arc: Add GNU/Linux support for ARC

ARC Linux targets differences from baremetal:

- No support for hardware single instruction stepping.
- Different access rules to registers.
- Use of another instruction for breakpoints.

v2: Changes after Tom's remarks [1]
 arc-linux-tdep.c
  - Use true/false instead of TRUE/FALSE.
  - arc_linux_sw_breakpoint_from_kind (): Break long lines into two.
  - arc_linux_sw_breakpoint_from_kind (): Remove starting blank line.
  - Use explicit number evaluation, e.g: if (a & b) -> if ((a & b) != 0)
 arc-tdep.c
  - Use explicit number evaluation, e.g: if (a & b) -> if ((a & b) != 0)
 gdb/configure.tgt
  - arc*-*-linux*): Remove "build_gdbserver=yes".

v3: Changes after Simon's remarks [2]
  arc-linux-tdep.c
  - Use "return trap_size" instead of cryptic "return 2".
  - Removed unnecessary curly braces.
  - Removed "void" from "_initialize_arc_linux_tdep (void)".

v5: Changes after Simon's remarks [3]
- Remove unnecessary empty lines.
- Replace "breakpoint uses" with "breakpoints use" in a comment.
- "return condition;" i.s.o. "if (condition) return true; else return false;"

[1] Tom's remarks
https://sourceware.org/pipermail/gdb-patches/2020-April/167887.html

[2] Simon's remarks on v2
https://sourceware.org/pipermail/gdb-patches/2020-May/168513.html

[3] Simon's remarks on v4
https://sourceware.org/pipermail/gdb-patches/2020-August/170994.html

gdb/ChangeLog:

2020-08-25  Anton Kolesov  <anton.kolesov@synopsys.com>

	* configure.tgt: ARC support for GNU/Linux.
	* Makefile.in (ALL_TARGET_OBJS): Likewise.
	* arc-linux-tdep.c: New file.
	* arc-tdep.h (ARC_STATUS32_L_MASK, ARC_STATUS32_DE_MASK): Declare.
	* arc-tdep.c (arc_write_pc): Use it.

6 files changed, 298 insertions, 2 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 2e97cdc..150a7af 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2020-08-25  Anton Kolesov  <anton.kolesov@synopsys.com>
+
+	* configure.tgt: ARC support for GNU/Linux.
+	* Makefile.in (ALL_TARGET_OBJS): Likewise.
+	* arc-linux-tdep.c: New file.
+	* arc-tdep.h (ARC_STATUS32_L_MASK, ARC_STATUS32_DE_MASK): Declare.
+	* arc-tdep.c (arc_write_pc): Use it.
+
 2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
 
