Thank you for offering your help. I can now definitely confirm that there is no computation difference between Go and C in my program. There is thus no red flag to use Go in our scientific application. What a relief.
The problem is that our C and Go programs don't store the values in the same location. I thus compared different values, but they were close enough to be confusing. It is impressive that the C and Go computation yields the exact same values and output when using %g. I could locate my values with a simple text search and diagnose their misplacement. I still need to determine which of the C or Go program is doing it wrong. This code translation process may have uncover a bug in the C program. Le jeudi 10 mars 2022 à 00:31:25 UTC+1, pdc...@gmail.com a écrit : > Hi Christopher, what input int64 is leading to this result (difference in > behavior between Go & C). Does it happen with any input? > I'm asking because I'm interested in playing around with this a bit, maybe > writing two trivial programs (one on each language) and comparing the > machine code generated from each one. I'm trying to venture into low level > debugging/reverse engineering and I think it will be a good exercise. > > Thank you! > > Best > > Pablo > > On Wed, Mar 9, 2022, 4:39 PM 'Dan Kortschak' via golang-nuts < > golan...@googlegroups.com> wrote: > >> On Wed, 2022-03-09 at 03:37 -0800, christoph...@gmail.com wrote: >> > I'm translating a scientific C program into Go that is doing some >> > 64bit floating point operations. >> > >> > In this process I check that the same input yields the same output. >> > Unfortunately they don't yield the same result, though the >> > computation is simple. It is as follow. I receive a 64bit integer >> > value. >> > >> > This value is converted to float64/double, and divided by 2^64. >> > The resulting number is multiplied by 1e8. >> > >> > With C I get 41 6E 84 FD 00 09 90 D7, with Go I get 41 6E 84 FD 00 09 >> > E6 8E. The last 15bits are different. The computation is performed >> > with the same computer. >> > >> > Could it be that the C program is performing the computation with >> > long double (80 bit) precision and that Go is doing it with 64bit >> > precision ? >> > >> > Is there something I could do about it because that might be a red >> > flag for replacing the C program with a Go program. >> >> This is not very surprising depending on the algorithms that are being >> used/the problem that is being solved. Some problems are fundamentally >> difficult to solve exactly and the nature of floating point makes them >> sensitive to the precise set of operations used, intermediate rounding >> and the order of operations (even for operations that are commutative >> in theory). As Robert said, knowing the C compiler will be important, >> and I'd go further, knowing which platform you are building the Go >> program on can be important due to differences in how floating point >> operations are rendered into machine code by the compiler, or even how >> the processor orders apparently commutative operations. >> >> Assuming the values that you've pasted above are big endian, then the >> Go value is within a 1e12th of the value calculate by C ( >> https://go.dev/play/p/dn7G2LI75RC). This is not terrible, and maybe >> that level of precision is all that can be promised by the algorithm >> (and believing digits smaller that 1e-12 is dangerous). Alternatively >> there is no fundamental limit at this point and there is a better more >> stable algorithm that you can use (though you are only four orders of >> magnitude from the machine epsilon >> https://en.wikipedia.org/wiki/Machine_epsilon, so be aware). >> >> Floats are tricky beasts and can easily trip people up. I would suggest >> that you read >> https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html (and >> the more friendly https://floating-point-gui.de/). >> >> >> >> -- >> You received this message because you are subscribed to the Google Groups >> "golang-nuts" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to golang-nuts...@googlegroups.com. >> > To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/ee58b6e5ad163014b256d249e21c875307fecddb.camel%40kortschak.io >> . >> > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/027222f6-8f42-4db8-ab5c-b34020cd42a6n%40googlegroups.com.
