1 files changed, 739 insertions, 0 deletions

diff --git a/src/lj_gdbjit.c b/src/lj_gdbjit.c
new file mode 100644
index 00000000..dfec188a
--- /dev/null
+++ b/src/lj_gdbjit.c
@@ -0,0 +1,739 @@
+/*
+** Client for the GDB JIT API.
+** Copyright (C) 2005-2009 Mike Pall. See Copyright Notice in luajit.h
+*/
+
+#define lj_gdbjit_c
+#define LUA_CORE
+
+#include "lj_obj.h"
+
+#if LJ_HASJIT
+
+#include "lj_gc.h"
+#include "lj_err.h"
+#include "lj_str.h"
+#include "lj_frame.h"
+#include "lj_jit.h"
+#include "lj_dispatch.h"
+
+/* This is not compiled in by default.
+** Enable with -DLUAJIT_USE_GDBJIT in the Makefile and recompile everything.
+*/
+#ifdef LUAJIT_USE_GDBJIT
+
+/* The GDB JIT API allows JIT compilers to pass debug information about
+** JIT-compiled code back to GDB. You need at least GDB 7.0 or higher
+** to see it in action.
+**
+** This is a passive API, so it works even when not running under GDB
+** or when attaching to an already running process. Alas, this implies
+** enabling it always has a non-negligible overhead -- do not use in
+** release mode!
+**
+** The LuaJIT GDB JIT client is rather minimal at the moment. It gives
+** each trace a symbol name and adds a source location and frame unwind
+** information. Obviously LuaJIT itself and any embedding C application
+** should be compiled with debug symbols, too (see the Makefile).
+**
+** Traces are named TRACE_1, TRACE_2, ... these correspond to the trace
+** numbers from -jv or -jdump. Use "break TRACE_1" or "tbreak TRACE_1" etc.
+** to set breakpoints on specific traces (even ahead of their creation).
+**
+** The source location for each trace allows listing the corresponding
+** source lines with the GDB command "list" (but only if the Lua source
+** has been loaded from a file). Currently this is always set to the
+** location where the trace has been started.
+**
+** Frame unwind information can be inspected with the GDB command
+** "info frame". This also allows proper backtraces across JIT-compiled
+** code with the GDB command "bt".
+**
+** You probably want to add the following settings to a .gdbinit file
+** (or add them to ~/.gdbinit):
+**   set disassembly-flavor intel
+**   set breakpoint pending on
+**
+** Here's a sample GDB session:
+** ------------------------------------------------------------------------
+
+$ cat >x.lua
+for outer=1,100 do
+  for inner=1,100 do end
+end
+^D
+
+$ luajit -jv x.lua
+[TRACE   1 x.lua:2]
+[TRACE   2 (1/3) x.lua:1 -> 1]
+
+$ gdb --quiet --args luajit x.lua
+(gdb) tbreak TRACE_1
+Function "TRACE_1" not defined.
+Temporary breakpoint 1 (TRACE_1) pending.
+(gdb) run
+Starting program: luajit x.lua
+
+Temporary breakpoint 1, TRACE_1 () at x.lua:2
+2         for inner=1,100 do end
+(gdb) list
+1       for outer=1,100 do
+2         for inner=1,100 do end
+3       end
+(gdb) bt
+#0  TRACE_1 () at x.lua:2
+#1  0x08053690 in lua_pcall [...]
+[...]
+#7  0x0806ff90 in main [...]
+(gdb) disass TRACE_1
+Dump of assembler code for function TRACE_1:
+0xf7fd9fba <TRACE_1+0>: mov    DWORD PTR ds:0xf7e0e2a0,0x1
+0xf7fd9fc4 <TRACE_1+10>:        movsd  xmm7,QWORD PTR [edx+0x20]
+[...]
+0xf7fd9ff8 <TRACE_1+62>:        jmp    0xf7fd2014
+End of assembler dump.
+(gdb) tbreak TRACE_2
+Function "TRACE_2" not defined.
