wrong gprof results with gcc -Ofast -pg
Question
I'm seeing strange profiling results with gcc -pg and gprof .
I can't find a better/smaller way to reproduce so I'm linking the actual code where I'm seeing the issue.
I'm using the code here , building gprof profiling results with make prof
I notice several errors, mainly:
- A function
shape_stream_testthat should never even be called is shown as being called heavily byai_best_move_recand taking up 15% of total run time in self time - Other functions legitimately called by
ai_best_move_reclikegrid_block_remove,grid_cpy,grid_newdon't even show up as children
Actual prof stats:
Flat profile:
Each sample counts as 0.01 seconds.
% cumulative self self total
time seconds seconds calls ms/call ms/call name
53.92 4.00 4.00 4231936 0.00 0.00 grid_eval
13.62 5.01 1.01 4467368 0.00 0.00 shape_stream_test
6.88 5.52 0.51 4690742 0.00 0.00 grid_block_center_top
6.07 5.97 0.45 4521016 0.00 0.00 grid_block_valid
5.80 6.40 0.43 4467368 0.00 0.00 grid_block_add
3.37 6.65 0.25 4467368 0.00 0.00 grid_block_drop
2.02 6.80 0.15 35 4.29 209.20 ai_best_move_rec
1.95 6.95 0.15 235469 0.00 0.00 grid_init
1.75 7.08 0.13 9212461 0.00 0.00 block_extreme
1.48 7.19 0.11 4467402 0.00 0.00 block_move
0.94 7.26 0.07 1654899 0.00 0.00 block_get
0.54 7.30 0.04 block_crust_get
0.40 7.33 0.03 grid_block_set_color
0.27 7.35 0.02 4467368 0.00 0.00 grid_block_remove
0.27 7.37 0.02 235501 0.00 0.00 block_new
0.27 7.39 0.02 grid_block_intersects
0.20 7.40 0.02 235469 0.00 0.00 grid_new
0.13 7.41 0.01 235467 0.00 0.00 shape_stream_peek
0.13 7.42 0.01 drop_amount
0.00 7.42 0.00 621 0.00 0.00 grid_clear_lines
0.00 7.42 0.00 587 0.00 0.00 grid_cpy
0.00 7.42 0.00 35 0.00 0.00 grid_print
0.00 7.42 0.00 34 0.00 0.00 block_init
0.00 7.42 0.00 34 0.00 0.00 block_print
0.00 7.42 0.00 34 0.00 0.00 game_move_print
0.00 7.42 0.00 34 0.00 0.00 grid_apply_moves
0.00 7.42 0.00 34 0.00 0.00 shape_stream_pop
0.00 7.42 0.00 7 0.00 0.00 shape_new
0.00 7.42 0.00 1 0.00 0.00 shape_stream_new
% the percentage of the total running time of the
time program used by this function.
cumulative a running sum of the number of seconds accounted
seconds for by this function and those listed above it.
self the number of seconds accounted for by this
seconds function alone. This is the major sort for this
listing.
calls the number of times this function was invoked, if
this function is profiled, else blank.
self the average number of milliseconds spent in this
ms/call function per call, if this function is profiled,
else blank.
total the average number of milliseconds spent in this
ms/call function and its descendents per call, if this
function is profiled, else blank.
name the name of the function. This is the minor sort
for this listing. The index shows the location of
the function in the gprof listing. If the index is
in parenthesis it shows where it would appear in
the gprof listing if it were to be printed.
