[TableGen][CodeEmitterGen] Add support for querying operand bit offsets

[iii]

Summary

In order to generate relocations or to apply fixups after the layout
has been computed, the targets need to know the offsets of the
respective operands. There are indirect ways to figure them out in some
cases, for example, on SystemZ, the first memory operand is always at
offset 2, and the second one is always at offset 4. But there are no
such tricks for the immediate operands on SystemZ, so one has to refer
to individual instruction encodings.

This information, however, is available to TableGen. Generate
the getOperandBitOffset() method to access it, and use it to simplify
getting memory operand offsets on SystemZ. This also paves the way for
implementing symbolic immediates on this platform.

For the multi-lit operands, getOperandBitOffset() returns the offset of
the first lit.

An alternative way to obtain offsets would be to pass them to the
encoder methods, but this would require reworking all targets. Also,
VarLenCodeEmitter already does this, but adopting it requires
reworking the respective targets without other significant benefits.

[craig.topper]

llvm/utils/TableGen/CodeEmitterGen.cpp

216

If I'm reading this right, this is the offset from the MSB of the encoding to the MSB of the field? Which works for SystemZ because the encoding bytes are emitted in big endian, but would not be useful for a little endian target?

[iii]

• Add little-endian support.
• Print operand names.
• Fix a typo in the #endif comment.

[iii]

llvm/utils/TableGen/CodeEmitterGen.cpp

216

You are right, I thought that reverseBitsForLittleEndianEncoding() would handle that, but it doesn't.
I've updated the patch and verified the update with PPC, which sets isLittleEndianEncoding.
For BCCTR (https://www.ibm.com/docs/en/aix/7.2?topic=set-bcctr-bcc-branch-conditional-count-register-instruction) we get:

switch (OpNum) {
case 0:
  // op: BI
  return 20;
}

which matches the drawing and the encoding in practice:

       0: 20 1c 22 4c   bcctr 1, 2, 3

                               bit 20
                                 |
                                 v
BitV  00100000    00011100    00100010    01001100
Bit#  76543210    54321098    32109876    10987654
      +++++++-    ...--+++    +++-----    ------++
Field   528  LK      BH       BO   BI       19  BO

[craig.topper]

I'm not sure when targets are supposed to set isLittleEndianEncoding(). RISC-V encoding is definitely little endian, but we don't set isLittleEndianEncoding().

[iii]

Hmm, yes, this indeed does not work for RISC-V. E.g., for LUI I get:

switch (OpNum) {
case 1:
  // op: imm20
  return 0;
case 0:
  // op: rd
  return 20;
}

which is in the wrong order:

0000000000000000 <.text>:
   0:	000c72b7          	lui	t0,0xc7

        b7 72 0c 00  # in memory

I'll need to find (or introduce) a different criterion.

[iii]

Right now there doesn't seem to be a generic way to get the instruction endianness.
The best example is ARM:

ARMDisassembler(const MCSubtargetInfo &STI, MCContext &Ctx,
                const MCInstrInfo *MCII)
    : MCDisassembler(STI, Ctx), MCII(MCII) {
  InstructionEndianness = STI.hasFeature(ARM::ModeBigEndianInstructions)
                              ? llvm::support::big
                              : llvm::support::little;

where it's separate from data endianness and is not available to TableGen.
Even though it's used only for disassembly, it demonstrates the level of flexibility that may be requred.

So I wonder if it would be acceptable to pass endianness as a parameter to getOperandBitOffset() like this?

--- a/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCCodeEmitter.cpp
+++ b/llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCCodeEmitter.cpp
@@ -55,6 +55,7 @@ private:
                                  SmallVectorImpl<MCFixup> &Fixups,
                                  const MCSubtargetInfo &STI) const;
   uint32_t getOperandBitOffset(const MCInst &MI, unsigned OpNum,
+                               bool IsLittleEndian,
                                const MCSubtargetInfo &STI) const;
 