+Temporary breakpoint 2 (TRACE_2) pending.
+(gdb) cont
+Continuing.
+
+Temporary breakpoint 2, TRACE_2 () at x.lua:1
+1       for outer=1,100 do
+(gdb) info frame
+Stack level 0, frame at 0xffffd7c0:
+ eip = 0xf7fd9f60 in TRACE_2 (x.lua:1); saved eip 0x8053690
+ called by frame at 0xffffd7e0
+ source language unknown.
+ Arglist at 0xffffd78c, args:
+ Locals at 0xffffd78c, Previous frame's sp is 0xffffd7c0
+ Saved registers:
+  ebx at 0xffffd7ac, ebp at 0xffffd7b8, esi at 0xffffd7b0, edi at 0xffffd7b4,
+  eip at 0xffffd7bc
+(gdb)
+
+** ------------------------------------------------------------------------
+*/
+
+/* -- GDB JIT API --------------------------------------------------------- */
+
+/* GDB JIT actions. */
+enum {
+  GDBJIT_NOACTION = 0,
+  GDBJIT_REGISTER,
+  GDBJIT_UNREGISTER
+};
+
+/* GDB JIT entry. */
+typedef struct GDBJITentry {
+  struct GDBJITentry *next_entry;
+  struct GDBJITentry *prev_entry;
+  const char *symfile_addr;
+  uint64_t symfile_size;
+} GDBJITentry;
+
+/* GDB JIT descriptor. */
+typedef struct GDBJITdesc {
+  uint32_t version;
+  uint32_t action_flag;
+  GDBJITentry *relevant_entry;
+  GDBJITentry *first_entry;
+} GDBJITdesc;
+
+GDBJITdesc __jit_debug_descriptor = {
+  1, GDBJIT_NOACTION, NULL, NULL
+};
+
+/* GDB sets a breakpoint at this function. */
+void LJ_NOINLINE __jit_debug_register_code()
+{
+  __asm__ __volatile__("");
+};
+
+/* -- In-memory ELF object definitions ------------------------------------ */
+
+/* ELF definitions. */
+typedef struct ELFheader {
+  uint8_t emagic[4];
+  uint8_t eclass;
+  uint8_t eendian;
+  uint8_t eversion;
+  uint8_t eosabi;
+  uint8_t eabiversion;
+  uint8_t epad[7];
+  uint16_t type;
+  uint16_t machine;
+  uint32_t version;
+  uintptr_t entry;
+  uintptr_t phofs;
+  uintptr_t shofs;
+  uint32_t flags;
+  uint16_t ehsize;
+  uint16_t phentsize;
+  uint16_t phnum;
+  uint16_t shentsize;
+  uint16_t shnum;
+  uint16_t shstridx;
+} ELFheader;
+
+typedef struct ELFsectheader {
+  uint32_t name;
+  uint32_t type;
+  uintptr_t flags;
+  uintptr_t addr;
+  uintptr_t ofs;
+  uintptr_t size;
+  uint32_t link;
+  uint32_t info;
+  uintptr_t align;
+  uintptr_t entsize;
+} ELFsectheader;
+
+#define ELFSECT_IDX_ABS         0xfff1
+
+enum {
+  ELFSECT_TYPE_PROGBITS = 1,
+  ELFSECT_TYPE_SYMTAB = 2,
+  ELFSECT_TYPE_STRTAB = 3,
+  ELFSECT_TYPE_NOBITS = 8
+};
+
+#define ELFSECT_FLAGS_WRITE     1
+#define ELFSECT_FLAGS_ALLOC     2
+#define ELFSECT_FLAGS_EXEC      4
+
+typedef struct ELFsymbol {
+#if LJ_64
+  uint32_t name;
+  uint8_t info;
+  uint8_t other;
+  uint16_t sectidx;
+  uintptr_t value;
+  uint64_t size;
