Optimize memory_xfer_partial for remote

Some analysis we did here showed that increasing the cap on the
transfer size in target.c:memory_xfer_partial could give 20% or more
improvement in remote load across JTAG.  Transfer sizes were capped
to 4K bytes because of performance problems encountered with the
restore command, documented here:

https://sourceware.org/ml/gdb-patches/2013-07/msg00611.html

and in commit 67c059c29e1f ("Improve performance of large restore
commands").

The 4K cap was introduced because in a case where the restore command
requested a 100MB transfer, memory_xfer_partial would repeatedy
allocate and copy an entire 100MB buffer in order to properly handle
breakpoint shadow instructions, even though memory_xfer_partial would
actually only write a small portion of the buffer contents.

A couple of alternative solutions were suggested:
* change the algorithm for handling the breakpoint shadow instructions
* throttle the transfer size up or down based on the previous actual
  transfer size

I tried implementing the throttling approach, and my implementation
reduced the performance in some cases.

This patch implements a new target function that returns that target's
limit on memory transfer size.  It defaults to ULONGEST_MAX bytes,
because for native targets there is no marshaling and thus no limit is
needed.  For remote targets it uses get_memory_write_packet_size.

gdb/ChangeLog:

	* remote.c (remote_get_memory_xfer_limit): New function.
	* target-delegates.c: Regenerate.
	* target.c (memory_xfer_partial): Call
	target_ops.to_get_memory_xfer_limit.
	* target.h (struct target_ops)
	<to_get_memory_xfer_limit>: New member.

1 files changed, 31 insertions, 0 deletions

diff --git a/gdb/target-delegates.c b/gdb/target-delegates.c
index 03aa2cc..2887033 100644
--- a/gdb/target-delegates.c
+++ b/gdb/target-delegates.c
@@ -2064,6 +2064,33 @@ debug_xfer_partial (struct target_ops *self, enum target_object arg1, const char
   return result;
 }
 
+static ULONGEST
+delegate_get_memory_xfer_limit (struct target_ops *self)
+{
+  self = self->beneath;
+  return self->to_get_memory_xfer_limit (self);
+}
+
+static ULONGEST
+tdefault_get_memory_xfer_limit (struct target_ops *self)
+{
+  return ULONGEST_MAX;
+}
+
+static ULONGEST
+debug_get_memory_xfer_limit (struct target_ops *self)
+{
+  ULONGEST result;
+  fprintf_unfiltered (gdb_stdlog, "-> %s->to_get_memory_xfer_limit (...)\n", debug_target.to_shortname);
+  result = debug_target.to_get_memory_xfer_limit (&debug_target);
+  fprintf_unfiltered (gdb_stdlog, "<- %s->to_get_memory_xfer_limit (", debug_target.to_shortname);
+  target_debug_print_struct_target_ops_p (&debug_target);
+  fputs_unfiltered (") = ", gdb_stdlog);
+  target_debug_print_ULONGEST (result);
+  fputs_unfiltered ("\n", gdb_stdlog);
+  return result;
+}
+
 static VEC(mem_region_s) *
 delegate_memory_map (struct target_ops *self)
 {
@@ -4223,6 +4250,8 @@ install_delegators (struct target_ops *ops)
     ops->to_get_thread_local_address = delegate_get_thread_local_address;
   if (ops->to_xfer_partial == NULL)
     ops->to_xfer_partial = delegate_xfer_partial;
+  if (ops->to_get_memory_xfer_limit == NULL)
+    ops->to_get_memory_xfer_limit = delegate_get_memory_xfer_limit;
   if (ops->to_memory_map == NULL)
     ops->to_memory_map = delegate_memory_map;
   if (ops->to_flash_erase == NULL)
@@ -4454,6 +4483,7 @@ install_dummy_methods (struct target_ops *ops)
   ops->to_goto_bookmark = tdefault_goto_bookmark;
   ops->to_get_thread_local_address = tdefault_get_thread_local_address;
   ops->to_xfer_partial = tdefault_xfer_partial;
+  ops->to_get_memory_xfer_limit = tdefault_get_memory_xfer_limit;
   ops->to_memory_map = tdefault_memory_map;
   ops->to_flash_erase = tdefault_flash_erase;
   ops->to_flash_done = tdefault_flash_done;
@@ -4610,6 +4640,7 @@ init_debug_target (struct target_ops *ops)
   ops->to_goto_bookmark = debug_goto_bookmark;
   ops->to_get_thread_local_address = debug_get_thread_local_address;
   ops->to_xfer_partial = debug_xfer_partial;
+  ops->to_get_memory_xfer_limit = debug_get_memory_xfer_limit;
   ops->to_memory_map = debug_memory_map;
   ops->to_flash_erase = debug_flash_erase;
   ops->to_flash_done = debug_flash_done;