vendor/llvm/llvm-release_350-r216957

1 files changed, 1842 insertions, 0 deletions

diff --git a/lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp b/lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
new file mode 100644
index 0000000000000..ab3d1f774bc75
--- /dev/null
+++ b/lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
@@ -0,0 +1,1842 @@
+//===-- X86DisassemblerDecoder.c - Disassembler decoder -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is part of the X86 Disassembler.
+// It contains the implementation of the instruction decoder.
+// Documentation for the disassembler can be found in X86Disassembler.h.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdarg.h>   /* for va_*()       */
+#include <stdio.h>    /* for vsnprintf()  */
+#include <stdlib.h>   /* for exit()       */
+#include <string.h>   /* for memset()     */
+
+#include "X86DisassemblerDecoder.h"
+
+using namespace llvm::X86Disassembler;
+
+/// Specifies whether a ModR/M byte is needed and (if so) which
+/// instruction each possible value of the ModR/M byte corresponds to.  Once
+/// this information is known, we have narrowed down to a single instruction.
+struct ModRMDecision {
+  uint8_t modrm_type;
+  uint16_t instructionIDs;
+};
+
+/// Specifies which set of ModR/M->instruction tables to look at
+/// given a particular opcode.
+struct OpcodeDecision {
+  ModRMDecision modRMDecisions[256];
+};
+
+/// Specifies which opcode->instruction tables to look at given
+/// a particular context (set of attributes).  Since there are many possible
+/// contexts, the decoder first uses CONTEXTS_SYM to determine which context
+/// applies given a specific set of attributes.  Hence there are only IC_max
+/// entries in this table, rather than 2^(ATTR_max).
+struct ContextDecision {
+  OpcodeDecision opcodeDecisions[IC_max];
+};
+
+#include "X86GenDisassemblerTables.inc"
+
+#ifndef NDEBUG
+#define debug(s) do { Debug(__FILE__, __LINE__, s); } while (0)
+#else
+#define debug(s) do { } while (0)
+#endif
+
+
+/*
+ * contextForAttrs - Client for the instruction context table.  Takes a set of
+ *   attributes and returns the appropriate decode context.
+ *
+ * @param attrMask  - Attributes, from the enumeration attributeBits.
+ * @return          - The InstructionContext to use when looking up an
+ *                    an instruction with these attributes.
+ */
+static InstructionContext contextForAttrs(uint16_t attrMask) {
+  return static_cast<InstructionContext>(CONTEXTS_SYM[attrMask]);
+}
+
+/*
+ * modRMRequired - Reads the appropriate instruction table to determine whether
+ *   the ModR/M byte is required to decode a particular instruction.
+ *
+ * @param type        - The opcode type (i.e., how many bytes it has).
+ * @param insnContext - The context for the instruction, as returned by
+ *                      contextForAttrs.
+ * @param opcode      - The last byte of the instruction's opcode, not counting
+ *                      ModR/M extensions and escapes.
+ * @return            - true if the ModR/M byte is required, false otherwise.
+ */
+static int modRMRequired(OpcodeType type,
+                         InstructionContext insnContext,
+                         uint16_t opcode) {
+  const struct ContextDecision* decision = nullptr;
+
+  switch (type) {
+  case ONEBYTE:
+    decision = &ONEBYTE_SYM;
+    break;
+  case TWOBYTE:
+    decision = &TWOBYTE_SYM;
+    break;
+  case THREEBYTE_38:
+    decision = &THREEBYTE38_SYM;
+    break;
+  case THREEBYTE_3A:
+    decision = &THREEBYTE3A_SYM;
+    break;
+  case XOP8_MAP:
+    decision = &XOP8_MAP_SYM;
+    break;
+  case XOP9_MAP:
+    decision = &XOP9_MAP_SYM;
+    break;
+  case XOPA_MAP:
+    decision = &XOPA_MAP_SYM;
+    break;
+  }
+
+  return decision->opcodeDecisions[insnContext].modRMDecisions[opcode].
+    modrm_type != MODRM_ONEENTRY;
+}
+
+/*
+ * decode - Reads the appropriate instruction table to obtain the unique ID of
+ *   an instruction.
+ *
+ * @param type        - See modRMRequired().
+ * @param insnContext - See modRMRequired().
+ * @param opcode      - See modRMRequired().
+ * @param modRM       - The ModR/M byte if required, or any value if not.
+ * @return            - The UID of the instruction, or 0 on failure.
+ */
+static InstrUID decode(OpcodeType type,
+                       InstructionContext insnContext,
+                       uint8_t opcode,
+                       uint8_t modRM) {
+  const struct ModRMDecision* dec = nullptr;
+
+  switch (type) {
+  case ONEBYTE:
+    dec = &ONEBYTE_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case TWOBYTE:
+    dec = &TWOBYTE_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case THREEBYTE_38:
+    dec = &THREEBYTE38_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case THREEBYTE_3A:
+    dec = &THREEBYTE3A_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case XOP8_MAP:
+    dec = &XOP8_MAP_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case XOP9_MAP:
+    dec = &XOP9_MAP_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  case XOPA_MAP:
+    dec = &XOPA_MAP_SYM.opcodeDecisions[insnContext].modRMDecisions[opcode];
+    break;
+  }
+
+  switch (dec->modrm_type) {
+  default:
+    debug("Corrupt table!  Unknown modrm_type");
+    return 0;
+  case MODRM_ONEENTRY:
+    return modRMTable[dec->instructionIDs];
+  case MODRM_SPLITRM:
+    if (modFromModRM(modRM) == 0x3)
+      return modRMTable[dec->instructionIDs+1];
+    return modRMTable[dec->instructionIDs];
+  case MODRM_SPLITREG:
+    if (modFromModRM(modRM) == 0x3)
+      return modRMTable[dec->instructionIDs+((modRM & 0x38) >> 3)+8];
+    return modRMTable[dec->instructionIDs+((modRM & 0x38) >> 3)];
+  case MODRM_SPLITMISC:
+    if (modFromModRM(modRM) == 0x3)
+      return modRMTable[dec->instructionIDs+(modRM & 0x3f)+8];
+    return modRMTable[dec->instructionIDs+((modRM & 0x38) >> 3)];
+  case MODRM_FULL:
+    return modRMTable[dec->instructionIDs+modRM];
+  }
+}
+
+/*
+ * specifierForUID - Given a UID, returns the name and operand specification for
+ *   that instruction.
+ *
+ * @param uid - The unique ID for the instruction.  This should be returned by
+ *              decode(); specifierForUID will not check bounds.
+ * @return    - A pointer to the specification for that instruction.
+ */
+static const struct InstructionSpecifier *specifierForUID(InstrUID uid) {
+  return &INSTRUCTIONS_SYM[uid];
+}
+
+/*
+ * consumeByte - Uses the reader function provided by the user to consume one
+ *   byte from the instruction's memory and advance the cursor.
+ *
+ * @param insn  - The instruction with the reader function to use.  The cursor
+ *                for this instruction is advanced.
+ * @param byte  - A pointer to a pre-allocated memory buffer to be populated
+ *                with the data read.
+ * @return      - 0 if the read was successful; nonzero otherwise.
+ */
+static int consumeByte(struct InternalInstruction* insn, uint8_t* byte) {
+  int ret = insn->reader(insn->readerArg, byte, insn->readerCursor);
+
+  if (!ret)
+    ++(insn->readerCursor);
+
+  return ret;
+}
+
+/*
+ * lookAtByte - Like consumeByte, but does not advance the cursor.
+ *
+ * @param insn  - See consumeByte().
+ * @param byte  - See consumeByte().
+ * @return      - See consumeByte().
+ */
+static int lookAtByte(struct InternalInstruction* insn, uint8_t* byte) {
+  return insn->reader(insn->readerArg, byte, insn->readerCursor);
+}
+
+static void unconsumeByte(struct InternalInstruction* insn) {
+  insn->readerCursor--;
+}
+
+#define CONSUME_FUNC(name, type)                                  \
+  static int name(struct InternalInstruction* insn, type* ptr) {  \
+    type combined = 0;                                            \
+    unsigned offset;                                              \
+    for (offset = 0; offset < sizeof(type); ++offset) {           \
+      uint8_t byte;                                               \
+      int ret = insn->reader(insn->readerArg,                     \
+                             &byte,                               \
+                             insn->readerCursor + offset);        \
+      if (ret)                                                    \
+        return ret;                                               \
+      combined = combined | ((uint64_t)byte << (offset * 8));     \
+    }                                                             \
+    *ptr = combined;                                              \
+    insn->readerCursor += sizeof(type);                           \
+    return 0;                                                     \
+  }
+
+/*
+ * consume* - Use the reader function provided by the user to consume data
+ *   values of various sizes from the instruction's memory and advance the
+ *   cursor appropriately.  These readers perform endian conversion.
