コグノスケ

1993                                          2000
OPeNBooK  ---,    1996                   ,--> オープンブック（The Towerの版権を持つ）
             +--> オープンブック9003 ---+--> シノミクス
9003, inc ---'
1990

targetm.hard_regno_mode_ok() のRISC-V向け実装

// gcc/config/riscv/riscv.c

/* Implement TARGET_HARD_REGNO_MODE_OK.  */

static bool
riscv_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
{
  unsigned int nregs = riscv_hard_regno_nregs (regno, mode);

  if (GP_REG_P (regno))
    {
      if (!GP_REG_P (regno + nregs - 1))
	return false;
    }
  else if (FP_REG_P (regno))
    {
      if (!FP_REG_P (regno + nregs - 1))
	return false;

      if (GET_MODE_CLASS (mode) != MODE_FLOAT
	  && GET_MODE_CLASS (mode) != MODE_COMPLEX_FLOAT)
	return false;

      /* Only use callee-saved registers if a potential callee is guaranteed
	 to spill the requisite width.  */
      if (GET_MODE_UNIT_SIZE (mode) > UNITS_PER_FP_REG
	  || (!call_used_or_fixed_reg_p (regno)
	      && GET_MODE_UNIT_SIZE (mode) > UNITS_PER_FP_ARG))
	return false;
    }
  else if (VP_REG_P (regno))    //★★前回足した実装
    {
      return true;
    }
  else
    return false;

...

machine modeクラスの定義

// gcc/machmode.def

...

/* Basic integer modes.  We go up to TI in generic code (128 bits).
   TImode is needed here because the some front ends now genericly
   support __int128.  If the front ends decide to generically support
   larger types, then corresponding modes must be added here.  The
   name OI is reserved for a 256-bit type (needed by some back ends).
    */

//★★MODE_INTクラスになる

INT_MODE (QI, 1);
INT_MODE (HI, 2);
INT_MODE (SI, 4);
INT_MODE (DI, 8);
INT_MODE (TI, 16);

...


// gcc/config/riscv/riscv-modes.def

//★★MODE_FLOATクラスになる

FLOAT_MODE (TF, 16, ieee_quad_format);

//★★以前、追加した実装
//★★VECTOR_MODEの場合は少し特殊で、MODE_VECTOR + 最初の引数 クラスになる
//★★この例だとMODE_VECTOR_INTになる

VECTOR_MODE (INT, SI, 32);
VECTOR_MODE (INT, SI, 64);

machine modeとクラスの対応、早見方法

// build_gcc/insn-modes.c

const unsigned char mode_class[NUM_MACHINE_MODES] =
{
  MODE_RANDOM,             /* VOID */
  MODE_RANDOM,             /* BLK */
  MODE_CC,                 /* CC */
  MODE_INT,                /* BI */
  MODE_INT,                /* QI */
  MODE_INT,                /* HI */
  MODE_INT,                /* SI */
  MODE_INT,                /* DI */
  MODE_INT,                /* TI */
  MODE_FRACT,              /* QQ */
  MODE_FRACT,              /* HQ */
  MODE_FRACT,              /* SQ */
  MODE_FRACT,              /* DQ */
  MODE_FRACT,              /* TQ */
  MODE_UFRACT,             /* UQQ */
  MODE_UFRACT,             /* UHQ */
  MODE_UFRACT,             /* USQ */
  MODE_UFRACT,             /* UDQ */
  MODE_UFRACT,             /* UTQ */
  MODE_ACCUM,              /* HA */
  MODE_ACCUM,              /* SA */
  MODE_ACCUM,              /* DA */
  MODE_ACCUM,              /* TA */
  MODE_UACCUM,             /* UHA */
  MODE_UACCUM,             /* USA */
  MODE_UACCUM,             /* UDA */
  MODE_UACCUM,             /* UTA */
  MODE_FLOAT,              /* SF */
  MODE_FLOAT,              /* DF */
  MODE_FLOAT,              /* TF */

...

