--- Tim Prince <timothyprince@xxxxxxxxxxxxx> wrote:
> ranjith kumar wrote:
> > Hi,
> >
> > 1) Will "gcc" insert prefetch instructions
> > automatically on "pentium 4" processor?
> > Which flags should be enabled while compiling
> sothat
> > gcc automatically insert prefetch instructions?
> >
> > 2) Or programmer has to include some functions?
> > If so, what is the syntax of that function?
> >
> P4 isn't suitable for automatic compiler-generated
> prefetch. Default
> hardware prefetch (stride-based and cache line
> pairs) is quite
> effective.
Prefetch intrinsics are available with
> #include
> <xmmintrin.h>. Details on what works vary with
> steppings. The earliest
> P4 models could accelerate hardware prefetch by the
> program issuing 3
> cache lines of prefetch prior to entering a loop.
> Since Northwood, that
> doesn't work. Since Prescott, prefetch hints are
> ignored on P4, with
> prefetch going to L2 regardless of hints. Effect of
> prefetch on DTLB
> misses also is model dependent.
> Contrary to what certain Windows related docs say,
> _mm_prefetch() works
> the same on all compilers which implement it.
>
Hi,
1) What is the difference between "prefetchnta" and
"prefetchT1" instructions in case of pentium 4
processor.
In IA-32 optimization manual it was given that
prefetchT1,prefetchT2 and prefetchT3 are identical in
case of pentium 4 processor. Also prefetchnta fetches
the data into second level cache "minimizing cache
pollution". What does "minimizing cache pollution"
mean?
When I compared two programs, first one prefetching
data using "prefetchnta" and the second one using
"prefetchT0", I observed that second program was
executed faster. What could be the reason?
p.s: Below is the program which uses "prefetchT0". To
get program which uses "prefetchnta" send 0 as second
argument to fucntion in 22 line. I run then on
pentium4 processor with fedora core operating system.
1 #include<stdio.h>
2 #include<xmmintrin.h>
3 int main()
4 {
5
6 int i,j,k,h;
7 struct list
8 {
9 long double
w,w1,u,u1,x,x1,y,y1,z,z1;
10 long double e1,e2,e3,e4;
11 long double b1,b2,b3,b4,b5,b6;
12 long int a,b,c,d,e;
13 }l[5000];
14
15
16 int total;
17 for(h=0;h<9;h++)
18 for(j=0;j<99999;j++)
19 for(i=0;i<1000;i++)
20 {
21 //k=rand()%500;
22 _mm_prefetch((&l[(i+2)].a),3);
23
24
total+=(l[i*5].a)*(l[i*5].b)*(l[i*5].c)*(l[i*5].d)*(l[i*5].e);
25
26 //printf("\n %d ",total);
27 }
28
29 printf("\n %d ",total);
30 }
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