Code efficiency

[Gorman, John]

A good way to see which one is more effecient is to compile
the code and generate the assembler code. 

See which one produces the most lines of code (LOC). The one with
the least LOC is the one that is more efficient and will most
probably run faster.

Some things to remember with testing code is:

1. make sure the buffer is warmed up.
2. run the test on a large number of records (or loops)

John

|-----Original Message-----
|From: sinck@corp.quepasa.com [mailto:sinck@corp.quepasa.com]
|Sent: Monday, February 07, 2000 2:05 PM
|To: plug-devel@lists.PLUG.phoenix.az.us
|Subject: Code efficiency
|
|
|
|I was chatting with my carpool buddy on the way home (suprise), and he
|stated that coding something along the lines of
|
|    while (--i > 0)
|      {
|	  p++;
|      }
|
|would be faster than a classic
|
|    for (i = 0; i < 50; i++)
|      {
|	  p++;
|      }
|
|based on the predecrement already setting the zero flags in the
|appropriate registers and involving fewer instructions.
|
|Well, I thought to myself, self, "I don't believe that it would make a
|bit of difference using modern compilers."  Furthermore, myself went
|on: "I bet gcc -O2 and stripping make more difference in the size."  So
|I set out with the following .c file:
|
|
|---------opt.c----------
|#ifdef normal
|main ()
|{
|    int i,p;
|    p = 0;
|    for (i = 0; i < 50; i++)
|      {
|	  p++;
|      }
|}
|#endif
|
|
|
|#ifdef mikeopt
|main ()
|{
|    int i,p;
|
|    i = 51;
|    p = 0;
|
|    while (--i > 0)
|      {
|	  p++;
|      }
|}
|#endif
|
|
|#ifdef sinckopt
|main ()
|{
|    int i,p;
|    p = 0;
|
|    for (i=-1; ++i < 50;)
|      {
|	  p++;
|      }
|}
|#endif
|--------end opt.c----------
|
|Not the prettiest of C, but it works and gives the same number of
|loops.
|
|So, I built the said files (without -O2) and stripped them and 
|came up with:
|
|-rwxr-xr-x   1 sinck    sinck       11624 Feb  6 11:49 loop-mikeopt*
|-rwxr-xr-x   1 sinck    sinck        2976 Feb  6 11:49 
|loop-mikeopt-stripped*
|-rwxr-xr-x   1 sinck    sinck       11624 Feb  6 11:49 loop-normal*
|-rwxr-xr-x   1 sinck    sinck        2976 Feb  6 11:49 
|loop-normal-stripped*
|-rwxr-xr-x   1 sinck    sinck       11624 Feb  6 11:49 loop-sinckopt*
|-rwxr-xr-x   1 sinck    sinck        2976 Feb  6 11:49 
|loop-sinckopt-stripped*
|
|All the non-stripped are the same; all the stripped are the same.
|
|Now, with -O2 enabled:
|
|-rwxr-xr-x   1 sinck    sinck       11592 Feb  6 11:50 loop-mikeopt*
|-rwxr-xr-x   1 sinck    sinck        2944 Feb  6 11:50 
|loop-mikeopt-stripped*
|-rwxr-xr-x   1 sinck    sinck       11592 Feb  6 11:50 loop-normal*
|-rwxr-xr-x   1 sinck    sinck        2944 Feb  6 11:50 
|loop-normal-stripped*
|-rwxr-xr-x   1 sinck    sinck       11592 Feb  6 11:50 loop-sinckopt*
|-rwxr-xr-x   1 sinck    sinck        2944 Feb  6 11:50 
|loop-sinckopt-stripped*
|
|Smaller than previous, but still grouped the same.
|
|So, by decreasing filesizes:
|non-optimized non-stripped
|    optimized non-stripped
|non-optimized stripped
|    optimized stripped
|
|And yes, it did make a bit of difference.  Somewhere in the binary
|files produced, something is not the same.  :-)
|
|My moral would be thus: code for supportability and leave the magic to
|the compiler.
|
|And yes, I know I didn't benchmark them running, but I didn't really
|feel like trying to control the free parameters enough to get an
|accurate benchmarking. 
|
|This potentially uninformative tale brought to you by the letter C,
|the number 50, and special guest Mike.
|
|David
|
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