 	* arc-tdep.c (arc_check_for_hardware_loop): New.
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 66abac4..4808357 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -702,6 +702,7 @@ ALL_64_TARGET_OBS = \
 # All other target-dependent objects files (used with --enable-targets=all).
 ALL_TARGET_OBS = \
 	aarch32-tdep.o \
+	arc-linux-tdep.o \
 	arc-tdep.o \
 	arch/aarch32.o \
 	arch/arc.o \
diff --git a/gdb/arc-linux-tdep.c b/gdb/arc-linux-tdep.c
new file mode 100644
index 0000000..36f3245
--- /dev/null
+++ b/gdb/arc-linux-tdep.c
@@ -0,0 +1,278 @@
+/* Target dependent code for GNU/Linux ARC.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* GDB header files.  */
+#include "defs.h"
+#include "linux-tdep.h"
+#include "objfiles.h"
+#include "opcode/arc.h"
+#include "osabi.h"
+#include "solib-svr4.h"
+
+/* ARC header files.  */
+#include "opcodes/arc-dis.h"
+#include "arc-tdep.h"
+
+/* Implement the "cannot_fetch_register" gdbarch method.  */
+
+static int
+arc_linux_cannot_fetch_register (struct gdbarch *gdbarch, int regnum)
+{
+  /* Assume that register is readable if it is unknown.  */
+  switch (regnum)
+    {
+    case ARC_ILINK_REGNUM:
+    case ARC_RESERVED_REGNUM:
+    case ARC_LIMM_REGNUM:
+      return true;
+    case ARC_R30_REGNUM:
+    case ARC_R58_REGNUM:
+    case ARC_R59_REGNUM:
+      return !arc_mach_is_arcv2 (gdbarch);
+    }
+  return (regnum > ARC_BLINK_REGNUM) && (regnum < ARC_LP_COUNT_REGNUM);
+}
+
+/* Implement the "cannot_store_register" gdbarch method.  */
+
+static int
+arc_linux_cannot_store_register (struct gdbarch *gdbarch, int regnum)
+{
+  /* Assume that register is writable if it is unknown.  */
+  switch (regnum)
+    {
+    case ARC_ILINK_REGNUM:
+    case ARC_RESERVED_REGNUM:
+    case ARC_LIMM_REGNUM:
+    case ARC_PCL_REGNUM:
+      return true;
+    case ARC_R30_REGNUM:
+    case ARC_R58_REGNUM:
+    case ARC_R59_REGNUM:
+      return !arc_mach_is_arcv2 (gdbarch);
+    }
+  return (regnum > ARC_BLINK_REGNUM) && (regnum < ARC_LP_COUNT_REGNUM);
+}
+
+/* For ARC Linux, breakpoints use the 16-bit TRAP_S 1 instruction, which
+   is 0x3e78 (little endian) or 0x783e (big endian).  */
+
+static const gdb_byte arc_linux_trap_s_be[] = { 0x78, 0x3e };
+static const gdb_byte arc_linux_trap_s_le[] = { 0x3e, 0x78 };
+static const int trap_size = 2;   /* Number of bytes to insert "trap".  */
+
+/* Implement the "breakpoint_kind_from_pc" gdbarch method.  */
+
+static int
+arc_linux_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr)
+{
+  return trap_size;
+}
+
+/* Implement the "sw_breakpoint_from_kind" gdbarch method.  */
+
+static const gdb_byte *
+arc_linux_sw_breakpoint_from_kind (struct gdbarch *gdbarch,
+				   int kind, int *size)
+{
+  *size = kind;
+  return ((gdbarch_byte_order (gdbarch) == BFD_ENDIAN_BIG)
+	  ? arc_linux_trap_s_be
+	  : arc_linux_trap_s_le);
+}
+
+/* Implement the "software_single_step" gdbarch method.  */
+
+static std::vector<CORE_ADDR>
+arc_linux_software_single_step (struct regcache *regcache)
+{
+  struct gdbarch *gdbarch = regcache->arch ();
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+  struct disassemble_info di = arc_disassemble_info (gdbarch);
+
+  /* Read current instruction.  */
+  struct arc_instruction curr_insn;
+  arc_insn_decode (regcache_read_pc (regcache), &di, arc_delayed_print_insn,
+		   &curr_insn);
+  CORE_ADDR next_pc = arc_insn_get_linear_next_pc (curr_insn);
+
+  std::vector<CORE_ADDR> next_pcs;
+
+  /* For instructions with delay slots, the fall thru is not the
+     instruction immediately after the current instruction, but the one
+     after that.  */
+  if (curr_insn.has_delay_slot)
+    {
+      struct arc_instruction next_insn;
+      arc_insn_decode (next_pc, &di, arc_delayed_print_insn, &next_insn);
+      next_pcs.push_back (arc_insn_get_linear_next_pc (next_insn));
+    }
+  else
+    next_pcs.push_back (next_pc);
+
+  ULONGEST status32;
+  regcache_cooked_read_unsigned (regcache, gdbarch_ps_regnum (gdbarch),
+				 &status32);
+
+  if (curr_insn.is_control_flow)
+    {
+      CORE_ADDR branch_pc = arc_insn_get_branch_target (curr_insn);
+      if (branch_pc != next_pc)
+	next_pcs.push_back (branch_pc);
+    }
+  /* Is current instruction the last in a loop body?  */
+  else if (tdep->has_hw_loops)
+    {
+      /* If STATUS32.L is 1, then ZD-loops are disabled.  */
+      if ((status32 & ARC_STATUS32_L_MASK) == 0)
+	{
+	  ULONGEST lp_end, lp_start, lp_count;