Copyright (C) 2012-2014 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Call graph (explanation follows)
granularity: each sample hit covers 2 byte(s) for 0.13% of 7.42 seconds
index % time self children called name
<spontaneous>
[1] 98.7 0.00 7.32 ai_test [1]
0.15 7.17 35/35 ai_best_move_rec [2]
0.00 0.00 34/34 grid_apply_moves [21]
0.00 0.00 2/235469 grid_new [9]
0.00 0.00 35/35 grid_print [24]
0.00 0.00 34/34 game_move_print [27]
0.00 0.00 34/34 shape_stream_pop [28]
0.00 0.00 1/1 shape_stream_new [30]
-----------------------------------------------
235432 ai_best_move_rec [2]
0.15 7.17 35/35 ai_test [1]
[2] 98.7 0.15 7.17 35+235432 ai_best_move_rec [2]
4.00 0.00 4231936/4231936 grid_eval [3]
1.01 0.00 4467368/4467368 shape_stream_test [4]
0.51 0.11 4690742/4690742 grid_block_center_top [5]
0.45 0.09 4521016/4521016 grid_block_valid [6]
0.43 0.00 4467368/4467368 grid_block_add [7]
0.25 0.00 4467368/4467368 grid_block_drop [8]
0.02 0.15 235467/235469 grid_new [9]
0.11 0.00 4467368/4467402 block_move [12]
0.02 0.00 4467368/4467368 grid_block_remove [16]
0.02 0.00 235467/235501 block_new [17]
0.01 0.00 235467/235467 shape_stream_peek [19]
0.00 0.00 587/587 grid_cpy [23]
0.00 0.00 587/621 grid_clear_lines [22]
235432 ai_best_move_rec [2]
-----------------------------------------------
4.00 0.00 4231936/4231936 ai_best_move_rec [2]
[3] 53.9 4.00 0.00 4231936 grid_eval [3]
-----------------------------------------------
1.01 0.00 4467368/4467368 ai_best_move_rec [2]
[4] 13.6 1.01 0.00 4467368 shape_stream_test [4]
-----------------------------------------------
0.51 0.11 4690742/4690742 ai_best_move_rec [2]
[5] 8.4 0.51 0.11 4690742 grid_block_center_top [5]
0.07 0.00 4690742/9212461 block_extreme [11]
0.04 0.00 1056160/1654899 block_get [13]
-----------------------------------------------
0.45 0.09 4521016/4521016 ai_best_move_rec [2]
[6] 7.3 0.45 0.09 4521016 grid_block_valid [6]
0.06 0.00 4521016/9212461 block_extreme [11]
0.03 0.00 598739/1654899 block_get [13]
-----------------------------------------------
0.43 0.00 4467368/4467368 ai_best_move_rec [2]
[7] 5.8 0.43 0.00 4467368 grid_block_add [7]
-----------------------------------------------
0.25 0.00 4467368/4467368 ai_best_move_rec [2]
[8] 3.4 0.25 0.00 4467368 grid_block_drop [8]
0.00 0.00 635/9212461 block_extreme [11]
-----------------------------------------------
0.00 0.00 2/235469 ai_test [1]
0.02 0.15 235467/235469 ai_best_move_rec [2]
[9] 2.2 0.02 0.15 235469 grid_new [9]
0.15 0.00 235469/235469 grid_init [10]
-----------------------------------------------
0.15 0.00 235469/235469 grid_new [9]
[10] 2.0 0.15 0.00 235469 grid_init [10]
-----------------------------------------------
0.00 0.00 68/9212461 grid_apply_moves [21]
0.00 0.00 635/9212461 grid_block_drop [8]
0.06 0.00 4521016/9212461 grid_block_valid [6]
0.07 0.00 4690742/9212461 grid_block_center_top [5]
[11] 1.8 0.13 0.00 9212461 block_extreme [11]
-----------------------------------------------
0.00 0.00 34/4467402 grid_apply_moves [21]
0.11 0.00 4467368/4467402 ai_best_move_rec [2]
[12] 1.5 0.11 0.00 4467402 block_move [12]
-----------------------------------------------
0.03 0.00 598739/1654899 grid_block_valid [6]
0.04 0.00 1056160/1654899 grid_block_center_top [5]
[13] 0.9 0.07 0.00 1654899 block_get [13]
-----------------------------------------------
<spontaneous>
[14] 0.5 0.04 0.00 block_crust_get [14]
-----------------------------------------------
<spontaneous>
[15] 0.4 0.03 0.00 grid_block_set_color [15]
-----------------------------------------------
0.02 0.00 4467368/4467368 ai_best_move_rec [2]
[16] 0.3 0.02 0.00 4467368 grid_block_remove [16]
-----------------------------------------------
0.00 0.00 34/235501 grid_apply_moves [21]
0.02 0.00 235467/235501 ai_best_move_rec [2]
[17] 0.3 0.02 0.00 235501 block_new [17]
-----------------------------------------------
<spontaneous>
[18] 0.3 0.02 0.00 grid_block_intersects [18]
