GDB: Introduce limited array lengths while printing values

This commit introduces the idea of loading only part of an array in
order to print it, what I call "limited length" arrays.

The motivation behind this work is to make it possible to print slices
of very large arrays, where very large means bigger than
`max-value-size'.

Consider this GDB session with the current GDB:

  (gdb) set max-value-size 100
  (gdb) p large_1d_array
  value requires 400 bytes, which is more than max-value-size
  (gdb) p -elements 10 -- large_1d_array
  value requires 400 bytes, which is more than max-value-size

notice that the request to print 10 elements still fails, even though 10
elements should be less than the max-value-size.  With a patched version
of GDB:

  (gdb) p -elements 10 -- large_1d_array
  $1 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9...}

So now the print has succeeded.  It also has loaded `max-value-size'
worth of data into value history, so the recorded value can be accessed
consistently:

  (gdb) p -elements 10 -- $1
  $2 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9...}
  (gdb) p $1
  $3 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,
    20, 21, 22, 23, 24, <unavailable> <repeats 75 times>}
  (gdb)

Accesses with other languages work similarly, although for Ada only
C-style [] array element/dimension accesses use history.  For both Ada
and Fortran () array element/dimension accesses go straight to the
inferior, bypassing the value history just as with C pointers.

Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/gdb/printcmd.c b/gdb/printcmd.c
index 7f35513..9bda60d 100644
--- a/gdb/printcmd.c
+++ b/gdb/printcmd.c
@@ -1242,6 +1242,11 @@ print_command_parse_format (const char **expp, const char *cmdname,
 void
 print_value (value *val, const value_print_options &opts)
 {
+  /* This setting allows large arrays to be printed by limiting the
+     number of elements that are loaded into GDB's memory; we only
+     need to load as many array elements as we plan to print.  */
+  scoped_array_length_limiting limit_large_arrays (opts.print_max);
+
   int histindex = record_latest_value (val);
 
   annotate_value_history_begin (histindex, value_type (val));
@@ -1301,6 +1306,11 @@ process_print_command_args (const char *args, value_print_options *print_opts,
 
   if (exp != nullptr && *exp)
     {
+      /* This setting allows large arrays to be printed by limiting the
+	 number of elements that are loaded into GDB's memory; we only
+	 need to load as many array elements as we plan to print.  */
+      scoped_array_length_limiting limit_large_arrays (print_opts->print_max);
+
       /* VOIDPRINT is true to indicate that we do want to print a void
 	 value, so invert it for parse_expression.  */
       expression_up expr = parse_expression (exp, nullptr, !voidprint);
@@ -1489,6 +1499,12 @@ output_command (const char *exp, int from_tty)
 
   get_formatted_print_options (&opts, format);
   opts.raw = fmt.raw;
+
+  /* This setting allows large arrays to be printed by limiting the
+     number of elements that are loaded into GDB's memory; we only
+     need to load as many array elements as we plan to print.  */
+  scoped_array_length_limiting limit_large_arrays (opts.print_max);
+
   print_formatted (val, fmt.size, &opts, gdb_stdout);
 
   annotate_value_end ();