  MODE_COMPLEX_FLOAT,      /* SC */
  MODE_COMPLEX_FLOAT,      /* DC */
  MODE_COMPLEX_FLOAT,      /* TC */
  MODE_VECTOR_INT,         /* V32SI */
  MODE_VECTOR_INT,         /* V64SI */
};

targetm.hard_regno_mode_ok() のRISC-V向け実装、修正版

// gcc/config/riscv/riscv.c

/* Implement TARGET_HARD_REGNO_MODE_OK.  */

static bool
riscv_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
{
  unsigned int nregs = riscv_hard_regno_nregs (regno, mode);

  if (GP_REG_P (regno))
    {
      if (!GP_REG_P (regno + nregs - 1))
	return false;
    }
  else if (FP_REG_P (regno))
    {
      if (!FP_REG_P (regno + nregs - 1))
	return false;

      if (GET_MODE_CLASS (mode) != MODE_FLOAT
	  && GET_MODE_CLASS (mode) != MODE_COMPLEX_FLOAT)
	return false;

      /* Only use callee-saved registers if a potential callee is guaranteed
	 to spill the requisite width.  */
      if (GET_MODE_UNIT_SIZE (mode) > UNITS_PER_FP_REG
	  || (!call_used_or_fixed_reg_p (regno)
	      && GET_MODE_UNIT_SIZE (mode) > UNITS_PER_FP_ARG))
	return false;
    }
  else if (VP_REG_P (regno))    //★★前回足した実装
    {
      if (GET_MODE_CLASS (mode) != MODE_VECTOR_INT
	  && GET_MODE_CLASS (mode) != MODE_VECTOR_FLOAT)    //★★今回足した実装
	return false;

      return true;
    }
  else
    return false;

...

エラーが起きたときの281r.ira

...

      Popping a57(r107,l0)  -- assign reg 26
      Popping a58(r106,l0)  -- assign reg 27
      Popping a59(r105,l0)  -- assign reg 64    ★64はv0レジスタの番号
      Popping a60(r104,l0)  -- assign reg 65    ★65はv1レジスタの番号
      Popping a61(r165,l0)  -- assign reg 5

...

281r.iraでPopping ... を出力しているコード

// gcc/ira-color.c

/* Pop the coloring stack and assign hard registers to the popped
   allocnos.  */
static void
pop_allocnos_from_stack (void)
{
  ira_allocno_t allocno;
  enum reg_class aclass;

  for (;allocno_stack_vec.length () != 0;)
    {
      allocno = allocno_stack_vec.pop ();
      aclass = ALLOCNO_CLASS (allocno);
      if (internal_flag_ira_verbose > 3 && ira_dump_file != NULL)
	{
	  fprintf (ira_dump_file, "      Popping");  //★★Poppingはここで出力
	  ira_print_expanded_allocno (allocno);
	  fprintf (ira_dump_file, "  -- ");
	}
      if (aclass == NO_REGS)
	{
	  ALLOCNO_HARD_REGNO (allocno) = -1;
	  ALLOCNO_ASSIGNED_P (allocno) = true;
	  ira_assert (ALLOCNO_UPDATED_HARD_REG_COSTS (allocno) == NULL);
	  ira_assert
	    (ALLOCNO_UPDATED_CONFLICT_HARD_REG_COSTS (allocno) == NULL);
	  if (internal_flag_ira_verbose > 3 && ira_dump_file != NULL)
	    fprintf (ira_dump_file, "assign memory\n");
	}
      else if (assign_hard_reg (allocno, false))    //★★レジスタの割当をする
	{
	  if (internal_flag_ira_verbose > 3 && ira_dump_file != NULL)
	    fprintf (ira_dump_file, "assign reg %d\n",
		     ALLOCNO_HARD_REGNO (allocno));    //★★assign reg 64はここで出力
	}


// gcc/ira-build.c

/* This recursive function outputs allocno A and if it is a cap the
   function outputs its members.  */
void
ira_print_expanded_allocno (ira_allocno_t a)
{
  basic_block bb;

