Author: Gerd Isenberg
Date: 03:28:39 10/22/02
Go up one level in this thread
On October 21, 2002 at 18:27:33, Dieter Buerssner wrote:
>On October 21, 2002 at 06:27:09, Gerd Isenberg wrote:
>
>>from AMD Athlon™ Processor x86 Code Optimization Guide:
>>Chapter 3 C Source-Level Optimizations
>>
>>Use Const Type Qualifier
>[...]
>
>My little programming ancectdote using const on AMD K6-2. This small function:
>
>typedef unsigned long long ul64;
>
>/* Two implemantations of the multiply with carry RNG.
> The only difference is the type of mul */
>
>static ul64 zseed = ((ul64)0x12345678UL<<32) | 0x87654321UL;
>
>unsigned long mwc32(void)
>{
> unsigned long l1, l2;
> ul64 res;
> static unsigned long mul=999996864UL;
> l1 = (unsigned long)(zseed & 0xffffffffUL);
> l2 = zseed>>32;
> res = l2+l1*(ul64)mul;
> zseed = res;
> return (unsigned long)(res & 0xffffffffUL);
>}
>
>preformed well. It is somewhat microptimized for gcc and procuces almost optimal
>assembly. (Part of the microoptimization was to use a variable for the constant
>mul).
>
>After I reviewed one of my programs, I changed the type of mul and added const.
>Much later, the program was running much slower. I did not suspect, to have it
>to do with that const and it took a long time to find the cause. More
>investigation showed, that that routine, with the const was about 8 times
>slower. Very strange. The assembly was exactly the same for both versions, with
>the only difference, that the variable mul now was in the code segment instead
>of the data segment. Data aligning was perfect in both cases. Adding some "dummy
>space" after the variable mul in the assembly, speed went back to normal. The
>behaviour could be reproduced on other K6-2 processors, but not on Pentium
>processors. On the later, both functions performed identically. The thread can
>be found via dejanews.
>
>Cheers,
>Dieter
Hi Dieter,
very stange behaviour of gcc (putting constant data to "romable" code segment)
with K6-2.
My intention on this topic was declaring member functions as const, if they
don't change the object (most likely some inlined getters, which simply return a
member variable). I can imagine that compilers may profit from this const
declarations.
Gerd
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