+#else
+  uint32_t name;
+  uintptr_t value;
+  uint32_t size;
+  uint8_t info;
+  uint8_t other;
+  uint16_t sectidx;
+#endif
+} ELFsymbol;
+
+enum {
+  ELFSYM_TYPE_FUNC = 2,
+  ELFSYM_TYPE_FILE = 4,
+  ELFSYM_BIND_LOCAL = 0 << 4,
+  ELFSYM_BIND_GLOBAL = 1 << 4,
+};
+
+/* DWARF definitions. */
+#define DW_CIE_VERSION  1
+
+enum {
+  DW_CFA_nop = 0x0,
+  DW_CFA_def_cfa = 0xc,
+  DW_CFA_def_cfa_offset = 0xe,
+  DW_CFA_advance_loc = 0x40,
+  DW_CFA_offset = 0x80
+};
+
+enum {
+  DW_EH_PE_udata4 = 3,
+  DW_EH_PE_textrel = 0x20
+};
+
+enum {
+  DW_TAG_compile_unit = 0x11
+};
+
+enum {
+  DW_children_no = 0,
+  DW_children_yes = 1
+};
+
+enum {
+  DW_AT_name = 0x03,
+  DW_AT_stmt_list = 0x10,
+  DW_AT_low_pc = 0x11,
+  DW_AT_high_pc = 0x12
+};
+
+enum {
+  DW_FORM_addr = 0x01,
+  DW_FORM_data4 = 0x06,
+  DW_FORM_string = 0x08
+};
+
+enum {
+  DW_LNS_extended_op = 0,
+  DW_LNS_copy = 1,
+  DW_LNS_advance_pc = 2,
+  DW_LNS_advance_line = 3
+};
+
+enum {
+  DW_LNE_end_sequence = 1,
+  DW_LNE_set_address = 2
+};
+
+enum {
+#if LJ_TARGET_X86
+  DW_REG_AX, DW_REG_CX, DW_REG_DX, DW_REG_BX,
+  DW_REG_SP, DW_REG_BP, DW_REG_SI, DW_REG_DI,
+  DW_REG_RA,
+#elif LJ_TARGET_X64
+  /* Yes, the order is strange, but correct. */
+  DW_REG_AX, DW_REG_DX, DW_REG_CX, DW_REG_BX,
+  DW_REG_SI, DW_REG_DI, DW_REG_BP, DW_REG_SP,
+  DW_REG_8, DW_REG_9, DW_REG_10, DW_REG_11,
+  DW_REG_12, DW_REG_13, DW_REG_14, DW_REG_15,
+  DW_REG_RA,
+#else
+#error "Unsupported target architecture"
+#endif
+};
+
+/* Minimal list of sections for the in-memory ELF object. */
+enum {
+  GDBJIT_SECT_NULL,
+  GDBJIT_SECT_text,
+  GDBJIT_SECT_eh_frame,
+  GDBJIT_SECT_shstrtab,
+  GDBJIT_SECT_strtab,
+  GDBJIT_SECT_symtab,
+  GDBJIT_SECT_debug_info,
+  GDBJIT_SECT_debug_abbrev,
+  GDBJIT_SECT_debug_line,
+  GDBJIT_SECT__MAX
+};
+
+enum {
+  GDBJIT_SYM_UNDEF,
+  GDBJIT_SYM_FILE,
+  GDBJIT_SYM_FUNC,
+  GDBJIT_SYM__MAX
+};
+
+/* In-memory ELF object. */
+typedef struct GDBJITobj {
+  ELFheader hdr;                        /* ELF header. */
+  ELFsectheader sect[GDBJIT_SECT__MAX]; /* ELF sections. */
+  ELFsymbol sym[GDBJIT_SYM__MAX];       /* ELF symbol table. */
+  uint8_t space[4096];                  /* Space for various section data. */
+} GDBJITobj;
+
+/* Combined structure for GDB JIT entry and ELF object. */
+typedef struct GDBJITentryobj {
+  GDBJITentry entry;
+  size_t sz;
+  GDBJITobj obj;
+} GDBJITentryobj;
+
+/* Template for in-memory ELF header. */
+static const ELFheader elfhdr_template = {
+  .emagic = { 0x7f, 'E', 'L', 'F' },
+  .eclass = LJ_64 ? 2 : 1,
+  .eendian = LJ_ENDIAN_SELECT(1, 2),
+  .eversion = 1,