+ *
+ * @param insn    - See consumeByte().
+ * @param ptr     - A pointer to a pre-allocated memory of appropriate size to
+ *                  be populated with the data read.
+ * @return        - See consumeByte().
+ */
+CONSUME_FUNC(consumeInt8, int8_t)
+CONSUME_FUNC(consumeInt16, int16_t)
+CONSUME_FUNC(consumeInt32, int32_t)
+CONSUME_FUNC(consumeUInt16, uint16_t)
+CONSUME_FUNC(consumeUInt32, uint32_t)
+CONSUME_FUNC(consumeUInt64, uint64_t)
+
+/*
+ * dbgprintf - Uses the logging function provided by the user to log a single
+ *   message, typically without a carriage-return.
+ *
+ * @param insn    - The instruction containing the logging function.
+ * @param format  - See printf().
+ * @param ...     - See printf().
+ */
+static void dbgprintf(struct InternalInstruction* insn,
+                      const char* format,
+                      ...) {
+  char buffer[256];
+  va_list ap;
+
+  if (!insn->dlog)
+    return;
+
+  va_start(ap, format);
+  (void)vsnprintf(buffer, sizeof(buffer), format, ap);
+  va_end(ap);
+
+  insn->dlog(insn->dlogArg, buffer);
+
+  return;
+}
+
+/*
+ * setPrefixPresent - Marks that a particular prefix is present at a particular
+ *   location.
+ *
+ * @param insn      - The instruction to be marked as having the prefix.
+ * @param prefix    - The prefix that is present.
+ * @param location  - The location where the prefix is located (in the address
+ *                    space of the instruction's reader).
+ */
+static void setPrefixPresent(struct InternalInstruction* insn,
+                                    uint8_t prefix,
+                                    uint64_t location)
+{
+  insn->prefixPresent[prefix] = 1;
+  insn->prefixLocations[prefix] = location;
+}
+
+/*
+ * isPrefixAtLocation - Queries an instruction to determine whether a prefix is
+ *   present at a given location.
+ *
+ * @param insn      - The instruction to be queried.
+ * @param prefix    - The prefix.
+ * @param location  - The location to query.
+ * @return          - Whether the prefix is at that location.
+ */
+static bool isPrefixAtLocation(struct InternalInstruction* insn,
+                               uint8_t prefix,
+                               uint64_t location)
+{
+  if (insn->prefixPresent[prefix] == 1 &&
+     insn->prefixLocations[prefix] == location)
+    return true;
+  else
+    return false;
+}
+
+/*
+ * readPrefixes - Consumes all of an instruction's prefix bytes, and marks the
+ *   instruction as having them.  Also sets the instruction's default operand,
+ *   address, and other relevant data sizes to report operands correctly.
+ *
+ * @param insn  - The instruction whose prefixes are to be read.
+ * @return      - 0 if the instruction could be read until the end of the prefix
+ *                bytes, and no prefixes conflicted; nonzero otherwise.
+ */
+static int readPrefixes(struct InternalInstruction* insn) {
+  bool isPrefix = true;
+  bool prefixGroups[4] = { false };
+  uint64_t prefixLocation;
+  uint8_t byte = 0;
+  uint8_t nextByte;
+
+  bool hasAdSize = false;
+  bool hasOpSize = false;
+
+  dbgprintf(insn, "readPrefixes()");
+
+  while (isPrefix) {
+    prefixLocation = insn->readerCursor;
+
+    /* If we fail reading prefixes, just stop here and let the opcode reader deal with it */
+    if (consumeByte(insn, &byte))
+      break;
+
+    /*
+     * If the byte is a LOCK/REP/REPNE prefix and not a part of the opcode, then
+     * break and let it be disassembled as a normal "instruction".
+     */
+    if (insn->readerCursor - 1 == insn->startLocation && byte == 0xf0)
+      break;
+
+    if (insn->readerCursor - 1 == insn->startLocation
+        && (byte == 0xf2 || byte == 0xf3)
+        && !lookAtByte(insn, &nextByte))
+    {
+      /*
+       * If the byte is 0xf2 or 0xf3, and any of the following conditions are
+       * met:
+       * - it is followed by a LOCK (0xf0) prefix
+       * - it is followed by an xchg instruction
+       * then it should be disassembled as a xacquire/xrelease not repne/rep.
+       */
+      if ((byte == 0xf2 || byte == 0xf3) &&
+          ((nextByte == 0xf0) |
+          ((nextByte & 0xfe) == 0x86 || (nextByte & 0xf8) == 0x90)))
+        insn->xAcquireRelease = true;
+      /*
+       * Also if the byte is 0xf3, and the following condition is met:
+       * - it is followed by a "mov mem, reg" (opcode 0x88/0x89) or
+       *                       "mov mem, imm" (opcode 0xc6/0xc7) instructions.
+       * then it should be disassembled as an xrelease not rep.
+       */
+      if (byte == 0xf3 &&
+          (nextByte == 0x88 || nextByte == 0x89 ||
+           nextByte == 0xc6 || nextByte == 0xc7))
+        insn->xAcquireRelease = true;
+      if (insn->mode == MODE_64BIT && (nextByte & 0xf0) == 0x40) {
+        if (consumeByte(insn, &nextByte))
+          return -1;
+        if (lookAtByte(insn, &nextByte))
+          return -1;
+        unconsumeByte(insn);
+      }
+      if (nextByte != 0x0f && nextByte != 0x90)
+        break;
+    }
+
+    switch (byte) {
+    case 0xf0:  /* LOCK */
+    case 0xf2:  /* REPNE/REPNZ */
+    case 0xf3:  /* REP or REPE/REPZ */
+      if (prefixGroups[0])
+        dbgprintf(insn, "Redundant Group 1 prefix");
+      prefixGroups[0] = true;
+      setPrefixPresent(insn, byte, prefixLocation);
+      break;
+    case 0x2e:  /* CS segment override -OR- Branch not taken */
+    case 0x36:  /* SS segment override -OR- Branch taken */
+    case 0x3e:  /* DS segment override */
+    case 0x26:  /* ES segment override */
+    case 0x64:  /* FS segment override */
+    case 0x65:  /* GS segment override */
+      switch (byte) {
+      case 0x2e:
+        insn->segmentOverride = SEG_OVERRIDE_CS;
+        break;
+      case 0x36:
+        insn->segmentOverride = SEG_OVERRIDE_SS;
+        break;
+      case 0x3e:
+        insn->segmentOverride = SEG_OVERRIDE_DS;
+        break;
+      case 0x26:
+        insn->segmentOverride = SEG_OVERRIDE_ES;
+        break;
+      case 0x64:
+        insn->segmentOverride = SEG_OVERRIDE_FS;
+        break;
+      case 0x65:
+        insn->segmentOverride = SEG_OVERRIDE_GS;
+        break;
+      default:
+        debug("Unhandled override");
+        return -1;
+      }
+      if (prefixGroups[1])
+        dbgprintf(insn, "Redundant Group 2 prefix");
+      prefixGroups[1] = true;
+      setPrefixPresent(insn, byte, prefixLocation);
+      break;
+    case 0x66:  /* Operand-size override */
+      if (prefixGroups[2])
+        dbgprintf(insn, "Redundant Group 3 prefix");
+      prefixGroups[2] = true;
+      hasOpSize = true;
+      setPrefixPresent(insn, byte, prefixLocation);
+      break;
+    case 0x67:  /* Address-size override */
+      if (prefixGroups[3])
+        dbgprintf(insn, "Redundant Group 4 prefix");
+      prefixGroups[3] = true;
+      hasAdSize = true;
+      setPrefixPresent(insn, byte, prefixLocation);
+      break;
+    default:    /* Not a prefix byte */
+      isPrefix = false;
+      break;
+    }
+
+    if (isPrefix)
+      dbgprintf(insn, "Found prefix 0x%hhx", byte);
+  }
+
+  insn->vectorExtensionType = TYPE_NO_VEX_XOP;
+
+  if (byte == 0x62) {
+    uint8_t byte1, byte2;
+
+    if (consumeByte(insn, &byte1)) {
+      dbgprintf(insn, "Couldn't read second byte of EVEX prefix");
+      return -1;
+    }
+
+    if (lookAtByte(insn, &byte2)) {
+      dbgprintf(insn, "Couldn't read third byte of EVEX prefix");
+      return -1;
+    }
+
+    if ((insn->mode == MODE_64BIT || (byte1 & 0xc0) == 0xc0) &&
