLoc(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.
-  if (isLoad && (BaseRegNum != RegOpNum))
+  // These are some of the types of expansions we perform here:
+  // 1) lw $8, sym        => lui $8, %hi(sym)
+  //                         lw $8, %lo(sym)($8)
+  // 2) lw $8, offset($9) => lui $8, %hi(offset)
+  //                         add $8, $8, $9
+  //                         lw $8, %lo(offset)($9)
+  // 3) lw $8, offset($8) => lui $at, %hi(offset)
+  //                         add $at, $at, $8
+  //                         lw $8, %lo(offset)($at)
+  // 4) sw $8, sym        => lui $at, %hi(sym)
+  //                         sw $8, %lo(sym)($at)
+  // 5) sw $8, offset($8) => lui $at, %hi(offset)
+  //                         add $at, $at, $8
+  //                         sw $8, %lo(offset)($at)
+  // 6) ldc1 $f0, sym     => lui $at, %hi(sym)
+  //                         ldc1 $f0, %lo(sym)($at)
+  //
+  // For load instructions we can use the destination register as a temporary
+  // if base and dst are different (examples 1 and 2) and if the base register
+  // is general purpose otherwise we must use $at (example 6) and error if it's
+  // not available. For stores we must use $at (examples 4 and 5) because we
+  // must not clobber the source register setting up the offset.
+  const MCInstrDesc &Desc = getInstDesc(Inst.getOpcode());
+  int16_t RegClassOp0 = Desc.OpInfo[0].RegClass;
+  unsigned RegClassIDOp0 =
+      getContext().getRegisterInfo()->getRegClass(RegClassOp0).getID();
+  bool IsGPR = (RegClassIDOp0 == Mips::GPR32RegClassID) ||
+               (RegClassIDOp0 == Mips::GPR64RegClassID);
+  if (isLoad && IsGPR && (BaseRegNum != RegOpNum))
     TmpRegNum = RegOpNum;
   else {
     int AT = getATReg(IDLoc);
     // At this point we need AT to perform the expansions and we exit if it is
     // not available.
     if (!AT)
       return;
     TmpRegNum =

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     $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
     li $8, ~7
 
     la $a0, 20
     la $7,65538
     la $a0, 20($a1)
     la $7,65538($8)
 
     .set noat
     lw  $t2, symbol($a0)
     .set at
     sw  $t2, symbol($t1)
 
     lw  $t2, 655483($a0)
     sw  $t2, 123456($t1)
+
+    ldc1 $f0, symbol
+    sdc1 $f0, symbol