Thanks for the advice, Sean.
I suspect it's an issue of double precision, but I cannot find what specific flags are involved.
Image colum from original program, unknown compiler optimizations: 0: 9.40411e-05 1: 0.000374236 2: 0.000421015 3: 0.000158423 4: 0.000421016 5: -6.64318e-05 6: 0.000225337
Image colum from my program, no compiler optimizations: 0: 9.40411e-05 1: 0.000388825 2: 0.000421015 3: 0.000158423 4: 0.000421016 5: 0.000374236 6: 0.000356633
Difference: 0: 0 1: -1.45885e-05 2: 0 3: 0 4: 0 5: -0.000440668 6: -0.000131296
Percentage wise, these differences can be quite significant. If I compile a single file from my version of the program (backprojectors.cpp) with the -O2 option, the differences disappear. I combined all of the options that the gcc man page said were included in -O1 and -O2, and the differences did NOT disappear, so I still don't know what specific options are making the difference in the original program.
Sean Cody wrote:
From gcc man page...
Important notes: -ffast-math results in code that is notnecessar- ily IEEE-compliant. -fstrict-aliasing is highly likely break non- standard-compliant programs. -malign-natural only works properly if the entire program is compiled with it, and none of the standard headers/libraries contain any code that changes alignment when this option is used.
If you are having precision issues with the doubles kill the above flags with your optimizations (eg. -fno-fast-math). I'm going to assume that v is global so it may not be scoping and from your description it _sounds_ more like precision issues. If it is precision issues start turning off selectively the options from the optimized flags and find out which one is mucking up the results.
I wouldn't assume it is the code generated until you can prove it by finding a particular optimization strategy which is producing the issue. In integer only cases I would argue the compiler is outputting correct code but with reals the size/space optimizations can do a _whole lot_ of damage.
If the values are within a valid range I would suggest liberal application of assertions, it is hard to say what you mean by correct values but if the numbers are off by a certain amount or are rounded up or down from what you expect then I would put money on precision issues.