[RndTbl] Problem with g++ optimization

Dan Martin ummar143 at cc.umanitoba.ca
Mon Sep 4 15:04:08 CDT 2006

I am modifying an open source C++ program.  I found that my output was 
not the same as the original, so I spent the last 3 days searching for 
the errors in my program, then making extensive back-modifications to 
make my program's output identical to the original.  I finally went back 
to the original, and made only minimal changes to allow compilation with 
my makefile.

The errors persisted.  Reasoning that the original was probably compiled 
with optimization, I tried that and sure enough the differences 
disappeared (some with -O and the rest disappeared using -O2).

Calculations done mainly with integer math were fine to begin with.  
Using -O, linear interpolation involving doubles yielded the same 
results, and all results including more complex interpolation and double 
to int conversions were the same using -O2.

I cannot verify my output other than by using the original program.

Which results are correct?  Am I to assume that optimization does 
something bad? [my program correct, the original compiled with 
optimizations incorrect].


Dr. Dan Martin, MD, CCFP, BSc, BCSc (Hon)

GP Hospital Practitioner
Computer Science grad student
ummar143 at cc.umanitoba.ca

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