protobuf/upb/pb/compile_decoder_x64.dasc

|//
|// upb - a minimalist implementation of protocol buffers.
|//
|// Copyright (c) 2011-2013 Google Inc.  See LICENSE for details.
|// Author: Josh Haberman <jhaberman@gmail.com>
|//
|// JIT compiler for upb_pbdecoder on x86-64.  Generates machine code from the
|// bytecode generated in compile_decoder.c.
|
|.arch x64
|.actionlist upb_jit_actionlist
|.globals UPB_JIT_GLOBAL_
|.globalnames upb_jit_globalnames
|
|// Calling conventions.  Note -- this will need to be changed for
|// Windows, which uses a different calling convention!
|.define ARG1_64,   rdi
|.define ARG2_8,    r6b  // DynASM's equivalent to "sil" -- low byte of esi.
|.define ARG2_32,   esi
|.define ARG2_64,   rsi
|.define ARG3_8,    dl
|.define ARG3_32,   edx
|.define ARG3_64,   rdx
|.define ARG4_64,   rcx
|.define ARG5_64,   r8
|.define XMMARG1,   xmm0
|
|// Register allocation / type map.
|// ALL of the code in this file uses these register allocations.
|// When we "call" within this file, we do not use regular calling
|// conventions, but of course when calling to user callbacks we must.
|.define PTR,       rbx                       // DECODER->ptr      (unsynced)
|.define DATAEND,   r12                       // DECODER->data_end (unsynced)
|.define CLOSURE,   r13                       // FRAME->closure    (unsynced)
|.type   FRAME,     upb_pbdecoder_frame, r14  // DECODER->top      (unsynced)
|.type   DECODER,   upb_pbdecoder, r15        // DECODER           (immutable)
|.define DELIMEND,  rbp
|
| // Spills unsynced registers back to memory.
|.macro commit_regs
|  mov  DECODER->top, FRAME
|  mov  DECODER->ptr, PTR
|  mov  DECODER->data_end, DATAEND
|  // We don't guarantee that delim_end is NULL when out of range like the
|  // interpreter does.
|  mov  DECODER->delim_end, DELIMEND
|  sub  DELIMEND, DECODER->buf
|  add  DELIMEND, DECODER->bufstart_ofs
|  mov  FRAME->end_ofs, DELIMEND
|  mov  FRAME->sink.closure, CLOSURE
|.endmacro
|
| // Loads unsynced registers from memory back into registers.
|.macro load_regs
|  mov  FRAME, DECODER->top
|  mov  PTR, DECODER->ptr
|  mov  DATAEND, DECODER->data_end
|  mov  CLOSURE, FRAME->sink.closure
|  mov  DELIMEND, FRAME->end_ofs
|  sub  DELIMEND, DECODER->bufstart_ofs
|  add  DELIMEND, DECODER->buf
|.endmacro
|
| // Calls an external C function at address "addr".
|.macro callp, addr
|    mov64  rax, (uintptr_t)addr
|
|    // Stack must be 16-byte aligned (x86-64 ABI requires this).
|    //
|    // OPT: possibly remove this by statically ensuring correct alignment.
|    //
|    // OPT: use "call rel32" where possible.
|    push   r12
|    mov    r12, rsp
|    and    rsp, 0xfffffffffffffff0UL   // Align stack.
|    call   rax
|    mov    rsp, r12
|    pop    r12
|.endmacro
|
|.macro ld64, val
|| {
|| uintptr_t v = (uintptr_t)val;
|| if (v > 0xffffffff) {
|    mov64  ARG2_64, v
|| } else if (v) {
|    mov    ARG2_32, v
|| } else {
|    xor    ARG2_32, ARG2_32
|| }
|| }
|.endmacro
|
|.macro load_handler_data, h, arg
|  ld64   upb_handlers_gethandlerdata(h, arg)
|.endmacro
|
|.macro chkeob, bytes, target
|| if (bytes == 1) {
|  cmp    PTR, DATAEND
|  je     target
|| } else {
|  mov    rcx, DATAEND
|  sub    rcx, PTR
|  cmp    rcx, bytes
|  jb     target
|| }
|.endmacro
|
|.macro chkneob, bytes, target
|| if (bytes == 1) {
|  cmp    PTR, DATAEND
|  jne    target
|| } else {
|  mov    rcx, DATAEND
|  sub    rcx, PTR
|  cmp    rcx, bytes
|  jae    target
|| }
|.endmacro

|.macro sethas, reg, hasbit
|| if (hasbit >= 0) {
|    or   byte [reg + ((uint32_t)hasbit / 8)], (1 << ((uint32_t)hasbit % 8))
|| }
|.endmacro
|
| // Decodes 32-bit varint into rdx, inlining 1 byte.
|.macro dv32
|  chkeob  1, >7
|  movzx   edx, byte [PTR]
|  test    dl, dl
|  jns     >8
|7:
|  call    ->decodev32_fallback
|8:
|  add     PTR, 1
|.endmacro

#define DECODE_EOF -3

static upb_func *gethandler(const upb_handlers *h, upb_selector_t sel) {
  return h ? upb_handlers_gethandler(h, sel) : NULL;
}