   // Called by the TableGen code to get the binary encoding of operand
@@ -174,7 +175,7 @@ uint64_t SystemZMCCodeEmitter::getDispOpValue(const MCInst &MI, unsigned OpNum,
   if (MO.isImm())
     return static_cast<uint64_t>(MO.getImm());
   if (MO.isExpr()) {
-    uint32_t BitOffset = getOperandBitOffset(MI, OpNum, STI);
+    uint32_t BitOffset = getOperandBitOffset(MI, OpNum, false, STI);
     Fixups.push_back(MCFixup::create(BitOffset >> 3, MO.getExpr(),
                                      (MCFixupKind)Kind, MI.getLoc()));
     assert(Fixups.size() <= 2 && "More than two memory operands in MI?");
diff --git a/llvm/utils/TableGen/CodeEmitterGen.cpp b/llvm/utils/TableGen/CodeEmitterGen.cpp
index dcac1efb7132..ddca6f0a2a68 100644
--- a/llvm/utils/TableGen/CodeEmitterGen.cpp
+++ b/llvm/utils/TableGen/CodeEmitterGen.cpp
@@ -211,12 +211,11 @@ bool CodeEmitterGen::addCodeToMergeInOperand(Record *R, BitsInit *BI,
     unsigned hiBit = loBit + N;
     unsigned loInstBit = beginInstBit - N + 1;
     if (!BitOffsetCaseEmitted) {
-      unsigned bitOffset = Target.isLittleEndianEncoding()
-                               ? beginInstBit
-                               : BI->getNumBits() - beginInstBit - 1;
       BitOffsetCase += "      case " + utostr(OpIdx) + ":\n";
       BitOffsetCase += "        // op: " + VarName + "\n";
-      BitOffsetCase += "        return " + utostr(bitOffset) + ";\n";
+      BitOffsetCase += "        return IsLittleEndian ? " +
+                       utostr(beginInstBit) + " : " +
+                       utostr(BI->getNumBits() - beginInstBit - 1) + ";\n";
       BitOffsetCaseEmitted = true;
     }
     if (UseAPInt) {
@@ -537,6 +536,7 @@ void CodeEmitterGen::run(raw_ostream &o) {
       << "uint32_t " << Target.getName()
       << "MCCodeEmitter::getOperandBitOffset(const MCInst &MI,\n"
       << "    unsigned OpNum,\n"
+      << "    bool IsLittleEndian,\n"
       << "    const MCSubtargetInfo &STI) const {\n"
       << "  switch (MI.getOpcode()) {\n";
     emitCaseMap(o, BitOffsetCaseMap);

[iii]

• Pass endianness explicitly.

[iii]

• Fix the handling of multi-lit instructions on little-endian. The code used to return the start of the last lit, which is not useful.

Example output with this change:

case RISCV::AUIPC:
case RISCV::JAL:
case RISCV::LUI: {
  switch (OpNum) {
  case 1:
    // op: imm20
    return IsLittleEndian ? 12 : 0;
  case 0:
    // op: rd
    return IsLittleEndian ? 7 : 20;
  }
  break;
}

case SystemZ::CFI:
case SystemZ::CGFI:
case SystemZ::CIH:
case SystemZ::CLFI:
case SystemZ::CLGFI:
case SystemZ::CLIH:
case SystemZ::IIHF:
case SystemZ::IILF:
case SystemZ::LGFI:
case SystemZ::LLIHF:
case SystemZ::LLILF: {
  switch (OpNum) {
  case 0:
    // op: R1
    return IsLittleEndian ? 36 : 8;
  case 1:
    // op: I2
    return IsLittleEndian ? 0 : 16;
  }
  break;
}

[uweigand]

In D155329#4509748, @iii wrote:

case SystemZ::LLILF: {
  switch (OpNum) {
  case 0:
    // op: R1
    return IsLittleEndian ? 36 : 8;
  case 1:
    // op: I2
    return IsLittleEndian ? 0 : 16;
  }
  break;
}

I guess this numbers are still a bit confusing to me. What exactly does IsLittleEndian specify here? TableGen doesn't really inherently know about the in-memory representation, it defines the instruction as a single integer. E.g. looking at how this instruction format is defined:

let Inst{47-40} = op{11-4};
let Inst{39-36} = R1;
let Inst{35-32} = op{3-0};
let Inst{31-0}  = I2;

we see it's represented as a single 48-bit integer. Its encoding into bytes in memory only happens in the platform-specific encodeInstruction routine here:

void SystemZMCCodeEmitter::encodeInstruction(const MCInst &MI,
                                             SmallVectorImpl<char> &CB,
                                             SmallVectorImpl<MCFixup> &Fixups,
                                             const MCSubtargetInfo &STI) const {
  MemOpsEmitted = 0;
  uint64_t Bits = getBinaryCodeForInstr(MI, Fixups, STI);
  unsigned Size = MCII.get(MI.getOpcode()).getSize();
  // Big-endian insertion of Size bytes.
  unsigned ShiftValue = (Size * 8) - 8;
  for (unsigned I = 0; I != Size; ++I) {
    CB.push_back(uint8_t(Bits >> ShiftValue));
    ShiftValue -= 8;
  }
}