+       ((~byte1 & 0xc) == 0xc) && ((byte2 & 0x4) == 0x4)) {
+      insn->vectorExtensionType = TYPE_EVEX;
+    }
+    else {
+      unconsumeByte(insn); /* unconsume byte1 */
+      unconsumeByte(insn); /* unconsume byte  */
+      insn->necessaryPrefixLocation = insn->readerCursor - 2;
+    }
+
+    if (insn->vectorExtensionType == TYPE_EVEX) {
+      insn->vectorExtensionPrefix[0] = byte;
+      insn->vectorExtensionPrefix[1] = byte1;
+      if (consumeByte(insn, &insn->vectorExtensionPrefix[2])) {
+        dbgprintf(insn, "Couldn't read third byte of EVEX prefix");
+        return -1;
+      }
+      if (consumeByte(insn, &insn->vectorExtensionPrefix[3])) {
+        dbgprintf(insn, "Couldn't read fourth byte of EVEX prefix");
+        return -1;
+      }
+
+      /* We simulate the REX prefix for simplicity's sake */
+      if (insn->mode == MODE_64BIT) {
+        insn->rexPrefix = 0x40
+                        | (wFromEVEX3of4(insn->vectorExtensionPrefix[2]) << 3)
+                        | (rFromEVEX2of4(insn->vectorExtensionPrefix[1]) << 2)
+                        | (xFromEVEX2of4(insn->vectorExtensionPrefix[1]) << 1)
+                        | (bFromEVEX2of4(insn->vectorExtensionPrefix[1]) << 0);
+      }
+
+      dbgprintf(insn, "Found EVEX prefix 0x%hhx 0x%hhx 0x%hhx 0x%hhx",
+              insn->vectorExtensionPrefix[0], insn->vectorExtensionPrefix[1],
+              insn->vectorExtensionPrefix[2], insn->vectorExtensionPrefix[3]);
+    }
+  }
+  else if (byte == 0xc4) {
+    uint8_t byte1;
+
+    if (lookAtByte(insn, &byte1)) {
+      dbgprintf(insn, "Couldn't read second byte of VEX");
+      return -1;
+    }
+
+    if (insn->mode == MODE_64BIT || (byte1 & 0xc0) == 0xc0) {
+      insn->vectorExtensionType = TYPE_VEX_3B;
+      insn->necessaryPrefixLocation = insn->readerCursor - 1;
+    }
+    else {
+      unconsumeByte(insn);
+      insn->necessaryPrefixLocation = insn->readerCursor - 1;
+    }
+
+    if (insn->vectorExtensionType == TYPE_VEX_3B) {
+      insn->vectorExtensionPrefix[0] = byte;
+      consumeByte(insn, &insn->vectorExtensionPrefix[1]);
+      consumeByte(insn, &insn->vectorExtensionPrefix[2]);
+
+      /* We simulate the REX prefix for simplicity's sake */
+
+      if (insn->mode == MODE_64BIT) {
+        insn->rexPrefix = 0x40
+                        | (wFromVEX3of3(insn->vectorExtensionPrefix[2]) << 3)
+                        | (rFromVEX2of3(insn->vectorExtensionPrefix[1]) << 2)
+                        | (xFromVEX2of3(insn->vectorExtensionPrefix[1]) << 1)
+                        | (bFromVEX2of3(insn->vectorExtensionPrefix[1]) << 0);
+      }
+
+      dbgprintf(insn, "Found VEX prefix 0x%hhx 0x%hhx 0x%hhx",
+                insn->vectorExtensionPrefix[0], insn->vectorExtensionPrefix[1],
+                insn->vectorExtensionPrefix[2]);
+    }
+  }
+  else if (byte == 0xc5) {
+    uint8_t byte1;
+
+    if (lookAtByte(insn, &byte1)) {
+      dbgprintf(insn, "Couldn't read second byte of VEX");
+      return -1;
+    }
+
+    if (insn->mode == MODE_64BIT || (byte1 & 0xc0) == 0xc0) {
+      insn->vectorExtensionType = TYPE_VEX_2B;
+    }
+    else {
+      unconsumeByte(insn);
+    }
+
+    if (insn->vectorExtensionType == TYPE_VEX_2B) {
+      insn->vectorExtensionPrefix[0] = byte;
+      consumeByte(insn, &insn->vectorExtensionPrefix[1]);
+
+      if (insn->mode == MODE_64BIT) {
+        insn->rexPrefix = 0x40
+                        | (rFromVEX2of2(insn->vectorExtensionPrefix[1]) << 2);
+      }
+
+      switch (ppFromVEX2of2(insn->vectorExtensionPrefix[1]))
+      {
+      default:
+        break;
+      case VEX_PREFIX_66:
+        hasOpSize = true;
+        break;
+      }
+
+      dbgprintf(insn, "Found VEX prefix 0x%hhx 0x%hhx",
+                insn->vectorExtensionPrefix[0],
+                insn->vectorExtensionPrefix[1]);
+    }
+  }
+  else if (byte == 0x8f) {
+    uint8_t byte1;
+
+    if (lookAtByte(insn, &byte1)) {
+      dbgprintf(insn, "Couldn't read second byte of XOP");
+      return -1;
+    }
+
+    if ((byte1 & 0x38) != 0x0) { /* 0 in these 3 bits is a POP instruction. */
+      insn->vectorExtensionType = TYPE_XOP;
+      insn->necessaryPrefixLocation = insn->readerCursor - 1;
+    }
+    else {
+      unconsumeByte(insn);
+      insn->necessaryPrefixLocation = insn->readerCursor - 1;
+    }
+
+    if (insn->vectorExtensionType == TYPE_XOP) {
+      insn->vectorExtensionPrefix[0] = byte;
+      consumeByte(insn, &insn->vectorExtensionPrefix[1]);
+      consumeByte(insn, &insn->vectorExtensionPrefix[2]);
+
+      /* We simulate the REX prefix for simplicity's sake */
+
+      if (insn->mode == MODE_64BIT) {
+        insn->rexPrefix = 0x40
+                        | (wFromXOP3of3(insn->vectorExtensionPrefix[2]) << 3)
+                        | (rFromXOP2of3(insn->vectorExtensionPrefix[1]) << 2)
+                        | (xFromXOP2of3(insn->vectorExtensionPrefix[1]) << 1)
+                        | (bFromXOP2of3(insn->vectorExtensionPrefix[1]) << 0);
+      }
+
+      switch (ppFromXOP3of3(insn->vectorExtensionPrefix[2]))
+      {
+      default:
+        break;
+      case VEX_PREFIX_66:
+        hasOpSize = true;
+        break;
+      }
+
+      dbgprintf(insn, "Found XOP prefix 0x%hhx 0x%hhx 0x%hhx",
+                insn->vectorExtensionPrefix[0], insn->vectorExtensionPrefix[1],
+                insn->vectorExtensionPrefix[2]);
+    }
+  }
+  else {
+    if (insn->mode == MODE_64BIT) {
+      if ((byte & 0xf0) == 0x40) {
+        uint8_t opcodeByte;
+
+        if (lookAtByte(insn, &opcodeByte) || ((opcodeByte & 0xf0) == 0x40)) {
+          dbgprintf(insn, "Redundant REX prefix");
+          return -1;
+        }
+
+        insn->rexPrefix = byte;
+        insn->necessaryPrefixLocation = insn->readerCursor - 2;
+
+        dbgprintf(insn, "Found REX prefix 0x%hhx", byte);
+      } else {
+        unconsumeByte(insn);
+        insn->necessaryPrefixLocation = insn->readerCursor - 1;
+      }
+    } else {
+      unconsumeByte(insn);
+      insn->necessaryPrefixLocation = insn->readerCursor - 1;
+    }
+  }
+
+  if (insn->mode == MODE_16BIT) {
+    insn->registerSize       = (hasOpSize ? 4 : 2);
+    insn->addressSize        = (hasAdSize ? 4 : 2);
+    insn->displacementSize   = (hasAdSize ? 4 : 2);
+    insn->immediateSize      = (hasOpSize ? 4 : 2);
+  } else if (insn->mode == MODE_32BIT) {
+    insn->registerSize       = (hasOpSize ? 2 : 4);
+    insn->addressSize        = (hasAdSize ? 2 : 4);
+    insn->displacementSize   = (hasAdSize ? 2 : 4);
+    insn->immediateSize      = (hasOpSize ? 2 : 4);
+  } else if (insn->mode == MODE_64BIT) {
+    if (insn->rexPrefix && wFromREX(insn->rexPrefix)) {
+      insn->registerSize       = 8;
+      insn->addressSize        = (hasAdSize ? 4 : 8);
+      insn->displacementSize   = 4;
+      insn->immediateSize      = 4;
+    } else if (insn->rexPrefix) {
+      insn->registerSize       = (hasOpSize ? 2 : 4);
+      insn->addressSize        = (hasAdSize ? 4 : 8);
+      insn->displacementSize   = (hasOpSize ? 2 : 4);
+      insn->immediateSize      = (hasOpSize ? 2 : 4);
+    } else {
+      insn->registerSize       = (hasOpSize ? 2 : 4);
+      insn->addressSize        = (hasAdSize ? 4 : 8);
+      insn->displacementSize   = (hasOpSize ? 2 : 4);
+      insn->immediateSize      = (hasOpSize ? 2 : 4);
+    }
+  }
+
+  return 0;
+}
+
+/*
+ * readOpcode - Reads the opcode (excepting the ModR/M byte in the case of
+ *   extended or escape opcodes).