/* Defines an "assembly label" for the current code generation offset.
 * This label exists *purely* for debugging purposes: it is emitted into
 * the .so, and printed as part of JIT debugging output when UPB_JIT_LOAD_SO is
 * defined.
 *
 * We would define this in the .c file except that it conditionally defines a
 * pclabel. */
static void asmlabel(jitcompiler *jc, const char *fmt, ...) {
#ifndef NDEBUG
  int ofs = jc->dynasm->section->ofs;
  UPB_ASSERT(ofs != jc->lastlabelofs);
  jc->lastlabelofs = ofs;
#endif

#ifndef UPB_JIT_LOAD_SO
  UPB_UNUSED(jc);
  UPB_UNUSED(fmt);
#else
  va_list args;
  va_start(args, fmt);
  char *str = upb_vasprintf(fmt, args);
  va_end(args);

  int pclabel = alloc_pclabel(jc);
  /* Normally we would prefer to allocate this inline with the codegen,
   * ie.
   *   |=>asmlabel(...)
   * But since we do this conditionally, only when UPB_JIT_LOAD_SO is defined,
   * we do it here instead. */
  |=>pclabel:
  upb_inttable_insert(&jc->asmlabels, pclabel, upb_value_ptr(str));
#endif
}

/* Should only be called when the associated handler is known to exist. */
static bool alwaysok(const upb_handlers *h, upb_selector_t sel) {
  upb_handlerattr attr = UPB_HANDLERATTR_INITIALIZER;
  bool ok = upb_handlers_getattr(h, sel, &attr);
  bool ret;

  UPB_ASSERT(ok);
  ret = upb_handlerattr_alwaysok(&attr);
  upb_handlerattr_uninit(&attr);
  return ret;
}