  //★★Poppingの後ろのaxx(rxx, lxx) を出力している

  fprintf (ira_dump_file, " a%d(r%d", ALLOCNO_NUM (a), ALLOCNO_REGNO (a));
  if ((bb = ALLOCNO_LOOP_TREE_NODE (a)->bb) != NULL)
    fprintf (ira_dump_file, ",b%d", bb->index);
  else
    fprintf (ira_dump_file, ",l%d", ALLOCNO_LOOP_TREE_NODE (a)->loop_num);
  if (ALLOCNO_CAP_MEMBER (a) != NULL)
    {
      fprintf (ira_dump_file, ":");
      ira_print_expanded_allocno (ALLOCNO_CAP_MEMBER (a));
    }
  fprintf (ira_dump_file, ")");
}


// gcc/ira-int.h

#define ALLOCNO_NUM(A) ((A)->num)
#define ALLOCNO_REGNO(A) ((A)->regno)
...
#define ALLOCNO_HARD_REGNO(A) ((A)->hard_regno)
...

281r.iraのPoppingの値

いずれもira_allocno_tのメンバを参照する。

（例）
Popping a59(r105,l0)  -- assign reg 64

  - a59   : num
  - r105  : regno
  - l0    : loop_num
  - reg 64: hard_regno

281r.iraでレジスタ割当を決めるコード

// gcc/ira-color.c

static bool
assign_hard_reg (ira_allocno_t a, bool retry_p)
{
  HARD_REG_SET conflicting_regs[2], profitable_hard_regs;
  int i, j, hard_regno, best_hard_regno, class_size;

...

  best_hard_regno = -1;

...

  /* We don't care about giving callee saved registers to allocnos no
     living through calls because call clobbered registers are
     allocated first (it is usual practice to put them first in
     REG_ALLOC_ORDER).  */
  mode = ALLOCNO_MODE (a);
  for (i = 0; i < class_size; i++)
    {
      hard_regno = ira_class_hard_regs[aclass][i];
#ifdef STACK_REGS
      if (no_stack_reg_p
	  && FIRST_STACK_REG <= hard_regno && hard_regno <= LAST_STACK_REG)
	continue;
#endif
      if (! check_hard_reg_p (a, hard_regno,
			      conflicting_regs, profitable_hard_regs))    //★★このチェックを通過すると、割り当てる候補になる
	continue;
      cost = costs[i];
      full_cost = full_costs[i];
      if (!HONOR_REG_ALLOC_ORDER)
	{

...

	}
      if (min_cost > cost)
	min_cost = cost;
      if (min_full_cost > full_cost
	  || (!HONOR_REG_ALLOC_ORDER && min_full_cost == full_cost
	      && best_hard_regno > hard_regno))
	{
	  min_full_cost = full_cost;
	  best_hard_regno = hard_regno;    //★★どのハードウェアレジスタに割り当てるか決まる
	  ira_assert (hard_regno >= 0);
	}
      if (internal_flag_ira_verbose > 5 && ira_dump_file != NULL)
	fprintf (ira_dump_file, "(%d=%d,%d) ", hard_regno, cost, full_cost);
    }

...

  if (! retry_p)
    restore_costs_from_copies (a);
  ALLOCNO_HARD_REGNO (a) = best_hard_regno;    //★★allocno->hard_regnoに格納
  ALLOCNO_ASSIGNED_P (a) = true;
  if (best_hard_regno >= 0)
    update_costs_from_copies (a, true, ! retry_p);
  ira_assert (ALLOCNO_CLASS (a) == aclass);
  /* We don't need updated costs anymore.  */
  ira_free_allocno_updated_costs (a);
  return best_hard_regno >= 0;    //★★best_hard_regnoに何か値が入っていれば成功
}