So I would have expected the getOperandBitOffset call to return one of the values explicitly called out in the instruction format definition, i.e. 39 or 36 for R1 in the above example. (I guess it doesn't matter which one, as the user will know the field length, so they are able to compute the other one.) Using that value, plus the total size of the current instruction (which can be gotten via MCII.get(MI.getOpcode()).getSize()), the caller should able to compute whatever they need.

[iii]

• Move the endianness handling to the target C++ code.

[uweigand]

The SystemZ parts LGTM. The TableGen changes also look reasonable to me, but I'd also like to wait for Craig's input on whether his concerns are resolved with this version.

It would be good to have a comment spelling out the explicit semantics of the new getOperandBitOffset routine that targets can rely on. Maybe the top of CodeEmitterGen.cpp would be a good place for this.

[iii]

• Document the new function.

[craig.topper]

llvm/utils/TableGen/CodeEmitterGen.cpp

12

MachineInstr should be MCInst here and in the new paragraph.

[iii]

• s/MachineInstr/MCInst/g

[craig.topper]

LGTM

[RKSimon]

@iii I'm seeing build warnings (which we treat as errors in most builds) due to this:

E:\llvm\ninja\lib\Target\SystemZ\SystemZGenMCCodeEmitter.inc(15414): warning C4060: switch statement contains no 'case' or 'default' labels

which came from:

    case SystemZ::CSCH:
    case SystemZ::HSCH:
    case SystemZ::IPK:
    case SystemZ::NNPA:
    case SystemZ::NOP_bare:
    case SystemZ::PALB:
    case SystemZ::PCC:
    case SystemZ::PCKMO:
    case SystemZ::PFPO:
    case SystemZ::PR:
    case SystemZ::PTFF:
    case SystemZ::PTLB:
    case SystemZ::RCHP:
    case SystemZ::RSCH:
    case SystemZ::SAL:
    case SystemZ::SAM24:
    case SystemZ::SAM31:
    case SystemZ::SAM64:
    case SystemZ::SCHM:
    case SystemZ::SCKPF:
    case SystemZ::TAM:
    case SystemZ::TEND:
    case SystemZ::TRAP2:
    case SystemZ::UPT:
    case SystemZ::XSCH: {
      switch (OpNum) {
      }
      break;
    }
  }
  std::string msg;
  raw_string_ostream Msg(msg);
  Msg << "Not supported instr[opcode]: " << MI << "[" << OpNum << "]";
  report_fatal_error(Msg.str().c_str());
}

#endif // GET_OPERAND_BIT_OFFSET

Please can you update this so that empty case map entries are not emitted?

[iii]

Thanks for letting me know, I'll fix this.

[iii]

https://reviews.llvm.org/D155805

llvm/lib/Target/SystemZ/MCTargetDesc/SystemZMCCodeEmitter.cpp

 #define DEBUG_TYPE "mccodeemitter"
 
 namespace {
 
 class SystemZMCCodeEmitter : public MCCodeEmitter {
   const MCInstrInfo &MCII;
   MCContext &Ctx;
 
-  mutable unsigned MemOpsEmitted;
-
 public:
   SystemZMCCodeEmitter(const MCInstrInfo &mcii, MCContext &ctx)
     : MCII(mcii), Ctx(ctx) {
   }
 
   ~SystemZMCCodeEmitter() override = default;
 
   // OVerride MCCodeEmitter.
   void encodeInstruction(const MCInst &MI, SmallVectorImpl<char> &CB,
                          SmallVectorImpl<MCFixup> &Fixups,
                          const MCSubtargetInfo &STI) const override;
 
 private:
   // Automatically generated by TableGen.
   uint64_t getBinaryCodeForInstr(const MCInst &MI,
                                  SmallVectorImpl<MCFixup> &Fixups,
                                  const MCSubtargetInfo &STI) const;
+  uint32_t getOperandBitOffset(const MCInst &MI, unsigned OpNum,
+                               bool IsLittleEndian,
+                               const MCSubtargetInfo &STI) const;
 
