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Groups > comp.lang.c.moderated > #391

Re: which one is faster?

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On Mon, 23 Apr 2012 08:32:16 -0500 (CDT), Keith Thompson
<kst-u@mib.org> wrote:

>echo ma <fatmck@gmail.com> writes:
>> on a 32bit system.
>>
>> #pragma pack(4)
>> struct TestStruct
>> {
>> 	unsigned short a;
>> 	unsigned short b;
>>    unsigned int c;
>> };
>> #pragma pack()
>> struct TestStruct t;
>> t.a = 0;      //step 1
>> t.b = 1;      //step 2
>> t.c = 2;      //step 3
>>
>> Questing is :  Are these 3 steps having the same perfomance time?
>
>Suppose you find out that one operation is faster than another on
>your system; what would you do with that information?  If you need
>to assign a value to t.c, assigning a value to t.a might be faster,
>but that doesn't do yhou much good if it's not the right thing to do.
>
>(Note that "#pragma pack" is non-standard and potentially unsafe;
>see <http://stackoverflow.com/q/8568432/827263>.  For your particular
>struct definition, it's unlikely to have any effect.)

The only real reasons for such tightly packed structures are to save
memory in *very* small systems, or to perform memory mapped device
access ... which brings up both coding issues such as register value
reuse and CPU/platform issues such as bus width, access alignment,
write-back caching, out-of-order execution, write combining, etc.

x86 being an exception, most 32-bit CPUs use memory mapped I/O
exclusively, and it often is desirable to create structures for
accessing groups of contiguous device "registers".  However, in such
cases, making a single 32-bit access to A+B in order to write a 16-bit
value to B would be incorrect (and potentially destructive).

As someone else said already, it would be nice if the OP had given a
more complete description of the intended use.

George
-- 
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Thread

which one is faster? echo ma <fatmck@gmail.com> - 2012-04-04 18:05 -0500
  Re: which one is faster? James Kuyper <jameskuyper@verizon.net> - 2012-04-23 08:31 -0500
  Re: which one is faster? "Joel C. Salomon" <joelcsalomon@gmail.com> - 2012-04-23 08:33 -0500
  Re: which one is faster? David Brown <david@westcontrol.removethisbit.com> - 2012-04-23 08:33 -0500
  Re: which one is faster? Thomas Richter <thor@math.tu-berlin.de> - 2012-04-23 08:31 -0500
    Re: which one is faster? Keith Thompson <kst-u@mib.org> - 2012-04-30 21:57 -0500
  Re: which one is faster? Barry Schwarz <schwarzb@dqel.com> - 2012-04-23 08:32 -0500
  Re: which one is faster? Keith Thompson <kst-u@mib.org> - 2012-04-23 08:32 -0500
    Re: which one is faster? George Neuner <gneuner2@comcast.net> - 2012-04-30 21:58 -0500

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