281r.iraでレジスタ割当候補を判断するコード

// gcc/ira-color.c

/* Return true if HARD_REGNO is ok for assigning to allocno A with
   PROFITABLE_REGS and whose objects have CONFLICT_REGS.  */
static inline bool
check_hard_reg_p (ira_allocno_t a, int hard_regno,
		  HARD_REG_SET *conflict_regs, HARD_REG_SET profitable_regs)
{
  int j, nwords, nregs;
  enum reg_class aclass;
  machine_mode mode;

  aclass = ALLOCNO_CLASS (a);
  mode = ALLOCNO_MODE (a);
  if (TEST_HARD_REG_BIT (ira_prohibited_class_mode_regs[aclass][mode],
			 hard_regno))    //★★このチェックを通過したら割当候補
    return false;
  /* Checking only profitable hard regs.  */
  if (! TEST_HARD_REG_BIT (profitable_regs, hard_regno))    //★★このチェックは基本的に通過するはず
    return false;

...


// gcc/ira-int.h

#define ALLOCNO_MODE(A) ((A)->mode)
...
#define ALLOCNO_CLASS(A) ((A)->aclass)
...

aclass, modeの値

ブレークに設定する条件はif hard_regno == 64

(gdb) p aclass
$4 = ALL_REGS

(gdb) p mode
$5 = E_SFmode

281r.iraでレジスタ割当候補を判断に使うira_prohibited_class_mode_regsを初期化するコード

// gcc/ira.c

static void
setup_prohibited_class_mode_regs (void)
ira_prohibited_class_mode_regs
{
  int j, k, hard_regno, cl, last_hard_regno, count;

  for (cl = (int) N_REG_CLASSES - 1; cl >= 0; cl--)
    {
      temp_hard_regset = reg_class_contents[cl] & ~no_unit_alloc_regs;
      for (j = 0; j < NUM_MACHINE_MODES; j++)
	{
	  count = 0;
	  last_hard_regno = -1;
	  CLEAR_HARD_REG_SET (ira_prohibited_class_mode_regs[cl][j]);
	  for (k = ira_class_hard_regs_num[cl] - 1; k >= 0; k--)
	    {
	      hard_regno = ira_class_hard_regs[cl][k];
	      if (!targetm.hard_regno_mode_ok (hard_regno, (machine_mode) j))    //★★hard_regno_mode_ok() がtrueであれば割当候補になる
		SET_HARD_REG_BIT (ira_prohibited_class_mode_regs[cl][j],
				  hard_regno);
	      else if (in_hard_reg_set_p (temp_hard_regset,
					  (machine_mode) j, hard_regno))
		{
		  last_hard_regno = hard_regno;
		  count++;
		}
	    }
	  ira_class_singleton[cl][j] = (count == 1 ? last_hard_regno : -1);
	}
    }
}


// gcc/config/riscv/riscv.h

enum reg_class
{
  NO_REGS,			/* no registers in set */
  SIBCALL_REGS,			/* registers used by indirect sibcalls */
  JALR_REGS,			/* registers used by indirect calls */
  GR_REGS,			/* integer registers */
  FP_REGS,			/* floating-point registers */
  VP_REGS,			/* vector registers */
  FRAME_REGS,			/* arg pointer and frame pointer */
  ALL_REGS,			/* all registers */
  LIM_REG_CLASSES		/* max value + 1 */
};

#define N_REG_CLASSES (int) LIM_REG_CLASSES

浮動小数点を60個使うプログラム

void _start(void)
{
	static volatile float gf[60];
	float f00, f01, f02, f03, f04, f05, f06, f07, f08, f09;
	float f10, f11, f12, f13, f14, f15, f16, f17, f18, f19;
	float f20, f21, f22, f23, f24, f25, f26, f27, f28, f29;
	float f30, f31, f32, f33, f34, f35, f36, f37, f38, f39;
	float f40, f41, f42, f43, f44, f45, f46, f47, f48, f49;
	float f50, f51, f52, f53, f54, f55, f56, f57, f58, f59;