-----------------------------------------------
0.01 0.00 235467/235467 ai_best_move_rec [2]
[19] 0.1 0.01 0.00 235467 shape_stream_peek [19]
-----------------------------------------------
<spontaneous>
[20] 0.1 0.01 0.00 drop_amount [20]
-----------------------------------------------
0.00 0.00 34/34 ai_test [1]
[21] 0.0 0.00 0.00 34 grid_apply_moves [21]
0.00 0.00 34/235501 block_new [17]
0.00 0.00 68/9212461 block_extreme [11]
0.00 0.00 34/4467402 block_move [12]
0.00 0.00 34/34 block_init [25]
0.00 0.00 34/621 grid_clear_lines [22]
-----------------------------------------------
0.00 0.00 34/621 grid_apply_moves [21]
0.00 0.00 587/621 ai_best_move_rec [2]
[22] 0.0 0.00 0.00 621 grid_clear_lines [22]
-----------------------------------------------
0.00 0.00 587/587 ai_best_move_rec [2]
[23] 0.0 0.00 0.00 587 grid_cpy [23]
-----------------------------------------------
0.00 0.00 35/35 ai_test [1]
[24] 0.0 0.00 0.00 35 grid_print [24]
-----------------------------------------------
0.00 0.00 34/34 grid_apply_moves [21]
[25] 0.0 0.00 0.00 34 block_init [25]
-----------------------------------------------
0.00 0.00 34/34 game_move_print [27]
[26] 0.0 0.00 0.00 34 block_print [26]
-----------------------------------------------
0.00 0.00 34/34 ai_test [1]
[27] 0.0 0.00 0.00 34 game_move_print [27]
0.00 0.00 34/34 block_print [26]
-----------------------------------------------
0.00 0.00 34/34 ai_test [1]
[28] 0.0 0.00 0.00 34 shape_stream_pop [28]
-----------------------------------------------
0.00 0.00 7/7 shapes_read [68]
[29] 0.0 0.00 0.00 7 shape_new [29]
-----------------------------------------------
0.00 0.00 1/1 ai_test [1]
[30] 0.0 0.00 0.00 1 shape_stream_new [30]
-----------------------------------------------
This table describes the call tree of the program, and was sorted by
the total amount of time spent in each function and its children.
Each entry in this table consists of several lines. The line with the
index number at the left hand margin lists the current function.
The lines above it list the functions that called this function,
and the lines below it list the functions this one called.
This line lists:
index A unique number given to each element of the table.
Index numbers are sorted numerically.
The index number is printed next to every function name so
it is easier to look up where the function is in the table.
% time This is the percentage of the `total' time that was spent
in this function and its children. Note that due to
different viewpoints, functions excluded by options, etc,
these numbers will NOT add up to 100%.
self This is the total amount of time spent in this function.
children This is the total amount of time propagated into this
function by its children.
called This is the number of times the function was called.
If the function called itself recursively, the number
only includes non-recursive calls, and is followed by
a `+' and the number of recursive calls.
name The name of the current function. The index number is
printed after it. If the function is a member of a
cycle, the cycle number is printed between the
function's name and the index number.
For the function's parents, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the function into this parent.
children This is the amount of time that was propagated from
the function's children into this parent.
called This is the number of times this parent called the
function `/' the total number of times the function
was called. Recursive calls to the function are not
included in the number after the `/'.
name This is the name of the parent. The parent's index
number is printed after it. If the parent is a
member of a cycle, the cycle number is printed between
the name and the index number.
If the parents of the function cannot be determined, the word
`<spontaneous>' is printed in the `name' field, and all the other
fields are blank.