+ *
+ * @param insn  - The instruction whose opcode is to be read.
+ * @return      - 0 if the opcode could be read successfully; nonzero otherwise.
+ */
+static int readOpcode(struct InternalInstruction* insn) {
+  /* Determine the length of the primary opcode */
+
+  uint8_t current;
+
+  dbgprintf(insn, "readOpcode()");
+
+  insn->opcodeType = ONEBYTE;
+
+  if (insn->vectorExtensionType == TYPE_EVEX)
+  {
+    switch (mmFromEVEX2of4(insn->vectorExtensionPrefix[1])) {
+    default:
+      dbgprintf(insn, "Unhandled mm field for instruction (0x%hhx)",
+                mmFromEVEX2of4(insn->vectorExtensionPrefix[1]));
+      return -1;
+    case VEX_LOB_0F:
+      insn->opcodeType = TWOBYTE;
+      return consumeByte(insn, &insn->opcode);
+    case VEX_LOB_0F38:
+      insn->opcodeType = THREEBYTE_38;
+      return consumeByte(insn, &insn->opcode);
+    case VEX_LOB_0F3A:
+      insn->opcodeType = THREEBYTE_3A;
+      return consumeByte(insn, &insn->opcode);
+    }
+  }
+  else if (insn->vectorExtensionType == TYPE_VEX_3B) {
+    switch (mmmmmFromVEX2of3(insn->vectorExtensionPrefix[1])) {
+    default:
+      dbgprintf(insn, "Unhandled m-mmmm field for instruction (0x%hhx)",
+                mmmmmFromVEX2of3(insn->vectorExtensionPrefix[1]));
+      return -1;
+    case VEX_LOB_0F:
+      insn->opcodeType = TWOBYTE;
+      return consumeByte(insn, &insn->opcode);
+    case VEX_LOB_0F38:
+      insn->opcodeType = THREEBYTE_38;
+      return consumeByte(insn, &insn->opcode);
+    case VEX_LOB_0F3A:
+      insn->opcodeType = THREEBYTE_3A;
+      return consumeByte(insn, &insn->opcode);
+    }
+  }
+  else if (insn->vectorExtensionType == TYPE_VEX_2B) {
+    insn->opcodeType = TWOBYTE;
+    return consumeByte(insn, &insn->opcode);
+  }
+  else if (insn->vectorExtensionType == TYPE_XOP) {
+    switch (mmmmmFromXOP2of3(insn->vectorExtensionPrefix[1])) {
+    default:
+      dbgprintf(insn, "Unhandled m-mmmm field for instruction (0x%hhx)",
+                mmmmmFromVEX2of3(insn->vectorExtensionPrefix[1]));
+      return -1;
+    case XOP_MAP_SELECT_8:
+      insn->opcodeType = XOP8_MAP;
+      return consumeByte(insn, &insn->opcode);
+    case XOP_MAP_SELECT_9:
+      insn->opcodeType = XOP9_MAP;
+      return consumeByte(insn, &insn->opcode);
+    case XOP_MAP_SELECT_A:
+      insn->opcodeType = XOPA_MAP;
+      return consumeByte(insn, &insn->opcode);
+    }
+  }
+
+  if (consumeByte(insn, &current))
+    return -1;
+
+  if (current == 0x0f) {
+    dbgprintf(insn, "Found a two-byte escape prefix (0x%hhx)", current);
+
+    if (consumeByte(insn, &current))
+      return -1;
+
+    if (current == 0x38) {
+      dbgprintf(insn, "Found a three-byte escape prefix (0x%hhx)", current);
+
+      if (consumeByte(insn, &current))
+        return -1;
+
+      insn->opcodeType = THREEBYTE_38;
+    } else if (current == 0x3a) {
+      dbgprintf(insn, "Found a three-byte escape prefix (0x%hhx)", current);
+
+      if (consumeByte(insn, &current))
+        return -1;
+
+      insn->opcodeType = THREEBYTE_3A;
+    } else {
+      dbgprintf(insn, "Didn't find a three-byte escape prefix");
+
+      insn->opcodeType = TWOBYTE;
+    }
+  }
+
+  /*
+   * At this point we have consumed the full opcode.
+   * Anything we consume from here on must be unconsumed.
+   */
+
+  insn->opcode = current;
+
+  return 0;
+}
+
+static int readModRM(struct InternalInstruction* insn);
+
+/*
+ * getIDWithAttrMask - Determines the ID of an instruction, consuming
+ *   the ModR/M byte as appropriate for extended and escape opcodes,
+ *   and using a supplied attribute mask.
+ *
+ * @param instructionID - A pointer whose target is filled in with the ID of the
+ *                        instruction.
+ * @param insn          - The instruction whose ID is to be determined.
+ * @param attrMask      - The attribute mask to search.
+ * @return              - 0 if the ModR/M could be read when needed or was not
+ *                        needed; nonzero otherwise.
+ */
+static int getIDWithAttrMask(uint16_t* instructionID,
+                             struct InternalInstruction* insn,
+                             uint16_t attrMask) {
+  bool hasModRMExtension;
+
+  InstructionContext instructionClass = contextForAttrs(attrMask);
+
+  hasModRMExtension = modRMRequired(insn->opcodeType,
+                                    instructionClass,
+                                    insn->opcode);
+
+  if (hasModRMExtension) {
+    if (readModRM(insn))
+      return -1;
+
+    *instructionID = decode(insn->opcodeType,
+                            instructionClass,
+                            insn->opcode,
+                            insn->modRM);
+  } else {
+    *instructionID = decode(insn->opcodeType,
+                            instructionClass,
+                            insn->opcode,
+                            0);
+  }
+
+  return 0;
+}
+
+/*
+ * is16BitEquivalent - Determines whether two instruction names refer to
+ * equivalent instructions but one is 16-bit whereas the other is not.
+ *
+ * @param orig  - The instruction that is not 16-bit
+ * @param equiv - The instruction that is 16-bit
+ */
+static bool is16BitEquivalent(const char* orig, const char* equiv) {
+  off_t i;
+
+  for (i = 0;; i++) {
+    if (orig[i] == '\0' && equiv[i] == '\0')
+      return true;
+    if (orig[i] == '\0' || equiv[i] == '\0')
+      return false;
+    if (orig[i] != equiv[i]) {
+      if ((orig[i] == 'Q' || orig[i] == 'L') && equiv[i] == 'W')
+        continue;
+      if ((orig[i] == '6' || orig[i] == '3') && equiv[i] == '1')
+        continue;
+      if ((orig[i] == '4' || orig[i] == '2') && equiv[i] == '6')
+        continue;
+      return false;
+    }
+  }
+}
+
+/*
+ * getID - Determines the ID of an instruction, consuming the ModR/M byte as
+ *   appropriate for extended and escape opcodes.  Determines the attributes and
+ *   context for the instruction before doing so.
+ *
+ * @param insn  - The instruction whose ID is to be determined.
+ * @return      - 0 if the ModR/M could be read when needed or was not needed;
+ *                nonzero otherwise.
