Use the existing instruction to determine the RISC-V breakpoint kind.

RISC-V supports instructions of varying lengths.  Standard existing
instructions in the base ISA are 4 bytes in length, but the 'C'
extension adds support for compressed, 2 byte instructions.  RISC-V
supports two different breakpoint instructions: EBREAK is a 4 byte
instruction in the base ISA, and C.EBREAK is a 2 byte instruction only
available on processors implementing the 'C' extension.  Using EBREAK
to set breakpoints on compressed instructions causes problems as the
second half of EBREAK will overwrite the first 2 bytes of the
following instruction breaking other threads in the process if their
PC is the following instruction.  Thus, breakpoints on compressed
instructions need to use C.EBREAK instead of EBREAK.

Previously, the riscv architecture checked the MISA register to
determine if the 'C' extension was available.  If so, it used C.EBREAK
for all breakpoints.  However, the MISA register is not necessarily
available to supervisor mode operating systems.  While native targets
could provide a fake MISA register value, this patch instead examines
the existing instruction at a breakpoint target to determine which
breakpoint instruction to use.  If the existing instruction is a
compressed instruction, C.EBREAK is used, otherwise EBREAK is used.

gdb/ChangeLog:

	* disasm-selftests.c (print_one_insn_test): Add bfd_arch_riscv to
	case with explicit breakpoint kind.
	* riscv-tdep.c (show_use_compressed_breakpoints): Remove
	'additional_info' and related logic.
	(riscv_debug_breakpoints): New variable.
	(riscv_breakpoint_kind_from_pc): Use the length of the existing
	instruction to determine the breakpoint kind.
	(_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints'
	flag.  Update description of 'set/show riscv
	use-compressed-breakpoints' flag.

3 files changed, 49 insertions, 20 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index df22c00874b..56645e960fc 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,16 @@
+2018-09-28  John Baldwin  <jhb@FreeBSD.org>
+
+	* disasm-selftests.c (print_one_insn_test): Add bfd_arch_riscv to
+	case with explicit breakpoint kind.
+	* riscv-tdep.c (show_use_compressed_breakpoints): Remove
+	'additional_info' and related logic.
+	(riscv_debug_breakpoints): New variable.
+	(riscv_breakpoint_kind_from_pc): Use the length of the existing
+	instruction to determine the breakpoint kind.
+	(_initialize_riscv_tdep): Add 'set/show debug riscv breakpoints'
+	flag.  Update description of 'set/show riscv
+	use-compressed-breakpoints' flag.
+
 2018-09-28  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	(NEWS): Mention changes to frame related commands.
diff --git a/gdb/disasm-selftests.c b/gdb/disasm-selftests.c
index f7d0ecca0e1..8cc267631e2 100644
--- a/gdb/disasm-selftests.c
+++ b/gdb/disasm-selftests.c
@@ -77,9 +77,10 @@ print_one_insn_test (struct gdbarch *gdbarch)
       /* fall through */
     case bfd_arch_nios2:
     case bfd_arch_score:
-      /* nios2 and score need to know the current instruction to select
-	 breakpoint instruction.  Give the breakpoint instruction kind
-	 explicitly.  */
+    case bfd_arch_riscv:
+      /* nios2, riscv, and score need to know the current instruction
+	 to select breakpoint instruction.  Give the breakpoint
+	 instruction kind explicitly.  */
       int bplen;
       insn = gdbarch_sw_breakpoint_from_kind (gdbarch, 4, &bplen);
       len = bplen;
diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 163c8ec231d..319e01b4abb 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -210,20 +210,9 @@ show_use_compressed_breakpoints (struct ui_file *file, int from_tty,
 				 struct cmd_list_element *c,
 				 const char *value)
 {
-  const char *additional_info;
-  struct gdbarch *gdbarch = target_gdbarch ();
-
-  if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO)
-    if (riscv_has_feature (gdbarch, 'C'))
-      additional_info = _(" (currently on)");
-    else
-      additional_info = _(" (currently off)");
-  else
-    additional_info = "";
-
   fprintf_filtered (file,
 		    _("Debugger's use of compressed breakpoints is set "
-		      "to %s%s.\n"), value, additional_info);
+		      "to %s.\n"), value);
 }
 
 /* The set and show lists for 'set riscv' and 'show riscv' prefixes.  */
@@ -284,6 +273,11 @@ show_riscv_debug_variable (struct ui_file *file, int from_tty,
 		    c->name, value);
 }
 
+/* When this is set to non-zero debugging information about breakpoint
+   kinds will be printed.  */
+
+static unsigned int riscv_debug_breakpoints = 0;
+
 /* When this is set to non-zero debugging information about inferior calls
    will be printed.  */
 
@@ -417,7 +411,18 @@ riscv_breakpoint_kind_from_pc (struct gdbarch *gdbarch, CORE_ADDR *pcptr)
 {
   if (use_compressed_breakpoints == AUTO_BOOLEAN_AUTO)
     {
-      if (riscv_has_feature (gdbarch, 'C'))
+      gdb_byte buf[1];
+
+      /* Read the opcode byte to determine the instruction length.  */
+      read_code (*pcptr, buf, 1);
+
+      if (riscv_debug_breakpoints)
+	fprintf_unfiltered
+	  (gdb_stdlog,
+	   "Using %s for breakpoint at %s (instruction length %d)\n",
+	   riscv_insn_length (buf[0]) == 2 ? "C.EBREAK" : "EBREAK",
+	   paddress (gdbarch, *pcptr), riscv_insn_length (buf[0]));
+      if (riscv_insn_length (buf[0]) == 2)
 	return 2;
       else
 	return 4;
@@ -2953,6 +2958,16 @@ _initialize_riscv_tdep (void)
 		  &showdebugriscvcmdlist, "show debug riscv ", 0,
 		  &showdebuglist);
 
+  add_setshow_zuinteger_cmd ("breakpoints", class_maintenance,
+			     &riscv_debug_breakpoints,  _("\
+Set riscv breakpoint debugging."), _("\
+Show riscv breakpoint debugging."), _("\
+When non-zero, print debugging information for the riscv specific parts\n\
+of the breakpoint mechanism."),
+			     NULL,
+			     show_riscv_debug_variable,
+			     &setdebugriscvcmdlist, &showdebugriscvcmdlist);
+
   add_setshow_zuinteger_cmd ("infcall", class_maintenance,
 			     &riscv_debug_infcall,  _("\
 Set riscv inferior call debugging."), _("\
@@ -2989,10 +3004,10 @@ of the stack unwinding mechanism."),
 				_("\
 Set debugger's use of compressed breakpoints."), _("	\
 Show debugger's use of compressed breakpoints."), _("\
-Debugging compressed code requires compressed breakpoints to be used. If\n \
-left to 'auto' then gdb will use them if $misa indicates the C extension\n \
-is supported. If that doesn't give the correct behavior, then this option\n\
-can be used."),
+Debugging compressed code requires compressed breakpoints to be used. If\n\
+left to 'auto' then gdb will use them if the existing instruction is a\n\
+compressed instruction. If that doesn't give the correct behavior, then\n\
+this option can be used."),
 				NULL,
 				show_use_compressed_breakpoints,
 				&setriscvcmdlist,