arc: Add hardware loop detection

For ARC there are registers that are not part of a required set in XML
target descriptions by default, but are almost always present on ARC
targets and are universally exposed by the ptrace interface.  Hardware
loop registers being one of them.

LP_START and LP_END auxiliary registers are hardware loop start and end.
Formally, they are optional, but it is hard to find an ARC configuration
that doesn't have them.  They are always present in processors that can
run GNU/Linux.  GDB needs to know about those registers to implement
proper software single stepping, since they affect  what instruction
will be next.

This commit adds the code to check for the existance of "lp_start" and
"lp_end" in XML target descriptions. If they exist, then the function
reports that the target supports hardware loops.

gdb/ChangeLog:

	* arc-tdep.c (arc_check_for_hardware_loop): New.
	* arc-tdep.h (gdbarch_tdep): New field has_hw_loops.

gdb/doc/ChangeLog:

	* gdb.texinfo (Synopsys ARC): Document LP_START, LP_END and BTA.

5 files changed, 56 insertions, 6 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index c7e8cfd..2e97cdc 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
 
+	* arc-tdep.c (arc_check_for_hardware_loop): New.
+	* arc-tdep.h (gdbarch_tdep): New field has_hw_loops.
+
+2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
+
 	* arc-tdep.h: Include "gdbarch.h".
 
 2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
diff --git a/gdb/arc-tdep.c b/gdb/arc-tdep.c
index cb4941d..8b4e285 100644
--- a/gdb/arc-tdep.c
+++ b/gdb/arc-tdep.c
@@ -2041,6 +2041,35 @@ arc_check_tdesc_feature (struct tdesc_arch_data *tdesc_data,
   return true;
 }
 
+/* Check for the existance of "lp_start" and "lp_end" in target description.
+   If both are present, assume there is hardware loop support in the target.
+   This can be improved by looking into "lpc_size" field of "isa_config"
+   auxiliary register.  */
+
+static bool
+arc_check_for_hw_loops (const struct target_desc *tdesc,
+			struct tdesc_arch_data *data)
+{
+  const auto feature_aux = tdesc_find_feature (tdesc, ARC_AUX_FEATURE_NAME);
+  const auto aux_regset = determine_aux_reg_feature_set ();
+
+  if (feature_aux == nullptr)
+    return false;
+
+  bool hw_loop_p = false;
+  const auto lp_start_name =
+    aux_regset->registers[ARC_LP_START_REGNUM - ARC_FIRST_AUX_REGNUM].names[0];
+  const auto lp_end_name =
+    aux_regset->registers[ARC_LP_END_REGNUM - ARC_FIRST_AUX_REGNUM].names[0];
+
+  hw_loop_p = tdesc_numbered_register (feature_aux, data,
+				       ARC_LP_START_REGNUM, lp_start_name);
+  hw_loop_p &= tdesc_numbered_register (feature_aux, data,
+				       ARC_LP_END_REGNUM, lp_end_name);
+
+  return hw_loop_p;
+}
+
 /* Initialize target description for the ARC.
 
    Returns true if input TDESC was valid and in this case it will assign TDESC
@@ -2163,13 +2192,15 @@ arc_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
     debug_printf ("arc: Architecture initialization.\n");
 
   if (!arc_tdesc_init (info, &tdesc, &tdesc_data))
-    return NULL;
+    return nullptr;
 
   /* Allocate the ARC-private target-dependent information structure, and the
      GDB target-independent information structure.  */
-  struct gdbarch_tdep *tdep = XCNEW (struct gdbarch_tdep);
+  gdb::unique_xmalloc_ptr<struct gdbarch_tdep> tdep
+    (XCNEW (struct gdbarch_tdep));
   tdep->jb_pc = -1; /* No longjmp support by default.  */
-  struct gdbarch *gdbarch = gdbarch_alloc (&info, tdep);
+  tdep->has_hw_loops = arc_check_for_hw_loops (tdesc, tdesc_data);
+  struct gdbarch *gdbarch = gdbarch_alloc (&info, tdep.release ());
 
   /* Data types.  */
   set_gdbarch_short_bit (gdbarch, 16);
@@ -2250,7 +2281,7 @@ arc_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
      It can override functions set earlier.  */
   gdbarch_init_osabi (info, gdbarch);
 
-  if (tdep->jb_pc >= 0)
+  if (gdbarch_tdep (gdbarch)->jb_pc >= 0)
     set_gdbarch_get_longjmp_target (gdbarch, arc_get_longjmp_target);
 
   /* Disassembler options.  Enforce CPU if it was specified in XML target
diff --git a/gdb/arc-tdep.h b/gdb/arc-tdep.h
index 5968abd..e752348 100644
--- a/gdb/arc-tdep.h
+++ b/gdb/arc-tdep.h
@@ -111,6 +111,9 @@ struct gdbarch_tdep
   /* Offset to PC value in jump buffer.  If this is negative, longjmp
      support will be disabled.  */
   int jb_pc;
+
+  /* Whether target has hardware (aka zero-delay) loops.  */
+  bool has_hw_loops;
 };
 
 /* Utility functions used by other ARC-specific modules.  */
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 6a81351..badf7ae 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,5 +1,9 @@
 2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
 
+	* gdb.texinfo (Synopsys ARC): Document LP_START, LP_END and BTA.
+
+2020-08-25  Shahab Vahedi  <shahab@synopsys.com>
+
 	* gdb.texinfo (Synopsys ARC): Update the documentation for ARC
 	Features.
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index a4f5b78..77c5d89 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -45298,8 +45298,15 @@ Extension core registers @samp{r32} through @samp{r59} are optional and their
 existence depends on the configuration.  When debugging GNU/Linux applications,
 i.e.@: user space debugging, these core registers are not available.
 
-The @samp{org.gnu.gdb.arc.aux} feature is required for all ARC targets.  It
-should at least contain @samp{pc} and @samp{status32} registers.
+The @samp{org.gnu.gdb.arc.aux} feature is required for all ARC targets.  Here
+is the list of registers pertinent to this feature:
+
+@itemize @minus
+@item
+mandatory: @samp{pc} and @samp{status32}.
+@item
+optional: @samp{lp_start}, @samp{lp_end}, and @samp{bta}.
+@end itemize
 
 @node ARM Features
 @subsection ARM Features