+ */
+static int getID(struct InternalInstruction* insn, const void *miiArg) {
+  uint16_t attrMask;
+  uint16_t instructionID;
+
+  dbgprintf(insn, "getID()");
+
+  attrMask = ATTR_NONE;
+
+  if (insn->mode == MODE_64BIT)
+    attrMask |= ATTR_64BIT;
+
+  if (insn->vectorExtensionType != TYPE_NO_VEX_XOP) {
+    attrMask |= (insn->vectorExtensionType == TYPE_EVEX) ? ATTR_EVEX : ATTR_VEX;
+
+    if (insn->vectorExtensionType == TYPE_EVEX) {
+      switch (ppFromEVEX3of4(insn->vectorExtensionPrefix[2])) {
+      case VEX_PREFIX_66:
+        attrMask |= ATTR_OPSIZE;
+        break;
+      case VEX_PREFIX_F3:
+        attrMask |= ATTR_XS;
+        break;
+      case VEX_PREFIX_F2:
+        attrMask |= ATTR_XD;
+        break;
+      }
+
+      if (zFromEVEX4of4(insn->vectorExtensionPrefix[3]))
+        attrMask |= ATTR_EVEXKZ;
+      if (bFromEVEX4of4(insn->vectorExtensionPrefix[3]))
+        attrMask |= ATTR_EVEXB;
+      if (aaaFromEVEX4of4(insn->vectorExtensionPrefix[3]))
+        attrMask |= ATTR_EVEXK;
+      if (lFromEVEX4of4(insn->vectorExtensionPrefix[3]))
+        attrMask |= ATTR_EVEXL;
+      if (l2FromEVEX4of4(insn->vectorExtensionPrefix[3]))
+        attrMask |= ATTR_EVEXL2;
+    }
+    else if (insn->vectorExtensionType == TYPE_VEX_3B) {
+      switch (ppFromVEX3of3(insn->vectorExtensionPrefix[2])) {
+      case VEX_PREFIX_66:
+        attrMask |= ATTR_OPSIZE;
+        break;
+      case VEX_PREFIX_F3:
+        attrMask |= ATTR_XS;
+        break;
+      case VEX_PREFIX_F2:
+        attrMask |= ATTR_XD;
+        break;
+      }
+
+      if (lFromVEX3of3(insn->vectorExtensionPrefix[2]))
+        attrMask |= ATTR_VEXL;
+    }
+    else if (insn->vectorExtensionType == TYPE_VEX_2B) {
+      switch (ppFromVEX2of2(insn->vectorExtensionPrefix[1])) {
+      case VEX_PREFIX_66:
+        attrMask |= ATTR_OPSIZE;
+        break;
+      case VEX_PREFIX_F3:
+        attrMask |= ATTR_XS;
+        break;
+      case VEX_PREFIX_F2:
+        attrMask |= ATTR_XD;
+        break;
+      }
+
+      if (lFromVEX2of2(insn->vectorExtensionPrefix[1]))
+        attrMask |= ATTR_VEXL;
+    }
+    else if (insn->vectorExtensionType == TYPE_XOP) {
+      switch (ppFromXOP3of3(insn->vectorExtensionPrefix[2])) {
+      case VEX_PREFIX_66:
+        attrMask |= ATTR_OPSIZE;
+        break;
+      case VEX_PREFIX_F3:
+        attrMask |= ATTR_XS;
+        break;
+      case VEX_PREFIX_F2:
+        attrMask |= ATTR_XD;
+        break;
+      }
+
+      if (lFromXOP3of3(insn->vectorExtensionPrefix[2]))
+        attrMask |= ATTR_VEXL;
+    }
+    else {
+      return -1;
+    }
+  }
+  else {
+    if (insn->mode != MODE_16BIT && isPrefixAtLocation(insn, 0x66, insn->necessaryPrefixLocation))
+      attrMask |= ATTR_OPSIZE;
+    else if (isPrefixAtLocation(insn, 0x67, insn->necessaryPrefixLocation))
+      attrMask |= ATTR_ADSIZE;
+    else if (isPrefixAtLocation(insn, 0xf3, insn->necessaryPrefixLocation))
+      attrMask |= ATTR_XS;
+    else if (isPrefixAtLocation(insn, 0xf2, insn->necessaryPrefixLocation))
+      attrMask |= ATTR_XD;
+  }
+
+  if (insn->rexPrefix & 0x08)
+    attrMask |= ATTR_REXW;
+
+  if (getIDWithAttrMask(&instructionID, insn, attrMask))
+    return -1;
+
+  /*
+   * JCXZ/JECXZ need special handling for 16-bit mode because the meaning
+   * of the AdSize prefix is inverted w.r.t. 32-bit mode.
+   */
+  if (insn->mode == MODE_16BIT && insn->opcode == 0xE3) {
+    const struct InstructionSpecifier *spec;
+    spec = specifierForUID(instructionID);
+
+    /*
+     * Check for Ii8PCRel instructions. We could alternatively do a
+     * string-compare on the names, but this is probably cheaper.
+     */
+    if (x86OperandSets[spec->operands][0].type == TYPE_REL8) {
+      attrMask ^= ATTR_ADSIZE;
+      if (getIDWithAttrMask(&instructionID, insn, attrMask))
+        return -1;
+    }
+  }
+
+  /* The following clauses compensate for limitations of the tables. */
+
+  if ((insn->mode == MODE_16BIT || insn->prefixPresent[0x66]) &&
+      !(attrMask & ATTR_OPSIZE)) {
+    /*
+     * The instruction tables make no distinction between instructions that
+     * allow OpSize anywhere (i.e., 16-bit operations) and that need it in a
+     * particular spot (i.e., many MMX operations).  In general we're
+     * conservative, but in the specific case where OpSize is present but not
+     * in the right place we check if there's a 16-bit operation.
+     */
+
+    const struct InstructionSpecifier *spec;
+    uint16_t instructionIDWithOpsize;
+    const char *specName, *specWithOpSizeName;
+
+    spec = specifierForUID(instructionID);
+
+    if (getIDWithAttrMask(&instructionIDWithOpsize,
+                          insn,
+                          attrMask | ATTR_OPSIZE)) {
+      /*
+       * ModRM required with OpSize but not present; give up and return version
+       * without OpSize set
+       */
+
+      insn->instructionID = instructionID;
+      insn->spec = spec;
+      return 0;
+    }
+
+    specName = GetInstrName(instructionID, miiArg);
+    specWithOpSizeName = GetInstrName(instructionIDWithOpsize, miiArg);
+
+    if (is16BitEquivalent(specName, specWithOpSizeName) &&
+        (insn->mode == MODE_16BIT) ^ insn->prefixPresent[0x66]) {
+      insn->instructionID = instructionIDWithOpsize;
+      insn->spec = specifierForUID(instructionIDWithOpsize);
+    } else {
+      insn->instructionID = instructionID;
+      insn->spec = spec;
+    }
+    return 0;
+  }
+
+  if (insn->opcodeType == ONEBYTE && insn->opcode == 0x90 &&
+      insn->rexPrefix & 0x01) {
+    /*
+     * NOOP shouldn't decode as NOOP if REX.b is set. Instead
+     * it should decode as XCHG %r8, %eax.
+     */
+
+    const struct InstructionSpecifier *spec;
+    uint16_t instructionIDWithNewOpcode;
+    const struct InstructionSpecifier *specWithNewOpcode;
+
+    spec = specifierForUID(instructionID);
+
+    /* Borrow opcode from one of the other XCHGar opcodes */
+    insn->opcode = 0x91;
+
+    if (getIDWithAttrMask(&instructionIDWithNewOpcode,
+                          insn,
+                          attrMask)) {
+      insn->opcode = 0x90;
+
+      insn->instructionID = instructionID;
+      insn->spec = spec;
+      return 0;
+    }
+
+    specWithNewOpcode = specifierForUID(instructionIDWithNewOpcode);
+
+    /* Change back */
+    insn->opcode = 0x90;
+
+    insn->instructionID = instructionIDWithNewOpcode;
+    insn->spec = specWithNewOpcode;
+
+    return 0;
+  }
+
+  insn->instructionID = instructionID;
+  insn->spec = specifierForUID(insn->instructionID);
+
+  return 0;
+}
+
+/*
+ * readSIB - Consumes the SIB byte to determine addressing information for an
+ *   instruction.
+ *
+ * @param insn  - The instruction whose SIB byte is to be read.
+ * @return      - 0 if the SIB byte was successfully read; nonzero otherwise.