	__asm__ volatile("nop;\n");

	f00 = gf[ 0]; f01 = gf[ 1]; f02 = gf[ 2]; f03 = gf[ 3]; f04 = gf[ 4];
	f05 = gf[ 5]; f06 = gf[ 6]; f07 = gf[ 7]; f08 = gf[ 8]; f09 = gf[ 9];
	f10 = gf[10]; f11 = gf[11]; f12 = gf[12]; f13 = gf[13]; f14 = gf[14];
	f15 = gf[15]; f16 = gf[16]; f17 = gf[17]; f18 = gf[18]; f19 = gf[19];
	f20 = gf[20]; f21 = gf[21]; f22 = gf[22]; f23 = gf[23]; f24 = gf[24];
	f25 = gf[25]; f26 = gf[26]; f27 = gf[27]; f28 = gf[28]; f29 = gf[29];
	f30 = gf[30]; f31 = gf[31]; f32 = gf[32]; f33 = gf[33]; f34 = gf[34];
	f35 = gf[35]; f36 = gf[36]; f37 = gf[37]; f38 = gf[38]; f39 = gf[39];
	f40 = gf[40]; f41 = gf[41]; f42 = gf[42]; f43 = gf[43]; f44 = gf[44];
	f45 = gf[45]; f46 = gf[46]; f47 = gf[47]; f48 = gf[48]; f49 = gf[49];
	f50 = gf[59]; f51 = gf[51]; f52 = gf[52]; f53 = gf[53]; f54 = gf[54];
	f55 = gf[55]; f56 = gf[56]; f57 = gf[57]; f58 = gf[58]; f59 = gf[59];

	__asm__ volatile("nop;\n");

	gf[ 0] = f00; gf[ 1] = f01; gf[ 2] = f02; gf[ 3] = f03; gf[ 4] = f04;
	gf[ 5] = f05; gf[ 6] = f06; gf[ 7] = f07; gf[ 8] = f08; gf[ 9] = f09;
	gf[10] = f10; gf[11] = f11; gf[12] = f12; gf[13] = f13; gf[14] = f14;
	gf[15] = f15; gf[16] = f16; gf[17] = f17; gf[18] = f18; gf[19] = f19;
	gf[20] = f20; gf[21] = f21; gf[22] = f22; gf[23] = f23; gf[24] = f24;
	gf[25] = f25; gf[26] = f26; gf[27] = f27; gf[28] = f28; gf[29] = f29;
	gf[30] = f30; gf[31] = f31; gf[32] = f32; gf[33] = f33; gf[34] = f34;
	gf[35] = f35; gf[36] = f36; gf[37] = f37; gf[38] = f38; gf[39] = f39;
	gf[40] = f40; gf[41] = f41; gf[42] = f42; gf[43] = f43; gf[44] = f44;
	gf[45] = f45; gf[46] = f46; gf[47] = f47; gf[48] = f48; gf[49] = f49;
	gf[50] = f50; gf[51] = f51; gf[52] = f52; gf[53] = f53; gf[54] = f54;
	gf[55] = f55; gf[56] = f56; gf[57] = f57; gf[58] = f58; gf[59] = f59;

	__asm__ volatile("nop;\n");
}

reloadでコンパイルエラー

$ riscv32-unknown-elf-gcc b.c -march=rv32imafcv -mabi=ilp32f -O3 -Wall -fno-builtin -fno-inline -nostdlib -g

b.c: In function '_start':
b.c:52:1: error: insn does not satisfy its constraints:
   52 | }
      | ^
(insn 562 17 18 2 (set (reg/v:SF 65 v1 [orig:104 f00 ] [104])
        (reg/v:SF 47 fa5 [orig:104 f00 ] [104])) "b.c":23:6 153 {*movsf_hardfloat}
     (nil))

during RTL pass: reload
dump file: b.c.282r.reload
b.c:52:1: internal compiler error: in extract_constrain_insn, at recog.c:2195
0x11b9e4d _fatal_insn(char const*, rtx_def const*, char const*, int, char const*)
        gcc/gcc/rtl-error.c:108
0x11b9ed1 _fatal_insn_not_found(rtx_def const*, char const*, int, char const*)
        gcc/gcc/rtl-error.c:118
0x1130298 extract_constrain_insn(rtx_insn*)
        gcc/gcc/recog.c:2195
0xeabdd8 check_rtl
        gcc/gcc/lra.c:2167
0xead5c0 lra(_IO_FILE*)
        gcc/gcc/lra.c:2604
0xe1a63b do_reload
        gcc/gcc/ira.c:5523
0xe1b078 execute
        gcc/gcc/ira.c:5709

