[mips][mc] Fix a crash when disassembling odd sized sections

[sdardis]

Summary

Make the MIPS disassembler consistent with the other targets in returning
a Size of zero when the input buffer cannot contain an instruction due
to it's size. Previously it reported the minimum instruction size when
it failed due to the buffer not being big enough for an instruction
causing llvm-objdump to crash when disassembling all sections.

[slthakur]

LGTM

llvm/trunk/lib/Target/Mips/Disassembler/MipsDisassembler.cpp

 
 DecodeStatus MipsDisassembler::getInstruction(MCInst &Instr, uint64_t &Size,
                                               ArrayRef<uint8_t> Bytes,
                                               uint64_t Address,
                                               raw_ostream &VStream,
                                               raw_ostream &CStream) const {
   uint32_t Insn;
   DecodeStatus Result;
+  Size = 0;
 
   if (IsMicroMips) {
     Result = readInstruction16(Bytes, Address, Size, Insn, IsBigEndian);
     if (Result == MCDisassembler::Fail)
       return MCDisassembler::Fail;
 
     if (hasMips32r6()) {
       DEBUG(dbgs() << "Trying MicroMipsR616 table (16-bit instructions):\n");
if (hasMips32r6() && isFP64()) {
       Result = decodeInstruction(DecoderTableMicroMips32r6FP6432, Instr, Insn,
                                  Address, this, STI);
       if (Result != MCDisassembler::Fail) {
         Size = 4;
         return Result;
       }
     }
 
-    // This is an invalid instruction. Let the disassembler move forward by the
-    // minimum instruction size.
+    // This is an invalid instruction. Claim that the Size is 2 bytes. Since
+    // microMIPS instructions have a minimum alignment of 2, the next 2 bytes
+    // could form a valid instruction. The two bytes we rejected as an
+    // instruction could have actually beeen an inline constant pool that is
+    // unconditionally branched over.
     Size = 2;
     return MCDisassembler::Fail;
   }
 
+  // Attempt to read the instruction so that we can attempt to decode it. If
+  // the buffer is not 4 bytes long, let the higher level logic figure out
+  // what to do with a size of zero and MCDisassembler::Fail.
   Result = readInstruction32(Bytes, Address, Size, Insn, IsBigEndian, false);
-  if (Result == MCDisassembler::Fail) {
-    Size = 4;
+  if (Result == MCDisassembler::Fail)
     return MCDisassembler::Fail;
-  }
+
+  // The only instruction size for standard encoded MIPS.
+  Size = 4;
 
   if (hasCOP3()) {
     DEBUG(dbgs() << "Trying COP3_ table (32-bit opcodes):\n");
     Result =
         decodeInstruction(DecoderTableCOP3_32, Instr, Insn, Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (hasMips32r6() && isGP64()) {
     DEBUG(dbgs() << "Trying Mips32r6_64r6 (GPR64) table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableMips32r6_64r6_GP6432, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (hasMips32r6() && isPTR64()) {
     DEBUG(dbgs() << "Trying Mips32r6_64r6 (PTR64) table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableMips32r6_64r6_PTR6432, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (hasMips32r6()) {
     DEBUG(dbgs() << "Trying Mips32r6_64r6 table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableMips32r6_64r632, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (hasMips2() && isPTR64()) {
     DEBUG(dbgs() << "Trying Mips32r6_64r6 (PTR64) table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableMips32_64_PTR6432, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (hasCnMips()) {
     DEBUG(dbgs() << "Trying CnMips table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableCnMips32, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   if (isGP64()) {
     DEBUG(dbgs() << "Trying Mips64 (GPR64) table (32-bit opcodes):\n");
     Result = decodeInstruction(DecoderTableMips6432, Instr, Insn,
                                Address, this, STI);
-    if (Result != MCDisassembler::Fail) {
-      Size = 4;
+    if (Result != MCDisassembler::Fail)
       return Result;
-    }
-  }
 
   DEBUG(dbgs() << "Trying Mips table (32-bit opcodes):\n");
   // Calling the auto-generated decoder function.
   Result =
       decodeInstruction(DecoderTableMips32, Instr, Insn, Address, this, STI);
-  if (Result != MCDisassembler::Fail) {
-    Size = 4;
+  if (Result != MCDisassembler::Fail)
     return Result;
-  }
 
-  Size = 4;
   return MCDisassembler::Fail;
 }
 
 static DecodeStatus DecodeCPU16RegsRegisterClass(MCInst &Inst,
                                                  unsigned RegNo,
                                                  uint64_t Address,
                                                  const void *Decoder) {
   return MCDisassembler::Fail;