+ */
+static int readSIB(struct InternalInstruction* insn) {
+  SIBIndex sibIndexBase = SIB_INDEX_NONE;
+  SIBBase sibBaseBase = SIB_BASE_NONE;
+  uint8_t index, base;
+
+  dbgprintf(insn, "readSIB()");
+
+  if (insn->consumedSIB)
+    return 0;
+
+  insn->consumedSIB = true;
+
+  switch (insn->addressSize) {
+  case 2:
+    dbgprintf(insn, "SIB-based addressing doesn't work in 16-bit mode");
+    return -1;
+  case 4:
+    sibIndexBase = SIB_INDEX_EAX;
+    sibBaseBase = SIB_BASE_EAX;
+    break;
+  case 8:
+    sibIndexBase = SIB_INDEX_RAX;
+    sibBaseBase = SIB_BASE_RAX;
+    break;
+  }
+
+  if (consumeByte(insn, &insn->sib))
+    return -1;
+
+  index = indexFromSIB(insn->sib) | (xFromREX(insn->rexPrefix) << 3);
+  if (insn->vectorExtensionType == TYPE_EVEX)
+    index |= v2FromEVEX4of4(insn->vectorExtensionPrefix[3]) << 4;
+
+  switch (index) {
+  case 0x4:
+    insn->sibIndex = SIB_INDEX_NONE;
+    break;
+  default:
+    insn->sibIndex = (SIBIndex)(sibIndexBase + index);
+    if (insn->sibIndex == SIB_INDEX_sib ||
+        insn->sibIndex == SIB_INDEX_sib64)
+      insn->sibIndex = SIB_INDEX_NONE;
+    break;
+  }
+
+  switch (scaleFromSIB(insn->sib)) {
+  case 0:
+    insn->sibScale = 1;
+    break;
+  case 1:
+    insn->sibScale = 2;
+    break;
+  case 2:
+    insn->sibScale = 4;
+    break;
+  case 3:
+    insn->sibScale = 8;
+    break;
+  }
+
+  base = baseFromSIB(insn->sib) | (bFromREX(insn->rexPrefix) << 3);
+
+  switch (base) {
+  case 0x5:
+  case 0xd:
+    switch (modFromModRM(insn->modRM)) {
+    case 0x0:
+      insn->eaDisplacement = EA_DISP_32;
+      insn->sibBase = SIB_BASE_NONE;
+      break;
+    case 0x1:
+      insn->eaDisplacement = EA_DISP_8;
+      insn->sibBase = (SIBBase)(sibBaseBase + base);
+      break;
+    case 0x2:
+      insn->eaDisplacement = EA_DISP_32;
+      insn->sibBase = (SIBBase)(sibBaseBase + base);
+      break;
+    case 0x3:
+      debug("Cannot have Mod = 0b11 and a SIB byte");
+      return -1;
+    }
+    break;
+  default:
+    insn->sibBase = (SIBBase)(sibBaseBase + base);
+    break;
+  }
+
+  return 0;
+}
+
+/*
+ * readDisplacement - Consumes the displacement of an instruction.
+ *
+ * @param insn  - The instruction whose displacement is to be read.
+ * @return      - 0 if the displacement byte was successfully read; nonzero
+ *                otherwise.
+ */
+static int readDisplacement(struct InternalInstruction* insn) {
+  int8_t d8;
+  int16_t d16;
+  int32_t d32;
+
+  dbgprintf(insn, "readDisplacement()");
+
+  if (insn->consumedDisplacement)
+    return 0;
+
+  insn->consumedDisplacement = true;
+  insn->displacementOffset = insn->readerCursor - insn->startLocation;
+
+  switch (insn->eaDisplacement) {
+  case EA_DISP_NONE:
+    insn->consumedDisplacement = false;
+    break;
+  case EA_DISP_8:
+    if (consumeInt8(insn, &d8))
+      return -1;
+    insn->displacement = d8;
+    break;
+  case EA_DISP_16:
+    if (consumeInt16(insn, &d16))
+      return -1;
+    insn->displacement = d16;
+    break;
+  case EA_DISP_32:
+    if (consumeInt32(insn, &d32))
+      return -1;
+    insn->displacement = d32;
+    break;
+  }
+
+  insn->consumedDisplacement = true;
+  return 0;
+}
+
+/*
+ * readModRM - Consumes all addressing information (ModR/M byte, SIB byte, and
+ *   displacement) for an instruction and interprets it.
+ *
+ * @param insn  - The instruction whose addressing information is to be read.
+ * @return      - 0 if the information was successfully read; nonzero otherwise.
+ */
+static int readModRM(struct InternalInstruction* insn) {
+  uint8_t mod, rm, reg;
+
+  dbgprintf(insn, "readModRM()");
+
+  if (insn->consumedModRM)
+    return 0;
+
+  if (consumeByte(insn, &insn->modRM))
+    return -1;
+  insn->consumedModRM = true;
+
+  mod     = modFromModRM(insn->modRM);
+  rm      = rmFromModRM(insn->modRM);
+  reg     = regFromModRM(insn->modRM);
+
+  /*
+   * This goes by insn->registerSize to pick the correct register, which messes
+   * up if we're using (say) XMM or 8-bit register operands.  That gets fixed in
+   * fixupReg().
+   */
+  switch (insn->registerSize) {
+  case 2:
+    insn->regBase = MODRM_REG_AX;
+    insn->eaRegBase = EA_REG_AX;
+    break;
+  case 4:
+    insn->regBase = MODRM_REG_EAX;
+    insn->eaRegBase = EA_REG_EAX;
+    break;
+  case 8:
+    insn->regBase = MODRM_REG_RAX;
+    insn->eaRegBase = EA_REG_RAX;
+    break;
+  }
+
+  reg |= rFromREX(insn->rexPrefix) << 3;
+  rm  |= bFromREX(insn->rexPrefix) << 3;
+  if (insn->vectorExtensionType == TYPE_EVEX) {
+    reg |= r2FromEVEX2of4(insn->vectorExtensionPrefix[1]) << 4;
+    rm  |=  xFromEVEX2of4(insn->vectorExtensionPrefix[1]) << 4;
+  }
+
+  insn->reg = (Reg)(insn->regBase + reg);
+
+  switch (insn->addressSize) {
+  case 2:
+    insn->eaBaseBase = EA_BASE_BX_SI;
+
+    switch (mod) {
+    case 0x0:
+      if (rm == 0x6) {
+        insn->eaBase = EA_BASE_NONE;
+        insn->eaDisplacement = EA_DISP_16;
+        if (readDisplacement(insn))
+          return -1;
+      } else {
+        insn->eaBase = (EABase)(insn->eaBaseBase + rm);
+        insn->eaDisplacement = EA_DISP_NONE;
+      }
+      break;
+    case 0x1:
+      insn->eaBase = (EABase)(insn->eaBaseBase + rm);
+      insn->eaDisplacement = EA_DISP_8;
+      insn->displacementSize = 1;
+      if (readDisplacement(insn))
+        return -1;
+      break;
+    case 0x2:
+      insn->eaBase = (EABase)(insn->eaBaseBase + rm);
+      insn->eaDisplacement = EA_DISP_16;
+      if (readDisplacement(insn))
+        return -1;
+      break;
+    case 0x3:
+      insn->eaBase = (EABase)(insn->eaRegBase + rm);
+      if (readDisplacement(insn))
+        return -1;
+      break;
+    }
+    break;
+  case 4:
+  case 8:
+    insn->eaBaseBase = (insn->addressSize == 4 ? EA_BASE_EAX : EA_BASE_RAX);
+
+    switch (mod) {
+    case 0x0:
+      insn->eaDisplacement = EA_DISP_NONE; /* readSIB may override this */
+      switch (rm) {
+      case 0x14:
+      case 0x4:
+      case 0xc:   /* in case REXW.b is set */
+        insn->eaBase = (insn->addressSize == 4 ?
