[PATCH] D25766: [ELF] - Implemented --section-ordering-file option.

[George Rimar via llvm-commits]

>> Minimizing the number of page faults is not a goal of this feature (although
>> it could be a measure to achieve it.) You want to measure the start up time
>> of the app. That's what this feature was originally designed for.
>
>Yes. The suggestion of looking at page faults is just for an easier to
>reproduce number to look at first.
>
>I am under the impression that this can also be useful for instruction
>cache on a running application. Which order file is used to optimize
>that is obviously different (the one created by
>function_reordering_plugin), but the linker option is the same.
>
>Cheers,
>Rafael

Yeah, I am looking on page faults because want to see the 
noticable difference, though I do not forget about looking at startup time.

Last my results are below.
As I mentioned I forward ported 6 years old icegring valgrind patch 
(https://bugzilla.mozilla.org/show_bug.cgi?id=549749) to produce sections list file for regular firefox build. 
Just in case I shared it here:
https://drive.google.com/file/d/0B_OWr6ld9gUmWjkwMmtLeDZxQTA/view
I tried both VM and real hardware tests of my lld patch with that file and saw no difference
in pagefaults and startup time. 

Finally I decided to use just vanila gold linker
which has this option implemented in case lld patch has some issues. 
I tested on old real hardware with HDD (not SSD) for that last time, I expected
to see launch time boost because of that. 
And I still do not observe any difference. 
For example next are average numbers I see from 5 sequentual runs
of ordered and unordered builds. I am lauching firefox and close it as soon as
I see full UI on screen. Yes, I am closing it manually, but average numbers are anyways are the same:

 Performance counter stats for '/home/umb/tests/firefox/mozilla-central/obj-x86_64-pc-linux-gnu/dist/bin/firefox -no-remote -profile /home/umb/tests/firefox/mozilla-central/obj-x86_64-pc-linux-gnu/tmp/scratch_user'
       9275,574798      task-clock (msec)         #    1,218 CPUs utilized          
            16 818      context-switches          #    0,002 M/sec                  
               936      cpu-migrations            #    0,101 K/sec                  
           116 713      page-faults               #    0,013 M/sec                  
    23 194 539 659      cycles                    #    2,501 GHz                      (49,46%)
   <not supported>      stalled-cycles-frontend  
   <not supported>      stalled-cycles-backend   
     8 179 492 869      instructions              #    0,35  insns per cycle          (74,69%)
     1 755 530 791      branches                  #  189,264 M/sec                    (74,94%)
       144 524 901      branch-misses             #    8,23% of all branches          (75,72%)

       7,615463859 seconds time elapsed
	   
Performance counter stats for '/home/umb/tests/firefox/ordered/mozilla-central/obj-x86_64-pc-linux-gnu/dist/bin/firefox -no-remote -profile /home/umb/tests/firefox/ordered/mozilla-central/obj-x86_64-pc-linux-gnu/tmp/scratch_user':
       9300,245987      task-clock (msec)         #    1,227 CPUs utilized          
            17 213      context-switches          #    0,002 M/sec                  
               966      cpu-migrations            #    0,104 K/sec                  
           116 913      page-faults               #    0,013 M/sec                  
    23 328 142 841      cycles                    #    2,508 GHz                      (49,94%)
   <not supported>      stalled-cycles-frontend  
   <not supported>      stalled-cycles-backend   
     8 367 279 963      instructions              #    0,36  insns per cycle          (75,55%)
     1 772 610 613      branches                  #  190,598 M/sec                    (74,98%)
       146 712 893      branch-misses             #    8,28% of all branches          (75,20%)

       7,580559286 seconds time elapsed

As you can see there is no real difference in pagefaults or time.

I prepared another patch that takes symbol list file instead of sections list file:  https://reviews.llvm.org/D26130,
tomorrow I am going to check combination of Rafael's script with this patch, probably that can change something.

George.