+#if defined(__linux__)
+  .eosabi = 0,  /* Nope, it's not 3. */
+#elif defined(__FreeBSD__)
+  .eosabi = 9,
+#elif defined(__NetBSD__)
+  .eosabi = 2,
+#elif defined(__OpenBSD__)
+  .eosabi = 12,
+#elif defined(__solaris__)
+  .eosabi = 6,
+#else
+  .eosabi = 0,
+#endif
+  .eabiversion = 0,
+  .epad = { 0, 0, 0, 0, 0, 0, 0 },
+  .type = 1,
+#if LJ_TARGET_X86
+  .machine = 3,
+#elif LJ_TARGET_X64
+  .machine = 62,
+#else
+#error "Unsupported target architecture"
+#endif
+  .version = 1,
+  .entry = 0,
+  .phofs = 0,
+  .shofs = offsetof(GDBJITobj, sect),
+  .flags = 0,
+  .ehsize = sizeof(ELFheader),
+  .phentsize = 0,
+  .phnum = 0,
+  .shentsize = sizeof(ELFsectheader),
+  .shnum = GDBJIT_SECT__MAX,
+  .shstridx = GDBJIT_SECT_shstrtab
+};
+
+/* -- In-memory ELF object generation ------------------------------------- */
+
+/* Context for generating the ELF object for the GDB JIT API. */
+typedef struct GDBJITctx {
+  uint8_t *p;           /* Pointer to next address in obj.space. */
+  uint8_t *startp;      /* Pointer to start address in obj.space. */
+  Trace *T;             /* Generate symbols for this trace. */
+  uintptr_t mcaddr;     /* Machine code address. */
+  MSize szmcode;        /* Size of machine code. */
+  MSize spadjp;         /* Stack adjustment for parent trace or interpreter. */
+  MSize spadj;          /* Stack adjustment for trace itself. */
+  BCLine lineno;        /* Starting line number. */
+  const char *filename; /* Starting file name. */
+  const char *trname;   /* Name of trace. */
+  size_t objsize;       /* Final size of ELF object. */
+  GDBJITobj obj;        /* In-memory ELF object. */
+} GDBJITctx;
+
+/* Add a zero-terminated string. */
+static uint32_t gdbjit_strz(GDBJITctx *ctx, const char *str)
+{
+  uint8_t *p = ctx->p;
+  uint32_t ofs = (uint32_t)(p - ctx->startp);
+  do {
+    *p++ = (uint8_t)*str;
+  } while (*str++);
+  ctx->p = p;
+  return ofs;
+}
+
+/* Add a ULEB128 value. */
+static void gdbjit_uleb128(GDBJITctx *ctx, uint32_t v)
+{
+  uint8_t *p = ctx->p;
+  for (; v >= 0x80; v >>= 7)
+    *p++ = (uint8_t)((v & 0x7f) | 0x80);
+  *p++ = (uint8_t)v;
+  ctx->p = p;
+}
+
+/* Add a SLEB128 value. */
+static void gdbjit_sleb128(GDBJITctx *ctx, int32_t v)
+{
+  uint8_t *p = ctx->p;
+  for (; (uint32_t)(v+0x40) >= 0x80; v >>= 7)
+    *p++ = (uint8_t)((v & 0x7f) | 0x80);
+  *p++ = (uint8_t)(v & 0x7f);
+  ctx->p = p;
+}
+
+/* Shortcuts to generate DWARF structures. */
+#define DB(x)           (*p++ = (x))
+#define DI8(x)          (*(int8_t *)p = (x), p++)
+#define DU16(x)         (*(uint16_t *)p = (x), p += 2)