+                        EA_BASE_sib : EA_BASE_sib64);
+        if (readSIB(insn) || readDisplacement(insn))
+          return -1;
+        break;
+      case 0x5:
+        insn->eaBase = EA_BASE_NONE;
+        insn->eaDisplacement = EA_DISP_32;
+        if (readDisplacement(insn))
+          return -1;
+        break;
+      default:
+        insn->eaBase = (EABase)(insn->eaBaseBase + rm);
+        break;
+      }
+      break;
+    case 0x1:
+      insn->displacementSize = 1;
+      /* FALLTHROUGH */
+    case 0x2:
+      insn->eaDisplacement = (mod == 0x1 ? EA_DISP_8 : EA_DISP_32);
+      switch (rm) {
+      case 0x14:
+      case 0x4:
+      case 0xc:   /* in case REXW.b is set */
+        insn->eaBase = EA_BASE_sib;
+        if (readSIB(insn) || readDisplacement(insn))
+          return -1;
+        break;
+      default:
+        insn->eaBase = (EABase)(insn->eaBaseBase + rm);
+        if (readDisplacement(insn))
+          return -1;
+        break;
+      }
+      break;
+    case 0x3:
+      insn->eaDisplacement = EA_DISP_NONE;
+      insn->eaBase = (EABase)(insn->eaRegBase + rm);
+      break;
+    }
+    break;
+  } /* switch (insn->addressSize) */
+
+  return 0;
+}
+
+#define GENERIC_FIXUP_FUNC(name, base, prefix)            \
+  static uint8_t name(struct InternalInstruction *insn,   \
+                      OperandType type,                   \
+                      uint8_t index,                      \
+                      uint8_t *valid) {                   \
+    *valid = 1;                                           \
+    switch (type) {                                       \
+    default:                                              \
+      debug("Unhandled register type");                   \
+      *valid = 0;                                         \
+      return 0;                                           \
+    case TYPE_Rv:                                         \
+      return base + index;                                \
+    case TYPE_R8:                                         \
+      if (insn->rexPrefix &&                              \
+         index >= 4 && index <= 7) {                      \
+        return prefix##_SPL + (index - 4);                \
+      } else {                                            \
+        return prefix##_AL + index;                       \
+      }                                                   \
+    case TYPE_R16:                                        \
+      return prefix##_AX + index;                         \
+    case TYPE_R32:                                        \
+      return prefix##_EAX + index;                        \
+    case TYPE_R64:                                        \
+      return prefix##_RAX + index;                        \
+    case TYPE_XMM512:                                     \
+      return prefix##_ZMM0 + index;                       \
+    case TYPE_XMM256:                                     \
+      return prefix##_YMM0 + index;                       \
+    case TYPE_XMM128:                                     \
+    case TYPE_XMM64:                                      \
+    case TYPE_XMM32:                                      \
+    case TYPE_XMM:                                        \
+      return prefix##_XMM0 + index;                       \
+    case TYPE_VK1:                                        \
+    case TYPE_VK8:                                        \
+    case TYPE_VK16:                                       \
+      return prefix##_K0 + index;                         \
+    case TYPE_MM64:                                       \
+    case TYPE_MM32:                                       \
+    case TYPE_MM:                                         \
+      if (index > 7)                                      \
+        *valid = 0;                                       \
+      return prefix##_MM0 + index;                        \
+    case TYPE_SEGMENTREG:                                 \
+      if (index > 5)                                      \
+        *valid = 0;                                       \
+      return prefix##_ES + index;                         \
+    case TYPE_DEBUGREG:                                   \
+      if (index > 7)                                      \
+        *valid = 0;                                       \
+      return prefix##_DR0 + index;                        \
+    case TYPE_CONTROLREG:                                 \
+      if (index > 8)                                      \
+        *valid = 0;                                       \
+      return prefix##_CR0 + index;                        \
+    }                                                     \
+  }
+
+/*
+ * fixup*Value - Consults an operand type to determine the meaning of the
+ *   reg or R/M field.  If the operand is an XMM operand, for example, an
+ *   operand would be XMM0 instead of AX, which readModRM() would otherwise
+ *   misinterpret it as.
+ *
+ * @param insn  - The instruction containing the operand.
+ * @param type  - The operand type.
+ * @param index - The existing value of the field as reported by readModRM().
+ * @param valid - The address of a uint8_t.  The target is set to 1 if the
+ *                field is valid for the register class; 0 if not.
+ * @return      - The proper value.
+ */
+GENERIC_FIXUP_FUNC(fixupRegValue, insn->regBase,    MODRM_REG)
+GENERIC_FIXUP_FUNC(fixupRMValue,  insn->eaRegBase,  EA_REG)
+
+/*
+ * fixupReg - Consults an operand specifier to determine which of the
+ *   fixup*Value functions to use in correcting readModRM()'ss interpretation.
+ *
+ * @param insn  - See fixup*Value().
+ * @param op    - The operand specifier.
+ * @return      - 0 if fixup was successful; -1 if the register returned was
+ *                invalid for its class.
+ */
+static int fixupReg(struct InternalInstruction *insn,
+                    const struct OperandSpecifier *op) {
+  uint8_t valid;
+
+  dbgprintf(insn, "fixupReg()");
+
+  switch ((OperandEncoding)op->encoding) {
+  default:
+    debug("Expected a REG or R/M encoding in fixupReg");
+    return -1;
+  case ENCODING_VVVV:
+    insn->vvvv = (Reg)fixupRegValue(insn,
+                                    (OperandType)op->type,
+                                    insn->vvvv,
+                                    &valid);
+    if (!valid)
+      return -1;
+    break;
+  case ENCODING_REG:
+    insn->reg = (Reg)fixupRegValue(insn,
+                                   (OperandType)op->type,
+                                   insn->reg - insn->regBase,
+                                   &valid);
+    if (!valid)
+      return -1;
+    break;
+  CASE_ENCODING_RM:
+    if (insn->eaBase >= insn->eaRegBase) {
+      insn->eaBase = (EABase)fixupRMValue(insn,
+                                          (OperandType)op->type,
+                                          insn->eaBase - insn->eaRegBase,
+                                          &valid);
+      if (!valid)
+        return -1;
+    }
+    break;
+  }
+
+  return 0;
+}
+
+/*
+ * readOpcodeRegister - Reads an operand from the opcode field of an
+ *   instruction and interprets it appropriately given the operand width.
+ *   Handles AddRegFrm instructions.
+ *
+ * @param insn  - the instruction whose opcode field is to be read.
+ * @param size  - The width (in bytes) of the register being specified.
+ *                1 means AL and friends, 2 means AX, 4 means EAX, and 8 means
+ *                RAX.
+ * @return      - 0 on success; nonzero otherwise.
+ */
+static int readOpcodeRegister(struct InternalInstruction* insn, uint8_t size) {
+  dbgprintf(insn, "readOpcodeRegister()");
+
+  if (size == 0)
+    size = insn->registerSize;
+
+  switch (size) {
+  case 1:
+    insn->opcodeRegister = (Reg)(MODRM_REG_AL + ((bFromREX(insn->rexPrefix) << 3)
+                                                  | (insn->opcode & 7)));
+    if (insn->rexPrefix &&
+        insn->opcodeRegister >= MODRM_REG_AL + 0x4 &&
+        insn->opcodeRegister < MODRM_REG_AL + 0x8) {
+      insn->opcodeRegister = (Reg)(MODRM_REG_SPL
+                                   + (insn->opcodeRegister - MODRM_REG_AL - 4));
+    }
+
+    break;
+  case 2:
+    insn->opcodeRegister = (Reg)(MODRM_REG_AX
+                                 + ((bFromREX(insn->rexPrefix) << 3)
+                                    | (insn->opcode & 7)));
+    break;
+  case 4:
+    insn->opcodeRegister = (Reg)(MODRM_REG_EAX
+                                 + ((bFromREX(insn->rexPrefix) << 3)
+                                    | (insn->opcode & 7)));
+    break;
+  case 8:
+    insn->opcodeRegister = (Reg)(MODRM_REG_RAX
+                                 + ((bFromREX(insn->rexPrefix) << 3)
+                                    | (insn->opcode & 7)));
+    break;
+  }
+
+  return 0;
+}
+
+/*
+ * readImmediate - Consumes an immediate operand from an instruction, given the
+ *   desired operand size.
+ *
+ * @param insn  - The instruction whose operand is to be read.
+ * @param size  - The width (in bytes) of the operand.
+ * @return      - 0 if the immediate was successfully consumed; nonzero
+ *                otherwise.
+ */
+static int readImmediate(struct InternalInstruction* insn, uint8_t size) {
+  uint8_t imm8;
+  uint16_t imm16;
+  uint32_t imm32;
+  uint64_t imm64;
+
+  dbgprintf(insn, "readImmediate()");
+
+  if (insn->numImmediatesConsumed == 2) {
+    debug("Already consumed two immediates");
+    return -1;
+  }
+
+  if (size == 0)
+    size = insn->immediateSize;
+  else
+    insn->immediateSize = size;
+  insn->immediateOffset = insn->readerCursor - insn->startLocation;
+
+  switch (size) {
+  case 1:
+    if (consumeByte(insn, &imm8))
+      return -1;
+    insn->immediates[insn->numImmediatesConsumed] = imm8;
+    break;
+  case 2:
+    if (consumeUInt16(insn, &imm16))
+      return -1;
+    insn->immediates[insn->numImmediatesConsumed] = imm16;
+    break;
+  case 4:
+    if (consumeUInt32(insn, &imm32))
+      return -1;
+    insn->immediates[insn->numImmediatesConsumed] = imm32;
+    break;
+  case 8:
+    if (consumeUInt64(insn, &imm64))
+      return -1;
+    insn->immediates[insn->numImmediatesConsumed] = imm64;
+    break;
+  }
+
+  insn->numImmediatesConsumed++;
+
+  return 0;
+}
+
+/*
+ * readVVVV - Consumes vvvv from an instruction if it has a VEX prefix.
