On 06 Nov 2007 18:32:01 +0300, Egor Pasko <egor.pasko@gmail.com> wrote:
> On the 0x386 day of Apache Harmony Aleksey Shipilev wrote:
> > On 11/6/07, Tim Ellison <t.p.ellison@gmail.com> wrote:
> > > > It seems to me that it could be improved further if some magic
> > > > implementing sqrt() will be used instead on native call.
> > >
> > > Looks like we have to go this path, since the hacked intrinsics are
> > > still 4x slower if I'm reading this properly.
> > You're reading right. We are still 4x slower than Sun 1.6.0_02
> >
> > > > Moreover, AFAIU the (3) approach is safe since IEEE754 compatibility
> > > > must be preserved only for strict mode, whereas (3) approach
> > > > implements fastpath for non-strict mode.
> > >
> > > Yes, I modified my microbench to use both strict and non-strict in the
> > > same run and there is a noticeable difference on Sun 6.0:
> > >       Math Result = 6.666661664588418E8 in 30ms
> > > StrictMath Result = 6.666661664588418E8 in 1012ms
> >
> > That's weird :S Here's what I've got on this modified microtest:
> > ========================================================
> > public class testSqrt {
> >
> > final static int count = 10000000;
> >
> > public static void main(String[] args) {
> >
> >    // warm-up
> >    double result = 0.0d;
> >    for (long i = 0; i < 1024*1024*10; i++) {
> >        result += Math.sqrt((double) i);
> >        result += StrictMath.sqrt((double) i);
> >    }
> >    System.out.println("Warmup finished: " + result);
> >
> >    long start;
> >
> >    // Timed run
> >    result = 0.0d;
> >    start = System.currentTimeMillis();
> >    for (int i = 0; i < count; i++) {
> >        result += Math.sqrt((double) i);
> >    }
> >
> >    System.out.println("Math Result = " + result + " in "
> >               + (System.currentTimeMillis() - start) + "ms");
> >
> >    // Timed run
> >    result = 0.0d;
> >    start = System.currentTimeMillis();
> >    for (int i = 0; i < count; i++) {
> >        result += StrictMath.sqrt((double) i);
> >    }
> >
> >    System.out.println("StrictMath Result = " + result + " in "
> >               + (System.currentTimeMillis() - start) + "ms");
> >
> >
> > }
> >
> > }
> > ========================================================
> >
> > java version "1.5.0_10"
> > Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_10-b03)
> > Java HotSpot(TM) Client VM (build 1.5.0_10-b03, mixed mode, sharing)
> >
> > Warmup finished: 4.527292719905836E10
> > Math Result = 2.1081849486439312E10 in 140ms
> > StrictMath Result = 2.1081849486439312E10 in 9235ms
> >
> > ----------------
> >
> > java version "1.6.0"
> > Java(TM) SE Runtime Environment (build 1.6.0-b105)
> > Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)
> >
> > Warmup finished: 4.527292719905836E10
> > Math Result = 2.1081849486439312E10 in 141ms
> > StrictMath Result = 2.1081849486439312E10 in 141ms
> >
> > ---------------
> >
> > java version "1.6.0_02"
> > Java(TM) SE Runtime Environment (build 1.6.0_02-b06)
> > Java HotSpot(TM) Client VM (build 1.6.0_02-b06, mixed mode)
> >
> > Warmup finished: 4.527292719905836E10
> > Math Result = 2.1081849486439312E10 in 156ms
> > StrictMath Result = 2.1081849486439312E10 in 140ms
> >
> > ---------------
> > Harmony (clean)
> >
> > Apache Harmony Launcher : (c) Copyright 1991, 2006 The Apache Software
> > Foundation or its licensors, as applicable.
> > java version "1.5.0"
> > pre-alpha : not complete or compatible
> > svn = r589548, (Nov  6 2007), Windows/ia32/msvc 1310, release build
> > http://harmony.apache.org
> >
> > Warmup finished: 4.527292719905836E10
> > Math Result = 2.1081849486439312E10 in 12078ms
> > StrictMath Result = 2.1081849486439312E10 in 12016ms
> >
> > ----------------
> > Harmony (patched)
> >
> > Apache Harmony Launcher : (c) Copyright 1991, 2006 The Apache Software
> > Foundation or its licensors, as applicable.
> > java version "1.5.0"
> > pre-alpha : not complete or compatible
> > svn = r589548, (Nov  6 2007), Windows/ia32/msvc 1310, release build
> > http://harmony.apache.org
> >
> > Warmup finished: 4.5272927199206406E10
> > Math Result = 2.1081849486508232E10 in 3406ms
> > StrictMath Result = 2.1081849486439312E10 in 12058ms
> >
> > ----------------
> > You see, Sun 1.6.0 behaves fast even in strict mode.
>
> Alexey, thanks, very useful!
>
> my 2c is that both Math and StrictMath require "correctly rounded" (to
> nearest) results for sqrt().
>
> BTW, the _mm_sqrt_s intrinsic looks like invokes the hardware
> implementation of sqrt which is khown to be slower than some modern
> software implementations.

Egor,

could you please provide link to the best software implementation?
AFAIK, with the provided patch we could receive good speedup for
Dacapo.lusearch bench. If I remember correctly with _mm_sqrt_s
intrinsic lusearch time decreased from ~2800 msec to 2100 msec. So, I
think it could be great to use this implementation while we haven't
good software implementation.

Thanks.
Vladimir

> --
> Egor Pasko
>
>