+#define DU32(x)         (*(uint32_t *)p = (x), p += 4)
+#define DADDR(x)        (*(uintptr_t *)p = (x), p += sizeof(uintptr_t))
+#define DUV(x)          (ctx->p = p, gdbjit_uleb128(ctx, (x)), p = ctx->p)
+#define DSV(x)          (ctx->p = p, gdbjit_sleb128(ctx, (x)), p = ctx->p)
+#define DSTR(str)       (ctx->p = p, gdbjit_strz(ctx, (str)), p = ctx->p)
+#define DALIGNNOP(s)    while ((uintptr_t)p & ((s)-1)) *p++ = DW_CFA_nop
+#define DSECT(name, stmt) \
+  { uint32_t *szp_##name = (uint32_t *)p; p += 4; stmt \
+    *szp_##name = (uint32_t)((p-(uint8_t *)szp_##name)-4); } \
+
+/* Initialize ELF section headers. */
+static void LJ_FASTCALL gdbjit_secthdr(GDBJITctx *ctx)
+{
+  ELFsectheader *sect;
+
+  *ctx->p++ = '\0';  /* Empty string at start of string table. */
+
+#define SECTDEF(id, tp, al) \
+  sect = &ctx->obj.sect[GDBJIT_SECT_##id]; \
+  sect->name = gdbjit_strz(ctx, "." #id); \
+  sect->type = ELFSECT_TYPE_##tp; \
+  sect->align = (al)
+
+  SECTDEF(text, NOBITS, 16);
+  sect->flags = ELFSECT_FLAGS_ALLOC|ELFSECT_FLAGS_EXEC;
+  sect->addr = ctx->mcaddr;
+  sect->ofs = 0;
+  sect->size = ctx->szmcode;
+
+  SECTDEF(eh_frame, PROGBITS, sizeof(uintptr_t));
+  sect->flags = ELFSECT_FLAGS_ALLOC;
+
+  SECTDEF(shstrtab, STRTAB, 1);
+  SECTDEF(strtab, STRTAB, 1);
+
+  SECTDEF(symtab, SYMTAB, sizeof(uintptr_t));
+  sect->ofs = offsetof(GDBJITobj, sym);
+  sect->size = sizeof(ctx->obj.sym);
+  sect->link = GDBJIT_SECT_strtab;
+  sect->entsize = sizeof(ELFsymbol);
+  sect->info = GDBJIT_SYM_FUNC;
+
+  SECTDEF(debug_info, PROGBITS, 1);
+  SECTDEF(debug_abbrev, PROGBITS, 1);
+  SECTDEF(debug_line, PROGBITS, 1);
+
+#undef SECTDEF
+}
+
+/* Initialize symbol table. */
+static void LJ_FASTCALL gdbjit_symtab(GDBJITctx *ctx)
+{
+  ELFsymbol *sym;
+
+  *ctx->p++ = '\0';  /* Empty string at start of string table. */
+
+  sym = &ctx->obj.sym[GDBJIT_SYM_FILE];
+  sym->name = gdbjit_strz(ctx, "JIT mcode");
+  sym->sectidx = ELFSECT_IDX_ABS;
+  sym->info = ELFSYM_TYPE_FILE|ELFSYM_BIND_LOCAL;
+
+  sym = &ctx->obj.sym[GDBJIT_SYM_FUNC];
+  sym->name = gdbjit_strz(ctx, ctx->trname);
+  sym->sectidx = GDBJIT_SECT_text;
+  sym->value = 0;
+  sym->size = ctx->szmcode;
+  sym->info = ELFSYM_TYPE_FUNC|ELFSYM_BIND_GLOBAL;
+}
+
+/* Initialize .eh_frame section. */
+static void LJ_FASTCALL gdbjit_ehframe(GDBJITctx *ctx)
+{
+  uint8_t *p = ctx->p;
+  uint8_t *framep = p;
+
+  /* Emit DWARF EH CIE. */
+  DSECT(CIE,
+    DU32(0);                    /* Offset to CIE itself. */
+    DB(DW_CIE_VERSION);
+    DSTR("zR");                 /* Augmentation. */
+    DUV(1);                     /* Code alignment factor. */