+ *
+ * @param insn  - The instruction whose operand is to be read.
+ * @return      - 0 if the vvvv was successfully consumed; nonzero
+ *                otherwise.
+ */
+static int readVVVV(struct InternalInstruction* insn) {
+  dbgprintf(insn, "readVVVV()");
+
+  int vvvv;
+  if (insn->vectorExtensionType == TYPE_EVEX)
+    vvvv = (v2FromEVEX4of4(insn->vectorExtensionPrefix[3]) << 4 |
+            vvvvFromEVEX3of4(insn->vectorExtensionPrefix[2]));
+  else if (insn->vectorExtensionType == TYPE_VEX_3B)
+    vvvv = vvvvFromVEX3of3(insn->vectorExtensionPrefix[2]);
+  else if (insn->vectorExtensionType == TYPE_VEX_2B)
+    vvvv = vvvvFromVEX2of2(insn->vectorExtensionPrefix[1]);
+  else if (insn->vectorExtensionType == TYPE_XOP)
+    vvvv = vvvvFromXOP3of3(insn->vectorExtensionPrefix[2]);
+  else
+    return -1;
+
+  if (insn->mode != MODE_64BIT)
+    vvvv &= 0x7;
+
+  insn->vvvv = static_cast<Reg>(vvvv);
+  return 0;
+}
+
+/*
+ * readMaskRegister - Reads an mask register from the opcode field of an
+ *   instruction.
+ *
+ * @param insn    - The instruction whose opcode field is to be read.
+ * @return        - 0 on success; nonzero otherwise.
+ */
+static int readMaskRegister(struct InternalInstruction* insn) {
+  dbgprintf(insn, "readMaskRegister()");
+
+  if (insn->vectorExtensionType != TYPE_EVEX)
+    return -1;
+
+  insn->writemask =
+      static_cast<Reg>(aaaFromEVEX4of4(insn->vectorExtensionPrefix[3]));
+  return 0;
+}
+
+/*
+ * readOperands - Consults the specifier for an instruction and consumes all
+ *   operands for that instruction, interpreting them as it goes.
+ *
+ * @param insn  - The instruction whose operands are to be read and interpreted.
+ * @return      - 0 if all operands could be read; nonzero otherwise.
+ */
+static int readOperands(struct InternalInstruction* insn) {
+  int hasVVVV, needVVVV;
+  int sawRegImm = 0;
+
+  dbgprintf(insn, "readOperands()");
+
+  /* If non-zero vvvv specified, need to make sure one of the operands
+     uses it. */
+  hasVVVV = !readVVVV(insn);
+  needVVVV = hasVVVV && (insn->vvvv != 0);
+
+  for (const auto &Op : x86OperandSets[insn->spec->operands]) {
+    switch (Op.encoding) {
+    case ENCODING_NONE:
+    case ENCODING_SI:
+    case ENCODING_DI:
+      break;
+    case ENCODING_REG:
+    CASE_ENCODING_RM:
+      if (readModRM(insn))
+        return -1;
+      if (fixupReg(insn, &Op))
+        return -1;
+      // Apply the AVX512 compressed displacement scaling factor.
+      if (Op.encoding != ENCODING_REG && insn->eaDisplacement == EA_DISP_8)
+        insn->displacement *= 1 << (Op.encoding - ENCODING_RM);
+      break;
+    case ENCODING_CB:
+    case ENCODING_CW:
+    case ENCODING_CD:
+    case ENCODING_CP:
+    case ENCODING_CO:
+    case ENCODING_CT:
+      dbgprintf(insn, "We currently don't hande code-offset encodings");
+      return -1;
+    case ENCODING_IB:
+      if (sawRegImm) {
+        /* Saw a register immediate so don't read again and instead split the
+           previous immediate.  FIXME: This is a hack. */
+        insn->immediates[insn->numImmediatesConsumed] =
+          insn->immediates[insn->numImmediatesConsumed - 1] & 0xf;
+        ++insn->numImmediatesConsumed;
+        break;
+      }
+      if (readImmediate(insn, 1))
+        return -1;
+      if (Op.type == TYPE_IMM3 &&
+          insn->immediates[insn->numImmediatesConsumed - 1] > 7)
+        return -1;
+      if (Op.type == TYPE_IMM5 &&
+          insn->immediates[insn->numImmediatesConsumed - 1] > 31)
+        return -1;
+      if (Op.type == TYPE_XMM128 ||
+          Op.type == TYPE_XMM256)
+        sawRegImm = 1;
+      break;
+    case ENCODING_IW:
+      if (readImmediate(insn, 2))
+        return -1;
+      break;
+    case ENCODING_ID:
+      if (readImmediate(insn, 4))
+        return -1;
+      break;
+    case ENCODING_IO:
+      if (readImmediate(insn, 8))
+        return -1;
+      break;
+    case ENCODING_Iv:
+      if (readImmediate(insn, insn->immediateSize))
+        return -1;
+      break;
+    case ENCODING_Ia:
+      if (readImmediate(insn, insn->addressSize))
+        return -1;
+      break;
+    case ENCODING_RB:
+      if (readOpcodeRegister(insn, 1))
+        return -1;
+      break;
+    case ENCODING_RW:
+      if (readOpcodeRegister(insn, 2))
+        return -1;
+      break;
+    case ENCODING_RD:
+      if (readOpcodeRegister(insn, 4))
+        return -1;
+      break;
+    case ENCODING_RO:
+      if (readOpcodeRegister(insn, 8))
+        return -1;
+      break;
+    case ENCODING_Rv:
+      if (readOpcodeRegister(insn, 0))
+        return -1;
+      break;
+    case ENCODING_FP:
+      break;
+    case ENCODING_VVVV:
+      needVVVV = 0; /* Mark that we have found a VVVV operand. */
+      if (!hasVVVV)
+        return -1;
+      if (fixupReg(insn, &Op))
+        return -1;
+      break;
+    case ENCODING_WRITEMASK:
+      if (readMaskRegister(insn))
+        return -1;
+      break;
+    case ENCODING_DUP:
+      break;
+    default:
+      dbgprintf(insn, "Encountered an operand with an unknown encoding.");
+      return -1;
+    }
+  }
+
+  /* If we didn't find ENCODING_VVVV operand, but non-zero vvvv present, fail */
+  if (needVVVV) return -1;
+
+  return 0;
+}
+
+/*
+ * decodeInstruction - Reads and interprets a full instruction provided by the
+ *   user.
+ *
+ * @param insn      - A pointer to the instruction to be populated.  Must be
+ *                    pre-allocated.
+ * @param reader    - The function to be used to read the instruction's bytes.
+ * @param readerArg - A generic argument to be passed to the reader to store
+ *                    any internal state.
+ * @param logger    - If non-NULL, the function to be used to write log messages
+ *                    and warnings.
+ * @param loggerArg - A generic argument to be passed to the logger to store
+ *                    any internal state.
+ * @param startLoc  - The address (in the reader's address space) of the first
+ *                    byte in the instruction.
+ * @param mode      - The mode (real mode, IA-32e, or IA-32e in 64-bit mode) to
+ *                    decode the instruction in.
+ * @return          - 0 if the instruction's memory could be read; nonzero if
+ *                    not.
+ */
+int llvm::X86Disassembler::decodeInstruction(
+    struct InternalInstruction *insn, byteReader_t reader,
+    const void *readerArg, dlog_t logger, void *loggerArg, const void *miiArg,
+    uint64_t startLoc, DisassemblerMode mode) {
+  memset(insn, 0, sizeof(struct InternalInstruction));
+
+  insn->reader = reader;
+  insn->readerArg = readerArg;
+  insn->dlog = logger;
+  insn->dlogArg = loggerArg;
+  insn->startLocation = startLoc;
+  insn->readerCursor = startLoc;
+  insn->mode = mode;
+  insn->numImmediatesConsumed = 0;
+
+  if (readPrefixes(insn)       ||
+      readOpcode(insn)         ||
+      getID(insn, miiArg)      ||
+      insn->instructionID == 0 ||
+      readOperands(insn))
+    return -1;
+
+  insn->operands = x86OperandSets[insn->spec->operands];
+
+  insn->length = insn->readerCursor - insn->startLocation;
+
+  dbgprintf(insn, "Read from 0x%llx to 0x%llx: length %zu",
+            startLoc, insn->readerCursor, insn->length);
+
+  if (insn->length > 15)
+    dbgprintf(insn, "Instruction exceeds 15-byte limit");
+
+  return 0;
+}