For the function's children, the fields have the following meanings:
self This is the amount of time that was propagated directly
from the child into the function.
children This is the amount of time that was propagated from the
child's children to the function.
called This is the number of times the function called
this child `/' the total number of times the child
was called. Recursive calls by the child are not
listed in the number after the `/'.
name This is the name of the child. The child's index
number is printed after it. If the child is a
member of a cycle, the cycle number is printed
between the name and the index number.
If there are any cycles (circles) in the call graph, there is an
entry for the cycle-as-a-whole. This entry shows who called the
cycle (as parents) and the members of the cycle (as children.)
The `+' recursive calls entry shows the number of function calls that
were internal to the cycle, and the calls entry for each member shows,
for that member, how many times it was called from other members of
the cycle.
Copyright (C) 2012-2014 Free Software Foundation, Inc.
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
Index by function name
[2] ai_best_move_rec [21] grid_apply_moves [3] grid_eval
[14] block_crust_get [7] grid_block_add [10] grid_init
[11] block_extreme [5] grid_block_center_top [9] grid_new
[13] block_get [8] grid_block_drop [24] grid_print
[25] block_init [18] grid_block_intersects [29] shape_new
[12] block_move [16] grid_block_remove [30] shape_stream_new
[17] block_new [15] grid_block_set_color [19] shape_stream_peek
[26] block_print [6] grid_block_valid [28] shape_stream_pop
[20] drop_amount [22] grid_clear_lines [4] shape_stream_test
[27] game_move_print [23] grid_cpy
Later I discovered I was combining the -Ofast and -pg flags, and removing -Ofast fixes these issues above.
My questions are:
Are these two flags incompatible? I couldn't find any such warning in the docs
If they are not incompatible, is this a gcc bug?
How can I work around this to see profiling results that match the most aggressive optimizations so that I don't waste time optimizing the wrong code?
1 answers
solution1
1 ACCPTED 2016-08-16 12:54:00
You want speed, right? -Ofast only matters at the bottom of the call stack. It can't fix things that only you can fix. It can only make them harder to find. First turn off the optimizer and fix the things you can fix. Here's an example of how some people do it. When you've gone as far as possible with that, turn on the optimizer and let it do its magic.
Small Flame :) Plenty of people say it's a waste of time to profile unoptimized code, but they never say why . They heard it in a lecture hall, read it on a blog, or from stackoverflow, from someone whose clear authority on the subject obviates the need to give a reason.
I think it's a case of wishful thinking: My code is basically perfect, so the only ways it could be speeded up are to run the compiler's optimizer. Then when the profiler shows no way to speed it up - it's obviously right and hurray for me!
Added in response to comments: Let me digress a little bit (sorry). I understand that one can go over the code carefully, invest effort in data representations, inline some functions, do other high-level optimizations, and measure speedups from doing so. Great. But if your reason for each code change is looking at and just thinking about the code, then as educated as your thinking is, it is still a guess. A guess may be right, but the question you should be asking is "What have I missed?" and "How do I find it?"
The method many people and I use is random pausing . The only tool it requires is a debugger, like GDB or any IDE. It differs from profiling in that it locates unnecessary computation directly , rather than taking measurements and counting on you to decipher them, filter out the noise, look at particular routines, and hunt inside them. It tells you down to the particular lines of code, and particular data, what the program is doing and why it's doing it. Then you can ask if there's a less wasteful way to do that, and chances are there is. What it does not do is tell you the precise fraction of time - you only get a very coarse measurement. What it does do is identify the problem. You might think you certainly can't depend on such an inaccurate method, and that's true if you only see the problem on one sample. But if you see it on more than one sample, you know it's big, and the fewer overall samples you took to see it more than once, the bigger it is. Here are the statistics behind it. And here's the kicker: this will find any problem any profiler will find, and more.
Then when you find and fix one problem, you can do it all again, because removal of one problem magnifies the remaining problems. In this way you can "ladder" the speedups until you can't any more. That's how you get maximal speed as in this example . Then, by all means, use -Ofast .
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