   // Called by the TableGen code to get the binary encoding of operand
   // MO in MI.  Fixups is the list of fixups against MI.
   uint64_t getMachineOpValue(const MCInst &MI, const MCOperand &MO,
                              SmallVectorImpl<MCFixup> &Fixups,
                              const MCSubtargetInfo &STI) const;
 
   // Return the displacement value for the OpNum operand. If it is a symbol,
   // add a fixup for it and return 0.
   uint64_t getDispOpValue(const MCInst &MI, unsigned OpNum,
                           SmallVectorImpl<MCFixup> &Fixups,
+                          const MCSubtargetInfo &STI,
                           SystemZ::FixupKind Kind) const;
 
   // Called by the TableGen code to get the binary encoding of an address.
   // The index or length, if any, is encoded first, followed by the base,
   // followed by the displacement.  In a 20-bit displacement,
   // the low 12 bits are encoded before the high 8 bits.
   template <unsigned N>
   uint64_t getLenEncoding(const MCInst &MI, unsigned OpNum,

 };
 
 } // end anonymous namespace
 
 void SystemZMCCodeEmitter::encodeInstruction(const MCInst &MI,
                                              SmallVectorImpl<char> &CB,
                                              SmallVectorImpl<MCFixup> &Fixups,
                                              const MCSubtargetInfo &STI) const {
-  MemOpsEmitted = 0;
   uint64_t Bits = getBinaryCodeForInstr(MI, Fixups, STI);
   unsigned Size = MCII.get(MI.getOpcode()).getSize();
   // Big-endian insertion of Size bytes.
   unsigned ShiftValue = (Size * 8) - 8;
   for (unsigned I = 0; I != Size; ++I) {
     CB.push_back(uint8_t(Bits >> ShiftValue));
     ShiftValue -= 8;
   }
 }
 
 uint64_t SystemZMCCodeEmitter::
 getMachineOpValue(const MCInst &MI, const MCOperand &MO,
                   SmallVectorImpl<MCFixup> &Fixups,
                   const MCSubtargetInfo &STI) const {
   if (MO.isReg())
     return Ctx.getRegisterInfo()->getEncodingValue(MO.getReg());
   if (MO.isImm())
     return static_cast<uint64_t>(MO.getImm());
   llvm_unreachable("Unexpected operand type!");
 }
 
-uint64_t SystemZMCCodeEmitter::
-getDispOpValue(const MCInst &MI, unsigned OpNum,
+uint64_t SystemZMCCodeEmitter::getDispOpValue(const MCInst &MI, unsigned OpNum,
                                               SmallVectorImpl<MCFixup> &Fixups,
+                                              const MCSubtargetInfo &STI,
                                               SystemZ::FixupKind Kind) const {
   const MCOperand &MO = MI.getOperand(OpNum);
-  if (MO.isImm()) {
-    ++MemOpsEmitted;
+  if (MO.isImm())
     return static_cast<uint64_t>(MO.getImm());
-  }
   if (MO.isExpr()) {
-    // All instructions follow the pattern where the first displacement has a
-    // 2 bytes offset, and the second one 4 bytes.
-    unsigned ByteOffs = MemOpsEmitted++ == 0 ? 2 : 4;
-    Fixups.push_back(MCFixup::create(ByteOffs, MO.getExpr(), (MCFixupKind)Kind,
-                                     MI.getLoc()));
+    uint32_t BitOffset = getOperandBitOffset(MI, OpNum, false, STI);
+    Fixups.push_back(MCFixup::create(BitOffset >> 3, MO.getExpr(),
+                                     (MCFixupKind)Kind, MI.getLoc()));
     assert(Fixups.size() <= 2 && "More than two memory operands in MI?");
     return 0;
   }
   llvm_unreachable("Unexpected operand type!");
 }
 
 template <unsigned N>
 uint64_t
 SystemZMCCodeEmitter::getLenEncoding(const MCInst &MI, unsigned OpNum,
                                      SmallVectorImpl<MCFixup> &Fixups,
                                      const MCSubtargetInfo &STI) const {
   return getMachineOpValue(MI, MI.getOperand(OpNum), Fixups, STI) - 1;
 }
 
