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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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plimit(1)                                                          User Commands                                                         plimit(1)

NAME
plimit - get or set the resource limits of running processes SYNOPSIS
plimit [-km] pid... plimit {-cdfnstv} soft,hard... pid... DESCRIPTION
If one or more of the cdfnstv options is specified, plimit sets the soft (current) limit and/or the hard (maximum) limit of the indicated resource(s) in the processes identified by the process-ID list, pid. Otherwise plimit reports the resource limits of the processes identi- fied by the process-ID list, pid. Only the owner of a process or the super-user is permitted either to get or to set the resource limits of a process. Only the super-user can increase the hard limit. OPTIONS
The following options are supported: -k On output, show file sizes in kilobytes (1024 bytes) rather than in 512-byte blocks. -m On output, show file and memory sizes in megabytes (1024*1024 bytes). The remainder of the options are used to change specified resource limits. They each accept an argument of the form: soft,hard where soft specifies the soft (current) limit and hard specifies the hard (maximum) limit. If the hard limit is not specified, the comma may be omitted. If the soft limit is an empty string, only the hard limit is set. Each limit is either the literal string unlimited, or a number, with an optional scaling factor, as follows: nk n kilobytes nm n megabytes (minutes for CPU time) nh n hours (for CPU time only) mm:ss minutes and seconds (for CPU time only) The soft limit cannot exceed the hard limit. -c soft,hard Set core file size limits (default unit is 512-byte blocks). -d soft,hard Set data segment (heap) size limits (default unit is kilobytes). -f soft,hard Set file size limits (default unit is 512-byte blocks). -n soft,hard Set file descriptor limits (no default unit). -s soft,hard Set stack segment size limits (default unit is kilobytes). -t soft,hard Set CPU time limits (default unit is seconds). -v soft,hard Set virtual memory size limits (default unit is kilobytes). OPERANDS
The following operands are supported. pid Process ID list. EXIT STATUS
plimit returns the exit value zero on success, non-zero on failure (such as no such process, permission denied, or invalid option). FILES
/proc/pid/* process information and control files ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWesu | +-----------------------------+-----------------------------+ SEE ALSO
ulimit(1), proc(1), getrlimit(2), setrlimit(2), proc(4), attributes(5), SunOS 5.10 8 Jun 1998 plimit(1)
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