/* Emit static assembly routines; code that does not vary based on the message
 * schema.  Since it's not input-dependent, we only need one single copy of it.
 * For the moment we generate a single copy per generated handlers.  Eventually
 * we should generate this code at compile time and link it into the binary so
 * we have one copy total.  To do that we'll want to be sure that it is within
 * 2GB of our JIT code, so that branches between the two are near (rel32).
 *
 * We'd put this assembly in a .s file directly, but DynASM's ability to
 * calculate structure offsets automatically is too useful to pass up (it's way
 * more convenient to write DECODER->sink than [rbx + 0x96], especially since
 * the latter would have to be changed whenever the structure is updated). */
static void emit_static_asm(jitcompiler *jc) {
  | // Trampolines for entering/exiting the JIT.  These are a bit tricky to
  | // support full resuming; when we suspend we copy the JIT's portion of
  | // the call stack into the upb_pbdecoder and restore it when we resume.
  asmlabel(jc, "enterjit");
  |->enterjit:
  |1:
  |  push  rbp
  |  push  r15
  |  push  r14
  |  push  r13
  |  push  r12
  |  push  rbx
  |
  |  mov   rbx, ARG2_64  // Preserve JIT method.
  |
  |  mov   DECODER, rdi
  |  callp upb_pbdecoder_resume  // Same args as us; reuse regs.
  |  test  eax, eax
  |  jns   >1
  |  mov   DECODER->saved_rsp, rsp
  |  mov   rax, rbx
  |  load_regs
  |
  |  // Test whether we have a saved stack to resume.
  |  mov   ARG3_64, DECODER->call_len
  |  test  ARG3_64, ARG3_64
  |  jnz   >2
  |
  |  call  rax
  |
  |  mov   rax, DECODER->size_param
  |  mov   qword DECODER->call_len, 0
  |1:
  |  pop   rbx
  |  pop   r12
  |  pop   r13
  |  pop   r14
  |  pop   r15
  |  pop   rbp
  |  ret
  |
  |2:
  |  // Resume decoder.
  |  mov   ARG2_64, DECODER->callstack
  |  sub   rsp, ARG3_64
  |  mov   ARG1_64, rsp
  |  callp memcpy  // Restore stack.
  |  ret  // Return to resumed function (not ->enterjit caller).
  |
  | // Other code can call this to suspend the JIT.
  | // To the calling code, it will appear that the function returns when
  | // the JIT resumes, and more buffer space will be available.
  | // Args: eax=the value that decode() should return.
  asmlabel(jc, "exitjit");
  |->exitjit:
  |  // Save the stack into DECODER->callstack.
  |  mov   ARG1_64, DECODER->callstack
  |  mov   ARG2_64, rsp
  |  mov   ARG3_64, DECODER->saved_rsp
  |  sub   ARG3_64, rsp
  |  mov   DECODER->call_len, ARG3_64  // Preserve len for next resume.
  |  mov   ebx, eax  // Preserve return value across memcpy.
  |  callp memcpy    // Copy stack into decoder.
  |  mov   eax, ebx  // This will be our return value.
  |
  |  // Must NOT do this before the memcpy(), otherwise memcpy() will
  |  // clobber the stack we are trying to save!
  |  mov   rsp, DECODER->saved_rsp
  |  pop   rbx
  |  pop   r12
  |  pop   r13
  |  pop   r14
  |  pop   r15
  |  pop   rbp
  |  ret
  |
  | // Like suspend() in the C decoder, except that the function appears
  | // (from the caller's perspective) not to return until the decoder is
  | // resumed.
  asmlabel(jc, "suspend");
  |->suspend:
  |  cmp   DECODER->ptr, PTR
  |  je    >1
  |  mov   DECODER->checkpoint, PTR
  |1:
  |  commit_regs
  |  mov   rdi, DECODER
  |  callp upb_pbdecoder_suspend
  |  jmp   ->exitjit
  |
  asmlabel(jc, "pushlendelim");
  |->pushlendelim:
  |1:
  |  mov   FRAME->sink.closure, CLOSURE
  |  mov   DECODER->checkpoint, PTR
  |  dv32
  |  mov   rcx, DELIMEND
  |  sub   rcx, PTR
  |  sub   rcx, rdx
  |  jb    >4   // Len is greater than enclosing message.
  |  mov   FRAME->end_ofs, rcx
  |  cmp   FRAME, DECODER->limit
  |  je    >3   // Stack overflow
  |  add   FRAME, sizeof(upb_pbdecoder_frame)
  |  mov   DELIMEND, PTR
  |  add   DELIMEND, rdx
  |  mov   dword FRAME->groupnum, 0
  |  test  rcx, rcx
  |  jz    >2
  |  mov   DATAEND, DECODER->end
  |  cmp   PTR, DELIMEND
  |  ja    >2
  |  cmp   DELIMEND, DATAEND
  |  ja    >2
  |  mov   DATAEND, DELIMEND  // If DELIMEND >= PTR && DELIMEND < DATAEND
  |2:
  |  ret
  |3:
  |  // Stack overflow error.
  |  mov   PTR, DECODER->checkpoint  // Rollback to before the delim len.
  |  // Prepare seterr args.
  |  mov   ARG1_64, DECODER
  |  ld64  kPbDecoderStackOverflow
  |  callp upb_pbdecoder_seterr
  |  call  ->suspend
  |  jmp   <1
  |4:
  |  // Overextended len.
  |  mov   PTR, DECODER->checkpoint  // Rollback to before the delim len.
  |  // Prepare seterr args.
  |  mov   ARG1_64, DECODER
  |  ld64  kPbDecoderSubmessageTooLong
  |  callp upb_pbdecoder_seterr
  |  call  ->suspend
  |  jmp   <1
  |
  | // For getting a value that spans a buffer seam.  Falls back to C.
  |.macro getvalue_slow, func, bytes
  |  sub   rsp, 8           // Need stack space for func to write value to.
  |1:
  |  mov   qword [rsp], 0   // For parsing routines that only parse 32 bits.
  |  mov   ARG1_64, DECODER
  |  mov   ARG2_64, rsp
  |  mov   DECODER->checkpoint, PTR
  |  commit_regs
  |  callp func
  |  load_regs
  |  test  eax, eax
  |  jns   >2
  |  // Success; return parsed data (in rdx AND xmm0).
  |  mov   rdx, [rsp]