 uint64_t
 SystemZMCCodeEmitter::getDisp12Encoding(const MCInst &MI, unsigned OpNum,
                                         SmallVectorImpl<MCFixup> &Fixups,
                                         const MCSubtargetInfo &STI) const {
-  return getDispOpValue(MI, OpNum, Fixups, SystemZ::FixupKind::FK_390_12);
+  return getDispOpValue(MI, OpNum, Fixups, STI, SystemZ::FixupKind::FK_390_12);
 }
 
 uint64_t
 SystemZMCCodeEmitter::getDisp20Encoding(const MCInst &MI, unsigned OpNum,
                                         SmallVectorImpl<MCFixup> &Fixups,
                                         const MCSubtargetInfo &STI) const {
-  return getDispOpValue(MI, OpNum, Fixups, SystemZ::FixupKind::FK_390_20);
+  return getDispOpValue(MI, OpNum, Fixups, STI, SystemZ::FixupKind::FK_390_20);
 }
 
 uint64_t
 SystemZMCCodeEmitter::getPCRelEncoding(const MCInst &MI, unsigned OpNum,
                                        SmallVectorImpl<MCFixup> &Fixups,
                                        unsigned Kind, int64_t Offset,
                                        bool AllowTLS) const {
   SMLoc Loc = MI.getLoc();
SystemZMCCodeEmitter::getPCRelEncoding(const MCInst &MI, unsigned OpNum,
   if (AllowTLS && OpNum + 1 < MI.getNumOperands()) {
     const MCOperand &MOTLS = MI.getOperand(OpNum + 1);
     Fixups.push_back(MCFixup::create(
         0, MOTLS.getExpr(), (MCFixupKind)SystemZ::FK_390_TLS_CALL, Loc));
   }
   return 0;
 }
 
+#define GET_OPERAND_BIT_OFFSET
 #include "SystemZGenMCCodeEmitter.inc"
 
 MCCodeEmitter *llvm::createSystemZMCCodeEmitter(const MCInstrInfo &MCII,
                                                 MCContext &Ctx) {
   return new SystemZMCCodeEmitter(MCII, Ctx);
 }

llvm/utils/TableGen/CodeEmitterGen.cpp

 //===- CodeEmitterGen.cpp - Code Emitter Generator ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // CodeEmitterGen uses the descriptions of instructions and their fields to
 // construct an automated code emitter: a function that, given a MachineInstr,
 // returns the (currently, 32-bit unsigned) value of the instruction.
 //
 //===----------------------------------------------------------------------===//
 
 #include "CodeGenHwModes.h"
 #include "CodeGenInstruction.h"
 #include "CodeGenTarget.h"
 #include "InfoByHwMode.h"
 #include "VarLenCodeEmitterGen.h"
 #include "llvm/ADT/APInt.h"

 
 public:
   CodeEmitterGen(RecordKeeper &R) : Records(R) {}
 
   void run(raw_ostream &o);
 
 private:
   int getVariableBit(const std::string &VarName, BitsInit *BI, int bit);
-  std::string getInstructionCase(Record *R, CodeGenTarget &Target);
-  std::string getInstructionCaseForEncoding(Record *R, Record *EncodingDef,
-                                            CodeGenTarget &Target);
+  std::pair<std::string, std::string>
+  getInstructionCases(Record *R, CodeGenTarget &Target);
+  void addInstructionCasesForEncoding(Record *R, Record *EncodingDef,
+                                      CodeGenTarget &Target, std::string &Case,
+                                      std::string &BitOffsetCase);
   bool addCodeToMergeInOperand(Record *R, BitsInit *BI,
-                               const std::string &VarName,
-                               std::string &Case, CodeGenTarget &Target);
+                               const std::string &VarName, std::string &Case,
+                               std::string &BitOffsetCase,
+                               CodeGenTarget &Target);
 
   void emitInstructionBaseValues(
       raw_ostream &o, ArrayRef<const CodeGenInstruction *> NumberedInstructions,
       CodeGenTarget &Target, int HwMode = -1);
+  void
+  emitCaseMap(raw_ostream &o,
+              const std::map<std::string, std::vector<std::string>> &CaseMap);
   unsigned BitWidth = 0u;
   bool UseAPInt = false;
 };
 