+	  regcache_cooked_read_unsigned (regcache, ARC_LP_START_REGNUM,
+					 &lp_start);
+	  regcache_cooked_read_unsigned (regcache, ARC_LP_END_REGNUM, &lp_end);
+	  regcache_cooked_read_unsigned (regcache, ARC_LP_COUNT_REGNUM,
+					 &lp_count);
+
+	  if (arc_debug)
+	    {
+	      debug_printf ("arc-linux: lp_start = %s, lp_end = %s, "
+			    "lp_count = %s, next_pc = %s\n",
+			    paddress (gdbarch, lp_start),
+			    paddress (gdbarch, lp_end),
+			    pulongest (lp_count),
+			    paddress (gdbarch, next_pc));
+	    }
+
+	  if (next_pc == lp_end && lp_count > 1)
+	    {
+	      /* The instruction is in effect a jump back to the start of
+		 the loop.  */
+	      next_pcs.push_back (lp_start);
+	    }
+	}
+    }
+
+  /* Is this a delay slot?  Then next PC is in BTA register.  */
+  if ((status32 & ARC_STATUS32_DE_MASK) != 0)
+    {
+      ULONGEST bta;
+      regcache_cooked_read_unsigned (regcache, ARC_BTA_REGNUM, &bta);
+      next_pcs.push_back (bta);
+    }
+
+  return next_pcs;
+}
+
+/* Implement the "skip_solib_resolver" gdbarch method.
+
+   See glibc_skip_solib_resolver for details.  */
+
+static CORE_ADDR
+arc_linux_skip_solib_resolver (struct gdbarch *gdbarch, CORE_ADDR pc)
+{
+  /* For uClibc 0.9.26+.
+
+     An unresolved PLT entry points to "__dl_linux_resolve", which calls
+     "_dl_linux_resolver" to do the resolving and then eventually jumps to
+     the function.
+
+     So we look for the symbol `_dl_linux_resolver', and if we are there,
+     gdb sets a breakpoint at the return address, and continues.  */
+  struct bound_minimal_symbol resolver
+    = lookup_minimal_symbol ("_dl_linux_resolver", NULL, NULL);
+
+  if (arc_debug)
+    {
+      if (resolver.minsym != nullptr)
+	{
+	  CORE_ADDR res_addr = BMSYMBOL_VALUE_ADDRESS (resolver);
+	  debug_printf ("arc-linux: skip_solib_resolver (): "
+			"pc = %s, resolver at %s\n",
+			print_core_address (gdbarch, pc),
+			print_core_address (gdbarch, res_addr));
+	}
+      else
+	{
+	  debug_printf ("arc-linux: skip_solib_resolver (): "
+			"pc = %s, no resolver found\n",
+			print_core_address (gdbarch, pc));
+	}
+    }
+
+  if (resolver.minsym != nullptr && BMSYMBOL_VALUE_ADDRESS (resolver) == pc)
+    {
+      /* Find the return address.  */
+      return frame_unwind_caller_pc (get_current_frame ());
+    }
+  else
+    {
+      /* No breakpoint required.  */
+      return 0;
+    }
+}
+
+/* Initialization specific to Linux environment.  */
+
+static void
+arc_linux_init_osabi (struct gdbarch_info info, struct gdbarch *gdbarch)
+{
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+
+  if (arc_debug)
+    debug_printf ("arc-linux: GNU/Linux OS/ABI initialization.\n");
+
+  /* If we are using Linux, we have in uClibc
+     (libc/sysdeps/linux/arc/bits/setjmp.h):
+
+     typedef int __jmp_buf[13+1+1+1];    //r13-r25, fp, sp, blink
+
+     Where "blink" is a stored PC of a caller function.
+   */
+  tdep->jb_pc = 15;
+
+  linux_init_abi (info, gdbarch);
+
+  /* Set up target dependent GDB architecture entries.  */
+  set_gdbarch_cannot_fetch_register (gdbarch, arc_linux_cannot_fetch_register);
+  set_gdbarch_cannot_store_register (gdbarch, arc_linux_cannot_store_register);
+  set_gdbarch_breakpoint_kind_from_pc (gdbarch,
+				       arc_linux_breakpoint_kind_from_pc);
+  set_gdbarch_sw_breakpoint_from_kind (gdbarch,
+				       arc_linux_sw_breakpoint_from_kind);
+  set_gdbarch_fetch_tls_load_module_address (gdbarch,
+					     svr4_fetch_objfile_link_map);
+  set_gdbarch_software_single_step (gdbarch, arc_linux_software_single_step);
+  set_gdbarch_skip_trampoline_code (gdbarch, find_solib_trampoline_target);
+  set_gdbarch_skip_solib_resolver (gdbarch, arc_linux_skip_solib_resolver);
+
+  /* GNU/Linux uses SVR4-style shared libraries, with 32-bit ints, longs
+     and pointers (ILP32).  */
+  set_solib_svr4_fetch_link_map_offsets (gdbarch,
+					 svr4_ilp32_fetch_link_map_offsets);
+}
+
+/* Suppress warning from -Wmissing-prototypes.  */
+extern initialize_file_ftype _initialize_arc_linux_tdep;
+
+void
+_initialize_arc_linux_tdep ()
+{
+  gdbarch_register_osabi (bfd_arch_arc, 0, GDB_OSABI_LINUX,
+			  arc_linux_init_osabi);
+}
diff --git a/gdb/arc-tdep.c b/gdb/arc-tdep.c
index 8b4e285..6f544bf 100644
--- a/gdb/arc-tdep.c
+++ b/gdb/arc-tdep.c
@@ -612,8 +612,7 @@ arc_write_pc (struct regcache *regcache, CORE_ADDR new_pc)
   regcache_cooked_read_unsigned (regcache, gdbarch_ps_regnum (gdbarch),
 				 &status32);
 