エラーの出力箇所を追う

// gcc/lra.c

/* Function checks RTL for correctness.	 If FINAL_P is true, it is
   done at the end of LRA and the check is more rigorous.  */
static void
check_rtl (bool final_p)
{
  basic_block bb;
  rtx_insn *insn;

  lra_assert (! final_p || reload_completed);
  FOR_EACH_BB_FN (bb, cfun)
    FOR_BB_INSNS (bb, insn)
    if (NONDEBUG_INSN_P (insn)
	&& GET_CODE (PATTERN (insn)) != USE
	&& GET_CODE (PATTERN (insn)) != CLOBBER
	&& GET_CODE (PATTERN (insn)) != ASM_INPUT)
      {
	if (final_p)
	  {
	    extract_constrain_insn (insn);    //★★これ
	    continue;
	  }


// gcc/recog.c

/* Do uncached extract_insn, constrain_operands and complain about failures.
   This should be used when extracting a pre-existing constrained instruction
   if the caller wants to know which alternative was chosen.  */
void
extract_constrain_insn (rtx_insn *insn)
{
  extract_insn (insn);    //★★INSN RTLからrecog_dataを作成する
  if (!constrain_operands (reload_completed, get_enabled_alternatives (insn)))    //★★ここでfalseが返る
    fatal_insn_not_found (insn);    //★★internal compile errorが出力される
}

エラーが発生するRTL

(insn 562 17 18 2 (set (reg/v:SF 65 v1 [orig:104 f00 ] [104])
        (reg/v:SF 47 fa5 [orig:104 f00 ] [104])) "b.c":23:6 145 {*movsf_hardfloat}
     (nil))

ブレークするために少し改造

// gcc/lra.c

/* Function checks RTL for correctness.	 If FINAL_P is true, it is
   done at the end of LRA and the check is more rigorous.  */
static void
check_rtl (bool final_p)
{
  basic_block bb;
  rtx_insn *insn;

  lra_assert (! final_p || reload_completed);
  FOR_EACH_BB_FN (bb, cfun)
    FOR_BB_INSNS (bb, insn)
    if (NONDEBUG_INSN_P (insn)
	&& GET_CODE (PATTERN (insn)) != USE
	&& GET_CODE (PATTERN (insn)) != CLOBBER
	&& GET_CODE (PATTERN (insn)) != ASM_INPUT)
      {
        if (INSN_UID(insn) == 562)      //★★ブレークポイントを仕掛けるために追加した部分
          printf("check point!!\n");    //★★ブレークポイントを仕掛けるために追加した部分

	if (final_p)
	  {
	    extract_constrain_insn (insn);
	    continue;
	  }

エラーが発生するconstraints

(gdb) p recog_data.constraints
$6 = {
  0x2d717f9 "=f,f,f,m,m,*f,*r,*r,*r,*m",
  0x2d71813 "f,G,m,f,G,*r,*f,*G*r,*m,*r",
...

エラーが起きているときのconstraints

// gcc/config/riscv/riscv.md

(define_insn "*movsf_hardfloat"
  [(set (match_operand:SF 0 "nonimmediate_operand" "=f,f,f,m,m,*f,*r,  *r,*r,*m")
	(match_operand:SF 1 "move_operand"         " f,G,m,f,G,*r,*f,*G*r,*m,*r"))]

...