  |  movsd xmm0, qword [rsp]
  |  add   rsp, 8
  |  sub   PTR, bytes       // Bias our buffer pointer to rejoin the fast-path.
  |  mov   DECODER->ptr, PTR
  |  ret
  |2:
  |  call  ->exitjit   // Return eax from decode function.
  |  jmp   <1
  |.endmacro
  |
  asmlabel(jc, "parse_unknown");
  | // Args: edx=fieldnum, cl=wire type
  |->parse_unknown:
  |  // OPT: handle directly instead of kicking to C.
  |  // Check for ENDGROUP.
  |  mov     ARG1_64, DECODER
  |  mov     ARG2_32, edx
  |  movzx   ARG3_32, cl
  |  commit_regs
  |  callp   upb_pbdecoder_skipunknown
  |  load_regs
  |  cmp     eax, DECODE_ENDGROUP
  |  jne     >1
  |  ret     // Return eax=DECODE_ENDGROUP, not zero
  |1:
  |  cmp     eax, DECODE_OK
  |  je      >1
  |  call    ->exitjit  // Return eax from decode function.
  |1:
  |  xor     eax, eax
  |  ret
  |
  | // Fallback functions for parsing single values.  These are used when the
  | // buffer doesn't contain enough remaining data for the fast path.  Each
  | // primitive type (v32, v64, f32, f64) has two functions: decode & skip.
  | // Decode functions return their value in rsi/esi.
  | //
  | // These functions leave PTR = value_end - fast_path_bytes, so that we can
  | // re-join the fast path which will add fast_path_bytes after the callback
  | // completes.  We also set DECODER->ptr to this value which is a signal to
  | // ->suspend that DECODER->checkpoint is up to date.
  asmlabel(jc, "skip_decode_f32_fallback");
  |->skipf32_fallback:
  |->decodef32_fallback:
  |  getvalue_slow upb_pbdecoder_decode_f32, 4
  |
  asmlabel(jc, "skip_decode_f64_fallback");
  |->skipf64_fallback:
  |->decodef64_fallback:
  |  getvalue_slow upb_pbdecoder_decode_f64, 8
  |
  | // Called for varint >= 1 byte.
  asmlabel(jc, "skip_decode_v32_fallback");
  |->skipv32_fallback:
  |->skipv64_fallback:
  |  chkeob   16, >1
  |  // With at least 16 bytes left, we can do a branch-less SSE version.
  |  movdqu   xmm0, [PTR]
  |  pmovmskb eax, xmm0   // bits 0-15 are continuation bits, 16-31 are 0.
  |  not      eax
  |  bsf      eax, eax
  |  cmp      al, 10
  |  jae      ->decode_varint_slow  // Error (>10 byte varint).
  |  add      PTR, rax    // bsf result is 0-based, so PTR=end-1, as desired.
  |  ret
  |
  |1:
  |  // With fewer than 16 bytes, we have to read byte by byte.
  |  lea      rcx, [PTR + 10]
  |  mov      rax, PTR    // Preserve PTR in case of fallback to slow path.
  |  cmp      rcx, DATAEND
  |  cmova    rcx, DATAEND    // rcx = MIN(DATAEND, PTR + 10)
  |2:
  |  cmp      rax, rcx
  |  je       ->decode_varint_slow
  |  test     byte [rax], 0x80
  |  jz       >3
  |  add      rax, 1
  |  jmp      <2
  |3:
  |  mov      PTR, rax  // PTR = varint_end - 1, as desired
  |  ret
  |
  | // Returns tag in edx
  asmlabel(jc, "decode_unknown_tag_fallback");
  |->decode_unknown_tag_fallback:
  |  sub   rsp, 16
  |1:
  |  cmp      PTR, DELIMEND
  |  jne      >2
  |  add      rsp, 16
  |  xor      eax, eax
  |  ret
  |2:
  |  // OPT: Have a medium-fast path before falling back to _slow.
  |  mov   ARG1_64, DECODER
  |  mov   ARG2_64, rsp
  |  commit_regs
  |  callp upb_pbdecoder_decode_varint_slow
  |  load_regs
  |  cmp   eax, 0
  |  jge   >3
  |  mov   edx, [rsp]   // Success; return parsed data.
  |  add   rsp, 16
  |  ret
  |3:
  |  call  ->exitjit   // Return eax from decode function.
  |  jmp   <1
  |
  | // Called for varint >= 1 byte.
  asmlabel(jc, "decode_v32_v64_fallback");
  |->decodev32_fallback:
  |->decodev64_fallback:
  |  chkeob   10, ->decode_varint_slow
  |  // OPT: do something faster than just calling the C version.
  |  mov      rdi, PTR
  |  callp    upb_vdecode_fast
  |  test     rax, rax
  |  je       ->decode_varint_slow  // Unterminated varint.
  |  mov      PTR, rax
  |  sub      PTR, 1
  |  mov      DECODER->ptr, PTR
  |  ret
  |
  asmlabel(jc, "decode_varint_slow");
  |->decode_varint_slow:
  |  // Slow path: end of buffer or error (varint length >= 10).
  |  getvalue_slow upb_pbdecoder_decode_varint_slow, 1
  |
  | // Args: rsi=expected tag, return=rax (DECODE_{OK,MISMATCH})
  asmlabel(jc, "checktag_fallback");
  |->checktag_fallback:
  |  sub      rsp, 8
  |  mov      [rsp], rsi  // Preserve expected tag.
  |1:
  |  mov      ARG1_64, DECODER
  |  commit_regs
  |  mov      DECODER->checkpoint, PTR
  |  callp    upb_pbdecoder_checktag_slow
  |  load_regs
  |  cmp      eax, 0
  |  jge      >2
  |  add      rsp, 8
  |  ret
  |2:
  |  call     ->exitjit
  |  mov      rsi, [rsp]
  |  cmp      PTR, DELIMEND
  |  jne      <1
  |  mov      eax, DECODE_EOF
  |  add      rsp, 8
  |  ret
  |
  | // Args: rsi=upb_inttable, rdx=key, return=rax (-1 if not found).
  | // Preserves: rcx, rdx
  | // OPT: Could write this in assembly if it's a hotspot.
  asmlabel(jc, "hashlookup");
  |->hashlookup:
  |  push   rcx
  |  push   rdx
  |  sub    rsp, 16
  |  mov    rdi, rsi
  |  mov    rsi, rdx
  |  mov    rdx, rsp
  |  callp  upb_inttable_lookup
  |  add    rsp, 16
  |  pop    rdx
  |  pop    rcx
  |  test   al, al
  |  jz     >2  // Unknown field.
  |  mov    rax, [rsp-32]  // Value from table.
  |  ret
  |2:
  |  xor    rax, rax
  |  not    rax
  |  ret
}

