[Bug exp/12705] New: Extremely poor performance when printing largearrays

["andrewg at cray dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12705

           Summary: Extremely poor performance when printing large arrays
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: critical
          Priority: P2
         Component: exp
        AssignedTo: unassigned@sourceware.org
        ReportedBy: andrewg@cray.com


I noticed that when I wanted to print out a very large array, the performance
was unbearably slow. This can be observed by setting print repeats and print
elements to 0 (unlimited) and then attempting to print a large array.

set print repeats 0
set print elements 0

I discovered this behavior while attempting to directly parse a very large
array's value directly using gdb/mi with the command -data-evaluate-expression.

The following is a patch which fixes this issue:

[andrewg@kaibab] gdb $ cvs diff -cp valprint.c                               
Index: valprint.c                                                            
===================================================================          
RCS file: /cvs/src/src/gdb/valprint.c,v                                      
retrieving revision 1.110                                                    
diff -c -p -r1.110 valprint.c                                                
*** valprint.c  14 Feb 2011 11:35:45 -0000      1.110                        
--- valprint.c  26 Apr 2011 16:57:39 -0000                                   
*************** val_print_array_elements (struct type *t                     
*** 1247,1261 ****                                                           

        rep1 = i + 1;                                                        
        reps = 1;                                                            
!       while (rep1 < len                                                    
             && value_available_contents_eq (val,                            
                                             embedded_offset + i * eltlen,   
                                             val,                            
                                             embedded_offset + rep1 * eltlen,
                                             eltlen))                        
!       {                                                                    
!         ++reps;                                                            
!         ++rep1;                                                            
        }                                                                    

        if (reps > options->repeat_count_threshold)
--- 1247,1267 ----

        rep1 = i + 1;
        reps = 1;
!
!       /* Only check for reps if repeat_count_threshold is not set to
!          UINT_MAX (unlimited) */
!       if (options->repeat_count_threshold < UINT_MAX)
!         {
!           while (rep1 < len
             && value_available_contents_eq (val,
                                             embedded_offset + i * eltlen,
                                             val,
                                             embedded_offset + rep1 * eltlen,
                                             eltlen))
!           {
!             ++reps;
!             ++rep1;
!           }
        }

        if (reps > options->repeat_count_threshold)

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