 // If the VarBitInit at position 'bit' matches the specified variable then
 // return the variable bit position.  Otherwise return -1.
 int CodeEmitterGen::getVariableBit(const std::string &VarName,
                                    BitsInit *BI, int bit) {
   if (VarBitInit *VBI = dyn_cast<VarBitInit>(BI->getBit(bit))) {
     if (VarInit *VI = dyn_cast<VarInit>(VBI->getBitVar()))
       if (VI->getName() == VarName)
         return VBI->getBitNum();
   } else if (VarInit *VI = dyn_cast<VarInit>(BI->getBit(bit))) {
     if (VI->getName() == VarName)
       return 0;
   }
 
   return -1;
 }
 
 // Returns true if it succeeds, false if an error.
 bool CodeEmitterGen::addCodeToMergeInOperand(Record *R, BitsInit *BI,
                                              const std::string &VarName,
                                              std::string &Case,
+                                             std::string &BitOffsetCase,
                                              CodeGenTarget &Target) {
   CodeGenInstruction &CGI = Target.getInstruction(R);
 
   // Determine if VarName actually contributes to the Inst encoding.
   int bit = BI->getNumBits()-1;
 
   // Scan for a bit that this contributed to.
   for (; bit >= 0; ) {
for (--tmpBit; tmpBit >= 0;) {
       if (varBit == -1 || varBit != (beginVarBit - N))
         break;
       ++N;
       --tmpBit;
     }
     ++numOperandLits;
   }
 
+  unsigned BitOffsetLE = -1, BitOffsetBE = -1;
   for (; bit >= 0; ) {
     int varBit = getVariableBit(VarName, BI, bit);
     
     // If this bit isn't from a variable, skip it.
     if (varBit == -1) {
       --bit;
       continue;
     }
-    
     // Figure out the consecutive range of bits covered by this operand, in
     // order to generate better encoding code.
     int beginInstBit = bit;
     int beginVarBit = varBit;
     int N = 1;
     for (--bit; bit >= 0;) {
       varBit = getVariableBit(VarName, BI, bit);
       if (varBit == -1 || varBit != (beginVarBit - N)) break;
       ++N;
       --bit;
     }
 
     std::string maskStr;
     int opShift;
 
     unsigned loBit = beginVarBit - N + 1;
     unsigned hiBit = loBit + N;
     unsigned loInstBit = beginInstBit - N + 1;
+    BitOffsetLE = loInstBit;
+    if (BitOffsetBE == -1)
+      BitOffsetBE = BI->getNumBits() - beginInstBit - 1;
     if (UseAPInt) {
       std::string extractStr;
       if (N >= 64) {
         extractStr = "op.extractBits(" + itostr(hiBit - loBit) + ", " +
                      itostr(loBit) + ")";
         Case += "      Value.insertBits(" + extractStr + ", " +
                 itostr(loInstBit) + ");\n";
       } else {
         extractStr = "op.extractBitsAsZExtValue(" + itostr(hiBit - loBit) +
if (UseAPInt) {
           Case += "      Value |= (op & " + maskStr + ") >> " +
                   itostr(-opShift) + ";\n";
         } else {
           Case += "      Value |= (op & " + maskStr + ");\n";
         }
       }
     }
   }
+
+  if (BitOffsetLE != -1 && BitOffsetBE != -1) {
+    BitOffsetCase += "      case " + utostr(OpIdx) + ":\n";
+    BitOffsetCase += "        // op: " + VarName + "\n";
+    BitOffsetCase += "        return IsLittleEndian ? " + utostr(BitOffsetLE) +
+                     " : " + utostr(BitOffsetBE) + ";\n";
+  }
+
   return true;
 }
 
-std::string CodeEmitterGen::getInstructionCase(Record *R,
-                                               CodeGenTarget &Target) {
-  std::string Case;
+std::pair<std::string, std::string>
+CodeEmitterGen::getInstructionCases(Record *R, CodeGenTarget &Target) {
+  std::string Case, BitOffsetCase;
+
+  auto append = [&](const char *S) {
+    Case += S;
+    BitOffsetCase += S;
+  };
+
   if (const RecordVal *RV = R->getValue("EncodingInfos")) {
     if (auto *DI = dyn_cast_or_null<DefInit>(RV->getValue())) {
       const CodeGenHwModes &HWM = Target.getHwModes();
       EncodingInfoByHwMode EBM(DI->getDef(), HWM);
-      Case += "      switch (HwMode) {\n";
-      Case += "      default: llvm_unreachable(\"Unhandled HwMode\");\n";
+      append("      switch (HwMode) {\n");
+      append("      default: llvm_unreachable(\"Unhandled HwMode\");\n");
       for (auto &KV : EBM) {
-        Case += "      case " + itostr(KV.first) + ": {\n";
-        Case += getInstructionCaseForEncoding(R, KV.second, Target);
-        Case += "      break;\n";
-        Case += "      }\n";
+        append(("      case " + itostr(KV.first) + ": {\n").c_str());
+        addInstructionCasesForEncoding(R, KV.second, Target, Case,
+                                       BitOffsetCase);
+        append("      break;\n");
+        append("      }\n");
       }
-      Case += "      }\n";
-      return Case;
+      append("      }\n");
+      return std::make_pair(std::move(Case), std::move(BitOffsetCase));
     }
   }
-  return getInstructionCaseForEncoding(R, R, Target);
+  addInstructionCasesForEncoding(R, R, Target, Case, BitOffsetCase);
+  return std::make_pair(std::move(Case), std::move(BitOffsetCase));
 }
 
