[mips] Partially fix PR34391

[sdardis]

Summary

Previously, the parsing of the 'subu $reg, ($reg,) imm' relied on a parser
which also rendered the operand to the instruction. In some cases the
general parser could construct an MCExpr which was not a MCConstantExpr
which MipsAsmParser was expecting.

Address this by altering the special handling to cope with unexpected inputs
and fine-tune the handling of cases where an register name that is not
available in the current ABI is regarded as not a match for the custom parser
but also not as ab outright error.

Also enforces the binutils restriction that only constants are accepted.

This partially resolves PR34391.

Thanks to Ed Maste for reporting the issue!

[sdardis]

Ping.

[nitesh.jain]

LGTM.

Thanks

[sdardis]

Found an issue when doing some final testing. One of the invalid tests (test/mc/mips2/invalid-mips3.s) reports a wrong operand error, as the parser uses the generic parser rather than the operand specific parser. It parses '$a4' as an immediate and rejects it, as the register matcher doesn't match $a4 as a register as the O32 abi is in use.

[sdardis]

Address failures by handling non-constants in the parser. Also handle the differences between register names/symbols between ABIs.

[arichardson]

This fixes the crashes we had when compiling some CheriBSD kernel assembly.

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

Operands.push_back(
       MipsOperand::CreateImm(Expr, S, getLexer().getLoc(), *this));
   return MatchOperand_Success;
 }
 
 OperandMatchResultTy
 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_ParseFail;
   int64_t Val = MCE->getValue();
   SMLoc E = SMLoc::getFromPointer(Parser.getTok().getLoc().getPointer() - 1);
   Operands.push_back(MipsOperand::CreateImm(
       MCConstantExpr::create(0 - Val, getContext()), S, E, *this));
   return MatchOperand_Success;
 }
 
 OperandMatchResultTy

test/MC/Mips/macro-aliases-invalid.s

+# RUN: not llvm-mc -arch=mips -mcpu=mips32r2 %s 2>%t1
+# RUN: FileCheck %s < %t1
+
+# Check that subu only rejects any non-constant values.
+
+.globl end
+  subu  $4, $4, %lo($start)   # CHECK: error: invalid operand for instruction
+  subu  $4, $4, $start        # CHECK: error: invalid operand for instruction
+  subu  $4, $4, %hi(end)      # CHECK: error: invalid operand for instruction
+  subu  $4, $4, end + 4       # CHECK: error: invalid operand for instruction
+  subu  $4, $4, end           # CHECK: error: invalid operand for instruction
+  subu  $4, $4, sp            # CHECK: error: invalid operand for instruction
+
+  subu  $4, %lo($start)       # CHECK: error: invalid operand for instruction
+  subu  $4, $start            # CHECK: error: invalid operand for instruction
+  subu  $4, %hi(end)          # CHECK: error: invalid operand for instruction
+  subu  $4, end + 4           # CHECK: error: invalid operand for instruction
+  subu  $4, end               # CHECK: error: invalid operand for instruction
+  subu  $4, sp                # CHECK: error: invalid operand for instruction
+
+$start:

test/MC/Mips/macro-aliases.s

+# 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
+