static void jitprimitive(jitcompiler *jc, opcode op,
                         const upb_handlers *h, upb_selector_t sel) {
  typedef enum { V32, V64, F32, F64, X } valtype_t;
  static valtype_t types[] = {
    X, F64, F32, V64, V64, V32, F64, F32, V64, X, X, X, X, V32, V32, F32, F64,
    V32, V64 };
  static char fastpath_bytes[] = { 1, 1, 4, 8 };
  const valtype_t vtype = types[op];
  const int fastbytes = fastpath_bytes[vtype];
  upb_func *handler = gethandler(h, sel);
  upb_fieldtype_t ftype;
  const upb_shim_data *data;

  if (handler) {
    |1:
    |  chkneob  fastbytes, >3
    |2:
    switch (vtype) {
    case V32:
      |  call   ->decodev32_fallback
      break;
    case V64:
      |  call   ->decodev64_fallback
      break;
    case F32:
      |  call   ->decodef32_fallback
      break;
    case F64:
      |  call   ->decodef64_fallback
      break;
    case X: break;
    }
    |  jmp    >4

    /* Fast path decode; for when check_bytes bytes are available. */
    |3:
    switch (op) {
    case OP_PARSE_SFIXED32:
    case OP_PARSE_FIXED32:
      |  mov    edx, dword [PTR]
      break;
    case OP_PARSE_SFIXED64:
    case OP_PARSE_FIXED64:
      |  mov    rdx, qword [PTR]
      break;
    case OP_PARSE_FLOAT:
      |  movss  xmm0, dword [PTR]
      break;
    case OP_PARSE_DOUBLE:
      |  movsd  xmm0, qword [PTR]
      break;
    default:
      /* Inline one byte of varint decoding. */
      |  movzx  edx, byte [PTR]
      |  test   dl, dl
      |  js     <2   // Fallback to slow path for >1 byte varint.
      break;
    }

    /* Second-stage decode; used for both fast and slow paths */
    /* (only needed for a few types). */
    |4:
    switch (op) {
    case OP_PARSE_SINT32:
      /* 32-bit zig-zag decode. */
      |  mov    eax, edx
      |  shr    edx, 1
      |  and    eax, 1
      |  neg    eax
      |  xor    edx, eax
      break;
    case OP_PARSE_SINT64:
      /* 64-bit zig-zag decode. */
      |  mov    rax, rdx
      |  shr    rdx, 1
      |  and    rax, 1
      |  neg    rax
      |  xor    rdx, rax
      break;
    case OP_PARSE_BOOL:
      |  test   rdx, rdx
      |  setne  dl
      break;
    default: break;
    }

    /* Call callback (or specialize if we can). */
    data = upb_shim_getdata(h, sel, &ftype);
    if (data) {
      switch (ftype) {
        case UPB_TYPE_INT64:
        case UPB_TYPE_UINT64:
          |  mov   [CLOSURE + data->offset], rdx
          break;
        case UPB_TYPE_INT32:
        case UPB_TYPE_UINT32:
        case UPB_TYPE_ENUM:
          |  mov   [CLOSURE + data->offset], edx
          break;
        case UPB_TYPE_DOUBLE:
          |  movsd  qword [CLOSURE + data->offset], XMMARG1
          break;
        case UPB_TYPE_FLOAT:
          |  movss  dword [CLOSURE + data->offset], XMMARG1
          break;
        case UPB_TYPE_BOOL:
          |  mov   [CLOSURE + data->offset], dl
          break;
        case UPB_TYPE_STRING:
        case UPB_TYPE_BYTES:
        case UPB_TYPE_MESSAGE:
          UPB_ASSERT(false); break;
      }
      |  sethas CLOSURE, data->hasbit
    } else if (handler) {
      |  mov    ARG1_64, CLOSURE
      |  load_handler_data h, sel
      |  callp  handler
      if (!alwaysok(h, sel)) {
        |  test   al, al
        |  jnz    >5
        |  call   ->suspend
        |  jmp    <1
        |5:
      }
    }

    /* We do this last so that the checkpoint is not advanced past the user's
     * data until the callback has returned success. */
    |  add    PTR, fastbytes
  } else {
    /* No handler registered for this value, just skip it. */
    |  chkneob  fastbytes, >3
    |2:
    switch (vtype) {
    case V32:
      |  call   ->skipv32_fallback
      break;
    case V64:
      |  call   ->skipv64_fallback
      break;
    case F32:
      |  call   ->skipf32_fallback
      break;
    case F64:
      |  call   ->skipf64_fallback
      break;
    case X: break;
    }

    /* Fast-path skip. */
    |3:
    if (vtype == V32 || vtype == V64) {
      |  test   byte [PTR], 0x80
      |  jnz    <2
    }
    |  add    PTR, fastbytes
  }
}

static void jitdispatch(jitcompiler *jc,
                        const upb_pbdecodermethod *method) {
  /* Lots of room for tweaking/optimization here. */

  const upb_inttable *dispatch = &method->dispatch;
  bool has_hash_entries = (dispatch->t.count > 0);