+    DSV(-(int32_t)sizeof(uintptr_t));  /* Data alignment factor. */
+    DB(DW_REG_RA);              /* Return address register. */
+    DB(1); DB(DW_EH_PE_textrel|DW_EH_PE_udata4);  /* Augmentation data. */
+    DB(DW_CFA_def_cfa); DUV(DW_REG_SP); DUV(sizeof(uintptr_t));
+    DB(DW_CFA_offset|DW_REG_RA); DUV(1);
+    DALIGNNOP(sizeof(uintptr_t));
+  )
+
+  /* Emit DWARF EH FDE. */
+  DSECT(FDE,
+    DU32((uint32_t)(p-framep)); /* Offset to CIE. */
+    DU32(0);                    /* Machine code offset relative to .text. */
+    DU32(ctx->szmcode);         /* Machine code length. */
+    DB(0);                      /* Augmentation data. */
+    /* Registers saved in CFRAME. */
+#if LJ_TARGET_X86
+    DB(DW_CFA_offset|DW_REG_BP); DUV(2);
+    DB(DW_CFA_offset|DW_REG_DI); DUV(3);
+    DB(DW_CFA_offset|DW_REG_SI); DUV(4);
+    DB(DW_CFA_offset|DW_REG_BX); DUV(5);
+#elif LJ_TARGET_X64
+    /* Add saved registers for x64 CFRAME. */
+#else
+#error "Unsupported target architecture"
+#endif
+    if (ctx->spadjp != ctx->spadj) {  /* Parent/interpreter stack frame size. */
+      DB(DW_CFA_def_cfa_offset); DUV(ctx->spadjp);
+      DB(DW_CFA_advance_loc|1);  /* Only an approximation. */
+    }
+    DB(DW_CFA_def_cfa_offset); DUV(ctx->spadj);  /* Trace stack frame size. */
+    DALIGNNOP(sizeof(uintptr_t));
+  )
+
+  ctx->p = p;
+}
+
+/* Initialize .debug_info section. */
+static void LJ_FASTCALL gdbjit_debuginfo(GDBJITctx *ctx)
+{
+  uint8_t *p = ctx->p;
+
+  DSECT(info,
+    DU16(2);                    /* DWARF version. */
+    DU32(0);                    /* Abbrev offset. */
+    DB(sizeof(uintptr_t));      /* Pointer size. */
+
+    DUV(1);                     /* Abbrev #1: DW_TAG_compile_unit. */
+    DSTR(ctx->filename);        /* DW_AT_name. */
+    DADDR(ctx->mcaddr);         /* DW_AT_low_pc. */
+    DADDR(ctx->mcaddr + ctx->szmcode);  /* DW_AT_high_pc. */
+    DU32(0);                    /* DW_AT_stmt_list. */
+  )
+
+  ctx->p = p;
+}
+
+/* Initialize .debug_abbrev section. */
+static void LJ_FASTCALL gdbjit_debugabbrev(GDBJITctx *ctx)
+{
+  uint8_t *p = ctx->p;
+
+  /* Abbrev #1: DW_TAG_compile_unit. */
+  DUV(1); DUV(DW_TAG_compile_unit);
+  DB(DW_children_no);
+  DUV(DW_AT_name);      DUV(DW_FORM_string);
+  DUV(DW_AT_low_pc);    DUV(DW_FORM_addr);
+  DUV(DW_AT_high_pc);   DUV(DW_FORM_addr);
+  DUV(DW_AT_stmt_list); DUV(DW_FORM_data4);
+  DB(0); DB(0);
+
+  ctx->p = p;
+}
+
+#define DLNE(op, s)     (DB(DW_LNS_extended_op), DUV(1+(s)), DB((op)))
+
+/* Initialize .debug_line section. */
+static void LJ_FASTCALL gdbjit_debugline(GDBJITctx *ctx)
+{
+  uint8_t *p = ctx->p;
+
+  DSECT(line,
