anonymous
  • anonymous
So I've got a simple C program that runs in 40 milliseconds on my x86 (1.6GHz Intel Atom). 40 milliseconds is not fast enough for me; I want it to happen in under 10 milliseconds. How do I optimize my C code? What are the sequence of steps that a programmer takes when optimizing code? How do I profile my program and find out what parts I need to refactor/ use a better algorithm, etc.
Computer Science
jamiebookeater
  • jamiebookeater
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anonymous
  • anonymous
so far all i've done is use gcc -O2 . what else can i do to optimize my program to run in my calculator?
nick67
  • nick67
did you try with this ? -Ofast "Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays."
anonymous
  • anonymous
Will you please provide the source code, so that I can have a look on it? You should not depend blindly on optimizations provided by compilers. Before optimizations, know when to optimize, what to optimize, and how to optimize. To answer first problem, if performance improvement is significant without TOOOOOOOOOO MUCH headach, go for it! To answer second problem, use a profiler. It will show you where most of the processing time is consumed in your program. Optimize that part first. For third one, you may use a better algorithm, or employ some *trikcy* fast solutions. It depends upon the case.

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anonymous
  • anonymous
I just tried -Ofast with no luck :( still giving me about 40 milliseconds. here is th e source: #include "prog.h" int main(int argc, char *argv[]) { switch (argc) { case 1: solve_from_stdin(); break; default: return -1; break; } return 0; }
anonymous
  • anonymous
Please provide prog.h as well! In case you are working on some secret project, you may use profiler(s) or ask other project members.
nick67
  • nick67
which way are you measuring the time ?
anonymous
  • anonymous
bash's builtin time command
nick67
  • nick67
did you try removing anything else but the pure main to have the minimum execution time ?
anonymous
  • anonymous
this is what prog.h looks like #ifndef _PROG_H_ #define _PROG_H_ unsigned int a(int *, int *, const unsigned int, const unsigned int); unsigned int b(int *, int *, const unsigned int, const unsigned int, const unsigned int); unsigned int c(int *, const unsigned int); void solve_from_stdin(void); #endif /* ifndef _PROG_H_ */ what prfiler should i use?
anonymous
  • anonymous
AMD APP Profiler is a free C/C++ Profiler
anonymous
  • anonymous
Intel Parallel Studio also contains a profiler.
nick67
  • nick67
In the header file you just have declarations, not definitions, so nobody can understand what the code really does; but could you try to execute a main() without the function call and report the measured execution time ? #include "prog.h" int main(int argc, char *argv[]) { switch (argc) { case 1: // solve_from_stdin(); break; default: return -1; break; } return 0; }
anonymous
  • anonymous
but those kits are both exclusive to Windows/visual studio :( without the solve_from_stdin(), time outputs 0.000 :-D so that one routine is taking 99.9% of cpu time :D
anonymous
  • anonymous
Intell Parallel Studio is available for LINUX as well.
anonymous
  • anonymous
that's awesome I'll check my package manager then
anonymous
  • anonymous
alright I have intel parallel studio xe in my package manager
nick67
  • nick67
ok, now reactivate the call to the function but eliminate any action in the function body, then go on reactivating part of the code in the function body until you can understand which part is taking more execution time
nick67
  • nick67
old times profiling style :-)
anonymous
  • anonymous
okay I deactivated procedure a() and I'm also getting 0.000 from bash
anonymous
  • anonymous
but procedure a() calls procedure b()
nick67
  • nick67
then reactivate procedure a() but not procedure b()
anonymous
  • anonymous
that decreased my time from 40 to 22 ms
nick67
  • nick67
does procedure a() do anything else apart calling procedure b() ?
anonymous
  • anonymous
procedure a also calls itself before calling procedure b
nick67
  • nick67
it sounds strange, it should loop forever, unless there is some kind of counter to avoid it
anonymous
  • anonymous
yeah at the start it tests for the value of (const int $1 + const int $2) / 2
nick67
  • nick67
is the function solve_from_stdin() recursive ?
anonymous
  • anonymous
here's procedure b() void merge_int(int* left, unsigned int len_left, int* right, unsigned int len_right, int* end) { unsigned int i, j, k; for (i = j = k = 0; i < len_left && j < len_right; ++k) { if (left[i] < right[j]) { end[k] = left[i]; ++i; } else { end[k] = right[j]; ++j; } } for (; i < len_left; ++i, ++k) { end[k] = left[i]; } for (; j < len_right; ++j, ++k) { end[k] = right[j]; } }
nick67
  • nick67
is the b() procedure called just once in the a() procedure ?
anonymous
  • anonymous
right just once
anonymous
  • anonymous
but since a() calls itself recursively, it ends up calling b quite a lot of times :)
nick67
  • nick67
if so you could try to directly write the b() procedure's content into the a() procedure body, so that you save one function call time (context saving time into the stack)
anonymous
  • anonymous
I got a seg fault :(
nick67
  • nick67
did you use correct types for left, right, end variables ?
anonymous
  • anonymous
alright I'm going to refurbish my code and use a different data structure and see how it goes...

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