Re: (a+b)+c should be replaced by a+(b+c)

[Scott Robert Ladd <coyote at coyotegulch dot com>]

Jakub Jelinek wrote:
> On Thu, Mar 25, 2004 at 09:21:48AM -0500, Scott Robert Ladd wrote:
>
> > Joost VandeVondele wrote:
> >
> > > BTW, timing of the code below on IBM SP4 with xlf90, would be useful to
> > > see how gfortran performs.
> >
> > Being in a benchmarking mood, I took your code and compiled it on a
> > 2.8GHz Pentium 4 (Northwood core). The results did not show gfortran in
> > a very good light:
> >
> > - - - - - - - - - - - - - - - - - - - - -
> >
> > Tycho$ ifort -O3 -tpp7 -xN -ipo -o matmuli matmul.for
> > IPO: using IR for /tmp/ifortyRX1Wg.o
> > IPO: performing single-file optimizations
> > matmul.for(6) : (col. 6) remark: LOOP WAS VECTORIZED.
> > matmul.for(7) : (col. 6) remark: LOOP WAS VECTORIZED.
> > matmul.for(8) : (col. 6) remark: LOOP WAS VECTORIZED.
> > Tycho:$ ./matmuli
> >   5.90410300000000        10.2399999999998
> > Tycho$ gfortran -o matmulg -O3 -ffast-math -march=pentium4 matmul.for
>
>
> You forgot -mfpmath=sse.  That is only the default for -m64.
>
> 	Jakub

Good point; I've been doing Opteron work for a week, and was getting 
used to not explicitly declaring certain flags.

Also, a minimized browser was playing a &%$!! Flash animation in the 
background, so I'll run numbers on a clean machine without the overhead.

And the compiler says:

 - - - - - - - - - - - - - - - -
Tycho$ gfortran -o matmulg -O3 -march=pentium4 -ffast-math matmul.for
Tycho$ ./matmulg
    64.9091330000000         10.2400000000000

Tycho$ gfortran -o matmulg -O3 -march=pentium4 -ffast-math -mfpmath=sse 
matmul.for
Tycho$ ./matmulg
    64.6051790000000         10.2399999999998

Tycho$ gfortran -o matmulg -O3 -march=pentium4 -mfpmath=sse matmul.for
Tycho$ ./matmulg
    64.7361590000000         10.2399999999998

Tycho$ gfortran -o matmulg -O3 -march=pentium4 matmul.for
Tycho$ ./matmulg
    64.7751530000000         10.2400000000000
Tycho$

 - - - - - - - - - - - - - - - -

[dry_sarcasm]
Well, we can see the -ffast-math *really* helps in this suituation, huh?
[/dry_sarcasm]


Nor did -mfpmath=sse show much value for this test. In my experience, 
-mfpmath=sse often fails to produce faster code (with gfortran or gcc)

What about Intel Fortran with their -mp1 and -mp options?

 - - - - - - - - - - - - - - - -

Tycho$ ifort -O3 -tpp7 -xN -ipo -o matmuli matmul.for
Tycho$ ./matmuli
   4.85226200000000        10.2399999999998

Tycho$ ifort -O3 -tpp7 -xN -ipo -mp1 -o matmuli matmul.for
Tycho:~/projects/spikes$ ./matmuli
   4.90425400000000        10.2399999999998

Tycho$ ifort -O3 -tpp7 -xN -ipo -mp -o matmuli matmul.for
Tycho$ ./matmuli
   66.0699560000000        10.2399999999998

 - - - - - - - - - - - - - - - -

Forcing Intel to stick with the "rules" does slow its performance. 
Certainly some food for thought...


--
Scott Robert Ladd
Coyote Gulch Productions (http://www.coyotegulch.com)
Software Invention for High-Performance Computing