Possible Code Performance Disparity using "parameter"

[John L. Males]

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Hello,

I am using GFortran 4.4.5.  I have discovered by accident an
issue in the performance of the generated code where at least
one variable changed from having a "parameter" attribute to not
causes at least more than 100% increase execution time for a
statical program.

The only reason the variable had the "parameter" attribute
removed was to enable the variable to have a different same
during different execution was to enable different values for
the statical analysis. This would avoid different versions of
code for the obvious different execution values normal in a
statical analysis.

integer (kind=1), parameter :: foo1
874.9 real	871.4 user	0.3 sys
vs
integer (kind=1) :: foo1
1838.4 real	1836.8 user	0.2 sys
whose value is for execution is
determined via get_command_argument.

Otherwise the variable is only read in the core part of both
variants of the code that uncovered this issue.  The code often 
involves at least 400000000000 iterations in a small loop for a
statical calculation.

The command line to compile was "gfortran -Wall -O2"

Any thoughts or comments with regards to this discovery?


Regards,

John L. Males
Toronto, Ontario
Canada
12 March 2012 18:03
<mailto:jlmales@gmail.com>


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2012-03-12 17:40:18.982824669-0400-EDT

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