SEGVするバイナリ、64バイト版

$ ls -la a.out

-rwxrwxr-x+ 1 katsuhiro katsuhiro 64 Jul  3 06:39 a.out

SEGVする64バイトバイナリ、実行

$ strace ./a.out

execve("./a.out", ["./a.out"], 0x7ffedf7dfa90 /* 47 vars */) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x400000001} ---
+++ killed by SIGSEGV +++
Segmentation fault

SEGVする64バイトバイナリ、readelf

$ readelf -a a.out
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x400000001
  Start of program headers:          8 (bytes into file)
  Start of section headers:          0 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         1
  Size of section headers:           0 (bytes)
  Number of section headers:         0
  Section header string table index: 0
There are no sections in this file.
There are no sections to group in this file.
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  NULL           0x00000001003e0002 0x0000000400000001 0x0000000000000008
                 0x0000000000000000 0x0038004000000000         0x1
There is no dynamic section in this file.
There are no relocations in this file.
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not curr
ently supported.
Dynamic symbol information is not available for displaying symbols.
No version information found in this file.

0文字でSEGVのバイナリサイズ

$ gcc -nostdlib a.c

/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000001000

$ ls -la a.out

-rwxr-xr-x 1 katsuhiro katsuhiro 9528 Jul  3 04:11 a.out

0文字でSEGVのバイナリサイズ、static版

$ echo -n  > a.c && gcc -nostdlib -static a.c

/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000

$ ls -la a.out

-rwxr-xr-x 1 katsuhiro katsuhiro 968 Jul  3 04:12 a.out

$ strace ./a.out

execve("./a.out", ["./a.out"], 0x7ffc92c2b390 /* 46 vars */) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x401000} ---
+++ killed by SIGSEGV +++
Segmentation fault

SEGVするバイナリ、stripする

$ strip -R ".note.gnu.build-id" -R ".comment" a.out

$ ls -la a.out
-rwxr-xr-x 1 katsuhiro katsuhiro 376 Jul  3 04:18 a.out

SEGVするバイナリ、手で削る

$ ls -la a.out

-rwxrwxr-x+ 1 katsuhiro katsuhiro 92 Jul  3 05:43 a.out

SEGVする92バイトバイナリ、実行

$ strace ./a.out

execve("./a.out", ["./a.out"], 0x7fff2b4f74a0 /* 47 vars */) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0x400000001} ---
+++ killed by SIGSEGV +++
Segmentation fault

SEGVする92バイトバイナリ、readelf

$ readelf -a a.out

ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x400000001
  Start of program headers:          36 (bytes into file)
  Start of section headers:          0 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         1
  Size of section headers:           0 (bytes)
  Number of section headers:         0
  Section header string table index: 0
There are no sections in this file.
There are no sections to group in this file.
Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  NULL           0x0000000000000000 0x0000000100380040 0x0000000000000000
                 0x0000000000000000 0x0000000000000000         0x1000
There is no dynamic section in this file.
There are no relocations in this file.
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Dynamic symbol information is not available for displaying symbols.
No version information found in this file.

16文字でSEGV

$ echo -n '*a;main(){*a=0;}' > a.c && gcc a.c

a.c:1:1: warning: data definition has no type or storage class
    1 | *a;main(){*a=0;}
      | ^
a.c:1:2: warning: type defaults to 'int' in declaration of 'a' [-Wimplicit-int]
    1 | *a;main(){*a=0;}
      |  ^
a.c:1:4: warning: return type defaults to 'int' [-Wimplicit-int]
    1 | *a;main(){*a=0;}
      |    ^~~~

$ ./a.out

Segmentation fault

5文字でSEGV

$ echo -n 'main;' > a.c && gcc a.c

a.c:1:1: warning: data definition has no type or storage class
    1 | main;
      | ^~~~
a.c:1:1: warning: type defaults to 'int' in declaration of 'main' [-Wimplicit-int]

$ ./a.out

Segmentation fault

SEGVしたくない場合

$ echo -n 'main=0xc3;' > a.c && gcc -z execstack a.c

a.c:1:1: warning: data definition has no type or storage class
    1 | main=0xc3;
      | ^~~~
a.c:1:1: warning: type defaults to 'int' in declaration of 'main' [-Wimplicit-int]

$ ./a.out

（SEGVしない）

0文字でSEGV

$ echo -n > a.c && gcc -nostdlib a.c

/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 0000000000001000

$ ./a.out

Segmentation fault