-  /* Mask for DE bit is 0x40.  */
-  if (status32 & 0x40)
+  if ((status32 & ARC_STATUS32_DE_MASK) != 0)
     {
       if (arc_debug)
 	{
diff --git a/gdb/arc-tdep.h b/gdb/arc-tdep.h
index e752348..6331d29 100644
--- a/gdb/arc-tdep.h
+++ b/gdb/arc-tdep.h
@@ -100,6 +100,11 @@ enum arc_regnum
    Longer registers are represented as pairs of 32-bit registers.  */
 #define ARC_REGISTER_SIZE  4
 
+/* STATUS32 register: hardware loops disabled bit.  */
+#define ARC_STATUS32_L_MASK (1 << 12)
+/* STATUS32 register: current instruction is a delay slot.  */
+#define ARC_STATUS32_DE_MASK (1 << 6)
+
 #define arc_print(fmt, args...) fprintf_unfiltered (gdb_stdlog, fmt, ##args)
 
 extern int arc_debug;
diff --git a/gdb/configure.tgt b/gdb/configure.tgt
index 8a26bde..a3e11c4 100644
--- a/gdb/configure.tgt
+++ b/gdb/configure.tgt
@@ -157,6 +157,11 @@ arc*-*-elf32)
 	gdb_target_obs="arc-newlib-tdep.o"
 	;;
 
+arc*-*-linux*)
+	# Target: ARC machine running Linux
+	gdb_target_obs="arc-linux-tdep.o linux-tdep.o solib-svr4.o"
+	;;
+
 arm*-wince-pe | arm*-*-mingw32ce*)
 	# Target: ARM based machine running Windows CE (win32)
 	gdb_target_obs="arm-wince-tdep.o windows-tdep.o"