OK, my compiler, gcc 2.95 does the optimization at -O2, so I ran the
tests here with the attached program, testing a constant of 256 and a
variable 'j' equal to 256.
I found a 1.7% slowdown with the variable compared to the constant.
The 256 value is rather large. For a length of 32, the difference was
14%!
So, seems I should back out the palloc0 usage and let the MemSet inline
at each location. Does that seems like the direction to head?
---------------------------------------------------------------------------
Bruce Momjian wrote:
> Tom Lane wrote:
> > Bruce Momjian <[EMAIL PROTECTED]> writes:
> > > I have applied this patch to inline MemSet for newNode.
> > > I will make another commit to make more general use of palloc0.
> >
> > Refresh my memory on why either of these was a good idea?
>
> You are talking about the use of palloc0 in place of palloc/MemSet(0),
> not the use of palloc0 in newNode, right?
>
> > > This was already discussed on hackers.
> >
> > And this was not the approach agreed to, IIRC. What you've done has
> > eliminated the possibility of optimizing away the controlling tests
> > in MemSet.
>
> I see now:
>
>
>http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&selm=200210120006.g9C06i502964%40candle.pha.pa.us
>
> I thought someone had tested that there was little/no performance
> difference between these two statements:
>
> MemSet(ptr, 0, 256)
>
> vs.
>
> i = 256;
> MemSet(ptr, 0, i)
>
> However, seeing no email reply, I now assume no test was done.
>
> Neil, can you run such a test and let us know. It has to be with a
> compiler that optimizes out the MemSet length test when the len is a
> constant. I can't remember what compiler version/settings that was, but
> it may have been -O3 gcc 3.20.
>
> I can back out my changes, but it would be easier to see if there is a
> benefit before doing that. Personally, I am going to be surprised if a
> single conditional tests in MemSet (which is not in the assignment loop)
> will make any measurable difference.
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> [EMAIL PROTECTED] | (610) 359-1001
> + If your life is a hard drive, | 13 Roberts Road
> + Christ can be your backup. | Newtown Square, Pennsylvania 19073
>
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--
Bruce Momjian | http://candle.pha.pa.us
[EMAIL PROTECTED] | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
#define INT_ALIGN_MASK (sizeof(int) - 1)
#include <stdlib.h>
/*
* MemSet
* Exactly the same as standard library function memset(), but considerably
* faster for zeroing small word-aligned structures (such as parsetree nodes).
* This has to be a macro because the main point is to avoid function-call
* overhead. However, we have also found that the loop is faster than
* native libc memset() on some platforms, even those with assembler
* memset() functions. More research needs to be done, perhaps with
* platform-specific MEMSET_LOOP_LIMIT values or tests in configure.
*
* bjm 2002-10-08
*/
#define MemSet(start, val, len) \
do \
{ \
int * _start = (int *) (start); \
int _val = (val); \
size_t _len = (len); \
\
if ((((long) _start) & INT_ALIGN_MASK) == 0 && \
(_len & INT_ALIGN_MASK) == 0 && \
_val == 0 && \
_len <= MEMSET_LOOP_LIMIT) \
{ \
int * _stop = (int *) ((char *) _start + _len); \
while (_start < _stop) \
*_start++ = 0; \
} \
else \
memset((char *) _start, _val, _len); \
} while (0)
#define MEMSET_LOOP_LIMIT 1024
int
main(int argc, char **argv)
{
int i,j;
char *buf;
buf = malloc(256);
j = 32;
for (i = 0; i < 1000000; i++)
MemSet(buf, 0, j /* choose 256 or j here */);
return 0;
}
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