  /* Whether any of the fields for this message can have two wire types which
   * are both valid (packed & non-packed).
   *
   * OPT: populate this more precisely; not all messages with hash entries have
   * this characteristic. */
  bool has_multi_wiretype = has_hash_entries;

  |=>define_jmptarget(jc, &method->dispatch):
  |1:
  /* Decode the field tag. */
  |  mov     aword DECODER->checkpoint, PTR
  |  chkeob  2, >6
  |  movzx   edx, byte [PTR]
  |  test    dl, dl
  |  jns     >7    // Jump if first byte has no continuation bit.
  |  movzx   ecx, byte [PTR + 1]
  |  test    cl, cl
  |  js      >6    // Jump if second byte has continuation bit.
  |  // Confirmed two-byte varint.
  |  shl     ecx, 7
  |  and     edx, 0x7f
  |  or      edx, ecx
  |  add     PTR, 2
  |  jmp     >8
  |6:
  |  call    ->decode_unknown_tag_fallback
  |  test    eax, eax  // Hit DELIMEND?
  |  jnz     >8
  |  ret
  |7:
  |  add     PTR, 1
  |8:
  |  mov     ecx, edx
  |  shr     edx, 3
  |  and     cl, 7

  /* See comment attached to upb_pbdecodermethod.dispatch for layout of the
   * dispatch table. */
  |2:
  |  cmp     edx, dispatch->array_size
  if (has_hash_entries) {
    |  jae     >7
  } else {
    |  jae     >5
  }
  |  // OPT: Compact the lookup arr into 32-bit entries.
  if ((uintptr_t)dispatch->array > 0x7fffffff) {
    |  mov64 rax, (uintptr_t)dispatch->array
    |  mov   rax, qword [rax + rdx * 8]
  } else {
    |  mov   rax, qword [rdx * 8 + dispatch->array]
  }
  |3:
  |  // We take advantage of the fact that non-present entries are stored
  |  // as -1, which will result in wire types that will never match.
  |  cmp  al, cl
  if (has_multi_wiretype) {
    |  jne  >6
  } else {
    |  jne  >5
  }
  |  shr  rax, 16
  |
  |  // Load the machine code address from the table entry.
  |  // The table entry is relative to the dispatch->array jmptarget
  |  // (patchdispatch() took care of this) which is the same as
  |  // local label "4".  The "lea" is really just trying to do
  |  //    lea  rax, [>4 + rax]
  |  //
  |  // But we can't write that directly for some reason, so we use
  |  // rdx as a temporary.
  |  lea  rdx, [>4]
  |=>define_jmptarget(jc, dispatch->array):
  |4:
  |  add  rax, rdx
  |  ret
  |
  |5:
  |  // Field isn't in our table.
  |  call ->parse_unknown
  |  test eax, eax  // ENDGROUP?
  |  jz   <1
  |  lea  rax, [>9]  // ENDGROUP; Load address of OP_ENDMSG.
  |  ret

  if (has_multi_wiretype) {
    |6:
    |  // Primary wire type didn't match, check secondary wire type.
    |  cmp  ah, cl
    |  jne  <5
    |  // Secondary wire type is a match, look up fn + UPB_MAX_FIELDNUMBER.
    |  add   rdx, UPB_MAX_FIELDNUMBER
    |  // This key will never be in the array part, so do a hash lookup.
    UPB_ASSERT(has_hash_entries);
    |  ld64  dispatch
    |  jmp   ->hashlookup  // Tail call.
  }

  if (has_hash_entries) {
    |7:
    |  // Hash table lookup.
    |  ld64   dispatch
    |  call   ->hashlookup
    |  jmp    <3
  }
}

static void jittag(jitcompiler *jc, uint64_t tag, int n, int ofs,
                   const upb_pbdecodermethod *method) {
  /* Internally we parse unknown fields; if this runs us into DELIMEND we jump
   * to the corresponding DELIMEND target (either msg end or repeated field
   * end), which we find from the OP_CHECKDELIM which must have necessarily
   * preceded us. */
  uint32_t last_instruction = *(jc->pc - 2);
  int last_arg = (int32_t)last_instruction >> 8;
  uint32_t *delimend = (jc->pc - 1) + last_arg;
  const size_t ptr_words = sizeof(void*) / sizeof(uint32_t);

  UPB_ASSERT((last_instruction & 0xff) == OP_CHECKDELIM);

  if (getop(*(jc->pc - 1)) == OP_TAGN) {
    jc->pc += ptr_words;
  }

  |  chkneob n, >1

  |  // OPT: this is way too much fallback code to put here.
  |  // Reduce and/or move to a separate section to make better icache usage.
  |  ld64  tag
  |  call  ->checktag_fallback
  |  cmp   eax, DECODE_MISMATCH
  |  je    >3
  |  cmp   eax, DECODE_EOF
  |  je     =>jmptarget(jc, delimend)
  |  jmp   >5