-std::string CodeEmitterGen::getInstructionCaseForEncoding(Record *R, Record *EncodingDef,
-                                                          CodeGenTarget &Target) {
-  std::string Case;
+void CodeEmitterGen::addInstructionCasesForEncoding(
+    Record *R, Record *EncodingDef, CodeGenTarget &Target, std::string &Case,
+    std::string &BitOffsetCase) {
   BitsInit *BI = EncodingDef->getValueAsBitsInit("Inst");
 
   // Loop over all of the fields in the instruction, determining which are the
   // operands to the instruction.
   bool Success = true;
+  BitOffsetCase += "      switch (OpNum) {\n";
   for (const RecordVal &RV : EncodingDef->getValues()) {
     // Ignore fixed fields in the record, we're looking for values like:
     //    bits<5> RST = { ?, ?, ?, ?, ? };
     if (RV.isNonconcreteOK() || RV.getValue()->isComplete())
       continue;
 
-    Success &=
-        addCodeToMergeInOperand(R, BI, std::string(RV.getName()),
-                                Case, Target);
+    Success &= addCodeToMergeInOperand(R, BI, std::string(RV.getName()), Case,
+                                       BitOffsetCase, Target);
   }
+  BitOffsetCase += "      }\n";
 
   if (!Success) {
     // Dump the record, so we can see what's going on...
     std::string E;
     raw_string_ostream S(E);
     S << "Dumping record for previous error:\n";
     S << *R;
     PrintNote(E);
   }
 
   StringRef PostEmitter = R->getValueAsString("PostEncoderMethod");
   if (!PostEmitter.empty()) {
     Case += "      Value = ";
     Case += PostEmitter;
     Case += "(MI, Value";
     Case += ", STI";
     Case += ");\n";
   }
-  
-  return Case;
 }
 
 static void emitInstBits(raw_ostream &OS, const APInt &Bits) {
   for (unsigned I = 0; I < Bits.getNumWords(); ++I)
     OS << ((I > 0) ? ", " : "") << "UINT64_C(" << utostr(Bits.getRawData()[I])
        << ")";
 }
 
for (const CodeGenInstruction *CGI : NumberedInstructions) {
     }
     o << "    ";
     emitInstBits(o, Value);
     o << "," << '\t' << "// " << R->getName() << "\n";
   }
   o << "    UINT64_C(0)\n  };\n";
 }
 
+void CodeEmitterGen::emitCaseMap(
+    raw_ostream &o,
+    const std::map<std::string, std::vector<std::string>> &CaseMap) {
+  std::map<std::string, std::vector<std::string>>::const_iterator IE, EE;
+  for (IE = CaseMap.begin(), EE = CaseMap.end(); IE != EE; ++IE) {
+    const std::string &Case = IE->first;
+    const std::vector<std::string> &InstList = IE->second;
+
+    for (int i = 0, N = InstList.size(); i < N; i++) {
+      if (i)
+        o << "\n";
+      o << "    case " << InstList[i] << ":";
+    }
+    o << " {\n";
+    o << Case;
+    o << "      break;\n"
+      << "    }\n";
+  }
+}
+
 void CodeEmitterGen::run(raw_ostream &o) {
   emitSourceFileHeader("Machine Code Emitter", o);
 
   CodeGenTarget Target(Records);
   std::vector<Record*> Insts = Records.getAllDerivedDefinitions("Instruction");
 
