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Full Discussion: char constants vs. hard-coding
Top Forums Programming char constants vs. hard-coding Post 302227826 by otheus on Friday 22nd of August 2008 04:23:01 AM
Old 08-22-2008
Quote:
Originally Posted by redoubtable
Despite what everyone said I think speed differences between both cases are not mensurable.
It IS measurable. But even after 1/2 million invocations, it made almost no difference on a very slow (10-year old) machine.

Quote:
For one, memory is stored in the data segment in both cases thus it's accessed in the same way/speed.
mostly wrong. The '@' literal is embedded in the machine instructions itself (for x86 architectures), so that's in the code segment. The "const" designation for a variable means the compiler can optimize that variable, for instance, by also "hard coding" the value inside instructions. However, I did not turn on optimizations. In my code, I defined the const char to be inside the main() call, meaning it would go on the stack. Do nothing is on the data segment. Finally, the call to strchr places both arguments on the stack. So the price of having a constant in an immediate instruction type is practically nullified by this.

Quote:
Furthermore, this is highly platform/architecture/implementation dependent.
Architecture and processor, yes. For instance, while practically all processors have both an 'immediate' addressing and a 'direct' addressing mode, the difference in the number of clock cycles to process such an argument varies across architectures (surely), processor manufacturers (AMD vs Intel), and processor families (Pentium vs Celeron). There almost always IS a difference, but in very-large-pipelined architectures and efficient caching, that difference is statistically erased.

However, there's more dependency on the compiler. Whether the compiler chooses immediate mode or direct mode for literals, whether it uses direct or stack-indexed addressing addressing for constants, weather it passes the first argument in using a register or the last, etc, etc. The OS can come into play, too, especially with my program of 100000 lines of code. This likely meant there were page-traps during the execution. For this reason (and others), I took an average of several runs.

Quote:
We should call a meta-programmer to enlighten us with accurate specifications on the matter at hand.
WTF do you mean by a metaprogrammer??
 

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LEARN ABOUT PHP

posix_getrlimit

POSIX_GETRLIMIT(3)							 1							POSIX_GETRLIMIT(3)

posix_getrlimit - Return info about system resource limits

SYNOPSIS

       array posix_getrlimit (void  )

DESCRIPTION

       posix_getrlimit(3) returns an array of information about the current resource's soft and hard limits.

	Each  resource has an associated soft and hard limit. The soft limit is the value that the kernel enforces for the corresponding resource.
       The hard limit acts as a ceiling for the soft limit. An unprivileged process may only set its soft limit to a value  from  0  to  the  hard
       limit, and irreversibly lower its hard limit.

RETURN VALUES

	Returns an associative array of elements for each limit that is defined. Each limit has a soft and a hard limit.

       List of possible limits returned

       +-----------+---------------------------------------------------+
       |Limit name |						       |
       |	   |						       |
       |	   |		     Limit description		       |
       |	   |						       |
       +-----------+---------------------------------------------------+
       |   core    |						       |
       |	   |						       |
       |	   |  The  maximum  size of the core file. When 0, not |
       |	   | core files  are  created.	When  core  files  are |
       |	   | larger  than this size, they will be truncated at |
       |	   | this size. 				       |
       |	   |						       |
       | totalmem  |						       |
       |	   |						       |
       |	   |  The maximum size of the memory of  the  process, |
       |	   | in bytes.					       |
       |	   |						       |
       |virtualmem |						       |
       |	   |						       |
       |	   |  The  maximum  size of the virtual memory for the |
       |	   | process, in bytes. 			       |
       |	   |						       |
       |   data    |						       |
       |	   |						       |
       |	   |  The maximum size of the  data  segment  for  the |
       |	   | process, in bytes. 			       |
       |	   |						       |
       |  stack    |						       |
       |	   |						       |
       |	   |  The maximum size of the process stack, in bytes. |
       |	   |						       |
       |   rss	   |						       |
       |	   |						       |
       |	   |  The  maximum number of virtual pages resident in |
       |	   | RAM					       |
       |	   |						       |
       | maxproc   |						       |
       |	   |						       |
       |	   |  The maximum number of processes that can be cre- |
       |	   | ated for the real user ID of the calling process. |
       |	   |						       |
       | memlock   |						       |
       |	   |						       |
       |	   |  The  maximum  number of bytes of memory that may |
       |	   | be locked into RAM.			       |
       |	   |						       |
       |   cpu	   |						       |
       |	   |						       |
       |	   |  The amount of time the process is allowed to use |
       |	   | the CPU.					       |
       |	   |						       |
       | filesize  |						       |
       |	   |						       |
       |	   |  The  maximum  size  of  the data segment for the |
       |	   | process, in bytes. 			       |
       |	   |						       |
       |openfiles  |						       |
       |	   |						       |
       |	   |  One more than the maximum number	of  open  file |
       |	   | descriptors.				       |
       |	   |						       |
       +-----------+---------------------------------------------------+
EXAMPLES

       Example #1

	      Example use of posix_getrlimit(3)

	      <?php

	      $limits = posix_getrlimit();

	      print_r($limits);
	      ?>

	      The above example will output something similar to:

	      Array
	      (
		  [soft core] => 0
		  [hard core] => unlimited
		  [soft data] => unlimited
		  [hard data] => unlimited
		  [soft stack] => 8388608
		  [hard stack] => unlimited
		  [soft totalmem] => unlimited
		  [hard totalmem] => unlimited
		  [soft rss] => unlimited
		  [hard rss] => unlimited
		  [soft maxproc] => unlimited
		  [hard maxproc] => unlimited
		  [soft memlock] => unlimited
		  [hard memlock] => unlimited
		  [soft cpu] => unlimited
		  [hard cpu] => unlimited
		  [soft filesize] => unlimited
		  [hard filesize] => unlimited
		  [soft openfiles] => 1024
		  [hard openfiles] => 1024
	      )

NOTES

       Note

	       This is a not POSIX function, but is common on BSD and System V systems. If the system does not support this function, then it will
	      not be included at compile time. This may be checked with function_exists(3).

SEE ALSO

       man page GETRLIMIT(2).

PHP Documentation Group 													POSIX_GETRLIMIT(3)
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