  |1:
  switch (n) {
  case 1:
    |  cmp  byte [PTR], tag
    break;
  case 2:
    |  cmp  word [PTR], tag
    break;
  case 3:
    |   // OPT: Slightly more efficient code, but depends on an extra byte.
    |   // mov  eax, dword [PTR]
    |   // shl  eax, 8
    |   // cmp  eax, tag << 8
    |   cmp  word [PTR], (tag & 0xffff)
    |   jne  >2
    |   cmp  byte [PTR + 2], (tag >> 16)
    |2:
    break;
  case 4:
    |   cmp  dword [PTR], tag
    break;
  case 5:
    |   cmp  dword [PTR], (tag & 0xffffffff)
    |   jne  >3
    |   cmp  byte  [PTR + 4], (tag >> 32)
  }
  |  je    >4
  |3:
  if (ofs == 0) {
    |  call   =>jmptarget(jc, &method->dispatch)
    |  test   rax, rax
    |  jz     =>jmptarget(jc, delimend)
    |  jmp    rax
  } else {
    |  jmp    =>jmptarget(jc, jc->pc + ofs)
  }
  |4:
  |  add    PTR, n
  |5:
}

/* Compile the bytecode to x64. */
static void jitbytecode(jitcompiler *jc) {
  upb_pbdecodermethod *method = NULL;
  const upb_handlers *h = NULL;
  for (jc->pc = jc->group->bytecode; jc->pc < jc->group->bytecode_end; ) {
    int32_t instr = *jc->pc;
    opcode op = instr & 0xff;
    uint32_t arg = instr >> 8;
    int32_t longofs = arg;

    if (op != OP_SETDISPATCH) {
      /* Skipped for SETDISPATCH because it defines its own asmlabel for the
       * dispatch code it emits. */
      asmlabel(jc, "0x%lx.%s", pcofs(jc), upb_pbdecoder_getopname(op));

      /* Skipped for SETDISPATCH because it should point at the function
       * prologue, not the dispatch function that is emitted first.
       * TODO: optimize this to only define pclabels that are actually used. */
      |=>define_jmptarget(jc, jc->pc):
    }

    jc->pc++;

    switch (op) {
    case OP_STARTMSG: {
      upb_func *startmsg = gethandler(h, UPB_STARTMSG_SELECTOR);
      if (startmsg) {
        /* bool startmsg(void *closure, const void *hd) */
        |1:
        |  mov   ARG1_64, CLOSURE
        |  load_handler_data h, UPB_STARTMSG_SELECTOR
        |  callp startmsg
        if (!alwaysok(h, UPB_STARTMSG_SELECTOR)) {
          |  test  al, al
          |  jnz   >2
          |  call  ->suspend
          |  jmp   <1
          |2:
        }
      } else {
        | nop
      }
      break;
    }
    case OP_ENDMSG: {
      upb_func *endmsg = gethandler(h, UPB_ENDMSG_SELECTOR);
      |9:
      if (endmsg) {
        /* bool endmsg(void *closure, const void *hd, upb_status *status) */
        |  mov   ARG1_64, CLOSURE
        |  load_handler_data h, UPB_ENDMSG_SELECTOR
        |  mov   ARG3_64, DECODER->status
        |  callp endmsg
      }
      break;
    }
    case OP_SETDISPATCH: {
      uint32_t *op_pc = jc->pc - 1;
      const char *msgname;
      upb_inttable *dispatch;

      /* Load info for new method. */
      memcpy(&dispatch, jc->pc, sizeof(void*));
      jc->pc += sizeof(void*) / sizeof(uint32_t);
      /* The OP_SETDISPATCH bytecode contains a pointer that is
       * &method->dispatch; we want to go backwards and recover method. */
      method =
          (void*)((char*)dispatch - offsetof(upb_pbdecodermethod, dispatch));
      /* May be NULL, in which case no handlers for this message will be found.
       * OPT: we should do better by completely skipping the message in this
       * case instead of parsing it field by field.  We should also do the skip
       * in the containing message's code. */
      h = method->dest_handlers_;
      msgname = upb_msgdef_fullname(upb_handlers_msgdef(h));

      /* Emit dispatch code for new method. */
      asmlabel(jc, "0x%lx.dispatch.%s", pcofs(jc), msgname);
      jitdispatch(jc, method);

      /* Emit function prologue for new method. */
      asmlabel(jc, "0x%lx.parse.%s", pcofs(jc), msgname);
      |=>define_jmptarget(jc, op_pc):
      |=>define_jmptarget(jc, method):
      |  sub   rsp, 8

