Index: llvm/trunk/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
===================================================================
--- llvm/trunk/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
+++ llvm/trunk/lib/Target/Mips/AsmParser/MipsAsmParser.cpp
@@ -5859,14 +5859,21 @@
 MipsAsmParser::parseInvNum(OperandVector &Operands) {
   MCAsmParser &Parser = getParser();
   const MCExpr *IdVal;
-  // If the first token is '$' we may have register operand.
-  if (Parser.getTok().is(AsmToken::Dollar))
-    return MatchOperand_NoMatch;
+  // If the first token is '$' we may have register operand. We have to reject
+  // cases where it is not a register. Complicating the matter is that
+  // register names are not reserved across all ABIs.
+  // Peek past the dollar to see if it's a register name for this ABI.
   SMLoc S = Parser.getTok().getLoc();
+  if (Parser.getTok().is(AsmToken::Dollar)) {
+    return matchCPURegisterName(Parser.getLexer().peekTok().getString()) == -1
+               ? MatchOperand_ParseFail
+               : MatchOperand_NoMatch;
+  }
   if (getParser().parseExpression(IdVal))
     return MatchOperand_ParseFail;
   const MCConstantExpr *MCE = dyn_cast<MCConstantExpr>(IdVal);
-  assert(MCE && "Unexpected MCExpr type.");
+  if (!MCE)
+    return MatchOperand_NoMatch;
   int64_t Val = MCE->getValue();
   SMLoc E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
   Operands.push_back(MipsOperand::CreateImm(
Index: llvm/trunk/test/MC/Mips/macro-aliases-invalid-wrong-error.s
===================================================================
--- llvm/trunk/test/MC/Mips/macro-aliases-invalid-wrong-error.s
+++ llvm/trunk/test/MC/Mips/macro-aliases-invalid-wrong-error.s
@@ -0,0 +1,38 @@
+# RUN: not llvm-mc -arch=mips %s 2>%t1
+# RUN: FileCheck --check-prefix=O32 %s < %t1
+
+# RUN: not llvm-mc -arch=mips64 %s 2>%t1
+# RUN: FileCheck --check-prefix=N64 %s < %t1
+
+# Check that subu only rejects any non-constant values.
+
+.globl end
+  subu  $4, $4, %lo($start)   # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $4, $start        # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $a4, $a4          # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $4, %hi(end)      # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $4, end + 4       # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $4, end           # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $4, sp            # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+
+  subu  $4, %lo($start)       # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $start            # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, $a4               # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, %hi(end)          # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, end + 4           # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, end               # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+  subu  $4, sp                # O32: [[@LINE]]:{{[0-9]+}}: error: unexpected token in argument list
+                              # N64: [[@LINE-1]]:{{[0-9]+}}: error: unexpected token in argument list
+
+$start:
Index: llvm/trunk/test/MC/Mips/macro-aliases.s
===================================================================
--- llvm/trunk/test/MC/Mips/macro-aliases.s
+++ llvm/trunk/test/MC/Mips/macro-aliases.s
@@ -0,0 +1,35 @@
+# RUN: llvm-mc -arch=mips -mcpu=mips32r2 %s -show-inst | FileCheck %s
+
+# Test that subu accepts constant operands and inverts them when
+# rendering the operand.
+
+  subu  $4, $4, 4          # CHECK: ADDiu
+                           # CHECK; Imm:-4
+  subu  $gp, $gp, 4        # CHECK: ADDiu
+                           # CHECK; Imm:-4
+  subu  $sp, $sp, 4        # CHECK: ADDiu
+                           # CHECK; Imm:-4
+  subu  $4, $4, -4         # CHECK: ADDiu
+                           # CHECK; Imm:4
+  subu  $gp, $gp, -4       # CHECK: ADDiu
+                           # CHECK; Imm:4
+  subu  $sp, $sp, -4       # CHECK: ADDiu
+                           # CHECK; Imm:4
+  subu  $sp, $sp, -(4 + 4) # CHECK: ADDiu
+                           # CHECK: Imm:8
+
+  subu  $4, 8              # CHECK: ADDiu
+                           # CHECK; Imm:-8
+  subu  $gp, 8             # CHECK: ADDiu
+                           # CHECK; Imm:-8
+  subu  $sp, 8             # CHECK: ADDiu
+                           # CHECK; Imm:-8
+  subu  $4, -8             # CHECK: ADDiu
+                           # CHECK; Imm:8
+  subu  $gp, -8            # CHECK: ADDiu
+                           # CHECK; Imm:8
+  subu  $sp, -8            # CHECK: ADDiu
+                           # CHECK; Imm:8
+  subu  $sp, -(4 + 4)      # CHECK: ADDiu
+                           # CHECK: Imm:8
+