   // For little-endian instruction bit encodings, reverse the bit order
   Target.reverseBitsForLittleEndianEncoding();
if (!HwModes.empty()) {
         o << "  case " << I << ": InstBits = InstBits_" << HWM.getMode(I).Name
           << "; break;\n";
       }
       o << "  };\n";
     }
 
     // Map to accumulate all the cases.
     std::map<std::string, std::vector<std::string>> CaseMap;
+    std::map<std::string, std::vector<std::string>> BitOffsetCaseMap;
 
     // Construct all cases statement for each opcode
     for (Record *R : Insts) {
       if (R->getValueAsString("Namespace") == "TargetOpcode" ||
           R->getValueAsBit("isPseudo"))
         continue;
       std::string InstName =
           (R->getValueAsString("Namespace") + "::" + R->getName()).str();
-      std::string Case = getInstructionCase(R, Target);
+      std::string Case, BitOffsetCase;
+      std::tie(Case, BitOffsetCase) = getInstructionCases(R, Target);
 
-      CaseMap[Case].push_back(std::move(InstName));
+      CaseMap[Case].push_back(InstName);
+      BitOffsetCaseMap[BitOffsetCase].push_back(std::move(InstName));
     }
 
     // Emit initial function code
     if (UseAPInt) {
       int NumWords = APInt::getNumWords(BitWidth);
       o << "  const unsigned opcode = MI.getOpcode();\n"
         << "  if (Scratch.getBitWidth() != " << BitWidth << ")\n"
         << "    Scratch = Scratch.zext(" << BitWidth << ");\n"
         << "  Inst = APInt(" << BitWidth << ", ArrayRef(InstBits + opcode * "
         << NumWords << ", " << NumWords << "));\n"
         << "  APInt &Value = Inst;\n"
         << "  APInt &op = Scratch;\n"
         << "  switch (opcode) {\n";
     } else {
       o << "  const unsigned opcode = MI.getOpcode();\n"
         << "  uint64_t Value = InstBits[opcode];\n"
         << "  uint64_t op = 0;\n"
         << "  (void)op;  // suppress warning\n"
         << "  switch (opcode) {\n";
     }
 
     // Emit each case statement
-    std::map<std::string, std::vector<std::string>>::iterator IE, EE;
-    for (IE = CaseMap.begin(), EE = CaseMap.end(); IE != EE; ++IE) {
-      const std::string &Case = IE->first;
-      std::vector<std::string> &InstList = IE->second;
-
-      for (int i = 0, N = InstList.size(); i < N; i++) {
-        if (i)
-          o << "\n";
-        o << "    case " << InstList[i] << ":";
-      }
-      o << " {\n";
-      o << Case;
-      o << "      break;\n"
-        << "    }\n";
-    }
+    emitCaseMap(o, CaseMap);
 
     // Default case: unhandled opcode
     o << "  default:\n"
       << "    std::string msg;\n"
       << "    raw_string_ostream Msg(msg);\n"
       << "    Msg << \"Not supported instr: \" << MI;\n"
       << "    report_fatal_error(Msg.str().c_str());\n"
       << "  }\n";
     if (UseAPInt)
       o << "  Inst = Value;\n";
     else
       o << "  return Value;\n";
     o << "}\n\n";
+
+    o << "#ifdef GET_OPERAND_BIT_OFFSET\n"
+      << "#undef GET_OPERAND_BIT_OFFSET\n\n"
+      << "uint32_t " << Target.getName()
+      << "MCCodeEmitter::getOperandBitOffset(const MCInst &MI,\n"
+      << "    unsigned OpNum,\n"
+      << "    bool IsLittleEndian,\n"
+      << "    const MCSubtargetInfo &STI) const {\n"
+      << "  switch (MI.getOpcode()) {\n";
+    emitCaseMap(o, BitOffsetCaseMap);
+    o << "  }\n"
+      << "  std::string msg;\n"
+      << "  raw_string_ostream Msg(msg);\n"
+      << "  Msg << \"Not supported instr[opcode]: \" << MI << \"[\" << OpNum "
+         "<< \"]\";\n"
+      << "  report_fatal_error(Msg.str().c_str());\n"
+      << "}\n\n"
+      << "#endif // GET_OPERAND_BIT_OFFSET\n\n";
   }
 }
 
 } // end anonymous namespace
 
 static TableGen::Emitter::OptClass<CodeEmitterGen>
     X("gen-emitter", "Generate machine code emitter");