      break;
    }
    case OP_PARSE_DOUBLE:
    case OP_PARSE_FLOAT:
    case OP_PARSE_INT64:
    case OP_PARSE_UINT64:
    case OP_PARSE_INT32:
    case OP_PARSE_FIXED64:
    case OP_PARSE_FIXED32:
    case OP_PARSE_BOOL:
    case OP_PARSE_UINT32:
    case OP_PARSE_SFIXED32:
    case OP_PARSE_SFIXED64:
    case OP_PARSE_SINT32:
    case OP_PARSE_SINT64:
      jitprimitive(jc, op, h, arg);
      break;
    case OP_STARTSEQ:
    case OP_STARTSUBMSG:
    case OP_STARTSTR: {
      upb_func *start = gethandler(h, arg);
      if (start) {
        /* void *startseq(void *closure, const void *hd)
         * void *startsubmsg(void *closure, const void *hd)
         * void *startstr(void *closure, const void *hd, size_t size_hint) */
        |1:
        |  mov   ARG1_64, CLOSURE
        |  load_handler_data h, arg
        if (op == OP_STARTSTR) {
          |  mov    ARG3_64, DELIMEND
          |  sub    ARG3_64, PTR
        }
        |  callp start
        if (!alwaysok(h, arg)) {
          |  test  rax, rax
          |  jnz   >2
          |  call  ->suspend
          |  jmp   <1
          |2:
        }
        |  mov   CLOSURE, rax
      } else {
        /* TODO: nop is only required because of asmlabel(). */
        |  nop
      }
      break;
    }
    case OP_ENDSEQ:
    case OP_ENDSUBMSG:
    case OP_ENDSTR: {
      upb_func *end = gethandler(h, arg);
      if (end) {
        /* bool endseq(void *closure, const void *hd)
         * bool endsubmsg(void *closure, const void *hd)
         * bool endstr(void *closure, const void *hd) */
        |1:
        |  mov   ARG1_64, CLOSURE
        |  load_handler_data h, arg
        |  callp end
        if (!alwaysok(h, arg)) {
          |  test  al, al
          |  jnz   >2
          |  call  ->suspend
          |  jmp   <1
          |2:
        }
      } else {
        /* TODO: nop is only required because of asmlabel(). */
        |  nop
      }
      break;
    }
    case OP_STRING: {
      upb_func *str = gethandler(h, arg);
      |  cmp   PTR, DELIMEND
      |  je    >4
      |1:
      |  cmp   PTR, DATAEND
      |  jne   >2
      |  call  ->suspend
      |  jmp   <1
      |2:
      if (str) {
        /* size_t str(void *closure, const void *hd, const char *str,
         *            size_t n) */
        |  mov   ARG1_64, CLOSURE
        |  load_handler_data h, arg
        |  mov   ARG3_64, PTR
        |  mov   ARG4_64, DATAEND
        |  sub   ARG4_64, PTR
        |  mov   ARG5_64, qword DECODER->handle
        |  callp str
        |  add   PTR, rax
        if (!alwaysok(h, arg)) {
          |  cmp   PTR, DATAEND
          |  je    >3
          |  call  ->strret_fallback
          |3:
        }
      } else {
        |  mov   PTR, DATAEND
      }
      |  cmp   PTR, DELIMEND
      |  jne   <1
      |4:
      break;
    }
    case OP_PUSHTAGDELIM:
      |  mov   FRAME->sink.closure, CLOSURE
      |  // This shouldn't need to be read, because tag-delimited fields
      |  // shouldn't have an OP_SETDELIM after them.  But for the moment
      |  // non-packed repeated fields do OP_SETDELIM so they can share more
      |  // code with the packed code-path.  If this is changed later, this
      |  // store can be removed.
      |  mov   qword FRAME->end_ofs, 0
      |  cmp   FRAME, DECODER->limit
      |  je    ->err
      |  add   FRAME, sizeof(upb_pbdecoder_frame)
      |  mov   dword FRAME->groupnum, arg
      break;
    case OP_PUSHLENDELIM:
      |  call  ->pushlendelim
      break;
    case OP_POP:
      |  sub   FRAME, sizeof(upb_pbdecoder_frame)
      |  mov   CLOSURE, FRAME->sink.closure
      break;
    case OP_SETDELIM:
      /* OPT: experiment with testing vs old offset to optimize away. */
      |  mov   DATAEND, DECODER->end
      |  add   DELIMEND, FRAME->end_ofs
      |  cmp   DELIMEND, DECODER->buf
      |  jb    >1
      |  cmp   DELIMEND, DATAEND
      |  ja    >1   // OPT: try cmov.
      |  mov   DATAEND, DELIMEND
      |1:
      break;
    case OP_SETBIGGROUPNUM:
      |  mov   dword FRAME->groupnum, *jc->pc++
      break;
    case OP_CHECKDELIM:
      |  cmp  DELIMEND, PTR
      |  je   =>jmptarget(jc, jc->pc + longofs)
      break;
    case OP_CALL:
      |  call =>jmptarget(jc, jc->pc + longofs)
      break;
    case OP_BRANCH:
      |  jmp  =>jmptarget(jc, jc->pc + longofs);
      break;
    case OP_RET:
      |9:
      |  add  rsp, 8
      |  ret
      break;
    case OP_TAG1:
      jittag(jc, (arg >> 8) & 0xff, 1, (int8_t)arg, method);
      break;
    case OP_TAG2:
      jittag(jc, (arg >> 8) & 0xffff, 2, (int8_t)arg, method);
      break;
    case OP_TAGN: {
      uint64_t tag;
      memcpy(&tag, jc->pc, 8);
      jittag(jc, tag, arg >> 8, (int8_t)arg, method);
      break;
    }
    case OP_DISPATCH:
      |  call   =>jmptarget(jc, &method->dispatch)
      break;
    case OP_HALT:
      UPB_ASSERT(false);
    }
  }

  asmlabel(jc, "eof");
  |  nop
}