+    DU16(2);                    /* DWARF version. */
+    DSECT(header,
+      DB(1);                    /* Minimum instruction length. */
+      DB(1);                    /* is_stmt. */
+      DI8(0);                   /* Line base for special opcodes. */
+      DB(2);                    /* Line range for special opcodes. */
+      DB(3+1);                  /* Opcode base at DW_LNS_advance_line+1. */
+      DB(0); DB(1); DB(1);      /* Standard opcode lengths. */
+      /* Directory table. */
+      DB(0);
+      /* File name table. */
+      DSTR(ctx->filename); DUV(0); DUV(0); DUV(0);
+      DB(0);
+    )
+
+    DLNE(DW_LNE_set_address, sizeof(uintptr_t)); DADDR(ctx->mcaddr);
+    if (ctx->lineno) {
+      DB(DW_LNS_advance_line); DSV(ctx->lineno-1);
+    }
+    DB(DW_LNS_copy);
+    DB(DW_LNS_advance_pc); DUV(ctx->szmcode);
+    DLNE(DW_LNE_end_sequence, 0);
+  )
+
+  ctx->p = p;
+}
+
+#undef DLNE
+
+/* Undef shortcuts. */
+#undef DB
+#undef DI8
+#undef DU16
+#undef DU32
+#undef DADDR
+#undef DUV
+#undef DSV
+#undef DSTR
+#undef DALIGNNOP
+#undef DSECT
+
+/* Type of a section initializer callback. */
+typedef void (LJ_FASTCALL *GDBJITinitf)(GDBJITctx *ctx);
+
+/* Call section initializer and set the section offset and size. */
+static void gdbjit_initsect(GDBJITctx *ctx, int sect, GDBJITinitf initf)
+{
+  ctx->startp = ctx->p;
+  ctx->obj.sect[sect].ofs = (uintptr_t)((char *)ctx->p - (char *)&ctx->obj);
+  initf(ctx);
+  ctx->obj.sect[sect].size = (uintptr_t)(ctx->p - ctx->startp);
+}
+
+#define SECTALIGN(p, a) \
+  ((p) = (uint8_t *)(((uintptr_t)(p) + ((a)-1)) & ~(uintptr_t)((a)-1)))
+
+/* Build in-memory ELF object. */
+static void gdbjit_buildobj(GDBJITctx *ctx)
+{
+  GDBJITobj *obj = &ctx->obj;
+  /* Fill in ELF header and clear structures. */
+  memcpy(&obj->hdr, &elfhdr_template, sizeof(ELFheader));
+  memset(&obj->sect, 0, sizeof(ELFsectheader)*GDBJIT_SECT__MAX);
+  memset(&obj->sym, 0, sizeof(ELFsymbol)*GDBJIT_SYM__MAX);
+  /* Initialize sections. */
+  ctx->p = obj->space;
+  gdbjit_initsect(ctx, GDBJIT_SECT_shstrtab, gdbjit_secthdr);
+  gdbjit_initsect(ctx, GDBJIT_SECT_strtab, gdbjit_symtab);
+  gdbjit_initsect(ctx, GDBJIT_SECT_debug_info, gdbjit_debuginfo);
+  gdbjit_initsect(ctx, GDBJIT_SECT_debug_abbrev, gdbjit_debugabbrev);
+  gdbjit_initsect(ctx, GDBJIT_SECT_debug_line, gdbjit_debugline);
+  SECTALIGN(ctx->p, sizeof(uintptr_t));
+  gdbjit_initsect(ctx, GDBJIT_SECT_eh_frame, gdbjit_ehframe);
+  ctx->objsize = (size_t)((char *)ctx->p - (char *)obj);
+  lua_assert(ctx->objsize < sizeof(GDBJITobj));
+}
+
+#undef SECTALIGN
+
