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Re: food for optimizer developers


On Thu, Aug 12, 2010 at 08:47:34PM +0200, Toon Moene wrote:
> Steve Kargl wrote:
> 
> ># gfc4x   9.814  9.358  8.622  9.810  Note1  9.172  8.958  9.022
> 
> Column 5 compiled with -march=native -O2 -ffast-math
> 
> ># Note 1:  STOP DLAMC1 failure (10)
> 
> That's probably because a standard compile of the LAPACK sources only 
> compiles {S|D}LAM* with -O0.
> 
> The code is simply not written for any higher optimization (i.e., it 
> assumes the compiler more or less compiles it "literally").
> 

Your observation re-enforces the notion that doing 
benchmarks properly is difficult.  I forgot about
the lapack inquiry routines.  One would think that
some 20+ year after F90, that Dongarra and colleagues
would use the intrinsic numeric inquiry functions.
Although the accumulated time is small, DLAMCH() is
called 2642428 times during execution.  Everything
returned by DLAMCH() can be reduced to a compile
time constant. 

-- 
Steve


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