+/* -- Interface to GDB JIT API -------------------------------------------- */
+
+/* Add new entry to GDB JIT symbol chain. */
+static void gdbjit_newentry(lua_State *L, GDBJITctx *ctx)
+{
+  /* Allocate memory for GDB JIT entry and ELF object. */
+  MSize sz = (MSize)(sizeof(GDBJITentryobj) - sizeof(GDBJITobj) + ctx->objsize);
+  GDBJITentryobj *eo = lj_mem_newt(L, sz, GDBJITentryobj);
+  memcpy(&eo->obj, &ctx->obj, ctx->objsize);  /* Copy ELF object. */
+  eo->sz = sz;
+  ctx->T->gdbjit_entry = (void *)eo;
+  /* Link new entry to chain and register it. */
+  eo->entry.prev_entry = NULL;
+  eo->entry.next_entry = __jit_debug_descriptor.first_entry;
+  if (eo->entry.next_entry)
+    eo->entry.next_entry->prev_entry = &eo->entry;
+  eo->entry.symfile_addr = (const char *)&eo->obj;
+  eo->entry.symfile_size = ctx->objsize;
+  __jit_debug_descriptor.first_entry = &eo->entry;
+  __jit_debug_descriptor.relevant_entry = &eo->entry;
+  __jit_debug_descriptor.action_flag = GDBJIT_REGISTER;
+  __jit_debug_register_code();
+}
+
+/* Add debug info for newly compiled trace and notify GDB. */
+void lj_gdbjit_addtrace(jit_State *J, Trace *T, TraceNo traceno)
+{
+  GDBJITctx ctx;
+  lua_State *L = J->L;
+  GCproto *pt = &gcref(T->startpt)->pt;
+  TraceNo parent = T->ir[REF_BASE].op1;
+  uintptr_t pcofs = (uintptr_t)(T->snap[0].mapofs+T->snap[0].nslots);
+  const BCIns *startpc = (const BCIns *)(uintptr_t)T->snapmap[pcofs];
+  ctx.T = T;
+  ctx.mcaddr = (uintptr_t)T->mcode;
+  ctx.szmcode = T->szmcode;
+  ctx.spadjp = CFRAME_SIZE + (MSize)(parent ? J->trace[parent]->spadjust : 0);
+  ctx.spadj = CFRAME_SIZE + T->spadjust;
+  ctx.lineno = pt->lineinfo ? pt->lineinfo[startpc - pt->bc] : 0;
+  ctx.filename = strdata(pt->chunkname);
+  if (*ctx.filename == '@' || *ctx.filename == '=')
+    ctx.filename++;
+  else
+    ctx.filename = "(string)";
+  ctx.trname = lj_str_pushf(L, "TRACE_%d", traceno);
+  L->top--;
+  gdbjit_buildobj(&ctx);
+  gdbjit_newentry(L, &ctx);
+}
+
+/* Delete debug info for trace and notify GDB. */
+void lj_gdbjit_deltrace(jit_State *J, Trace *T)
+{
+  GDBJITentryobj *eo = (GDBJITentryobj *)T->gdbjit_entry;
+  if (eo) {
+    if (eo->entry.prev_entry)
+      eo->entry.prev_entry->next_entry = eo->entry.next_entry;
+    else
+      __jit_debug_descriptor.first_entry = eo->entry.next_entry;
+    if (eo->entry.next_entry)
+      eo->entry.next_entry->prev_entry = eo->entry.prev_entry;
+    __jit_debug_descriptor.relevant_entry = &eo->entry;
+    __jit_debug_descriptor.action_flag = GDBJIT_UNREGISTER;
+    __jit_debug_register_code();
+    lj_mem_free(J2G(J), eo, eo->sz);
+  }
+}
+
+#endif
+#endif