Re: [Boost-users] [Interval] Help debugging compiler optimization

Hi Degski, But if I assign 1/3 to a double and then print that to the console, then I would expect the compiler to be committed to the value shown in the console, but apparently it isn't, and still does optimization. Best, Tim On 07/03/2018 15:53, Mário Costa via Boost-users wrote:

Not sure this "random" aswer explains it:

1/3 it's a recurring decimal (https://en.wikipedia.org/wiki/Repeating_decimal https://en.wikipedia.org/wiki/Repeating_decimal), that means it is not possible to represent it within a limited number of bits (even in 64 bits).

So your "bug" example, I would say that it's not a bug. Using optimization in one of the cases the compiler can figure out, that the value is 1, by analyzing constant values assigned to v2 and lazy expanding the expression (template expansions, etc), (which is correct math value). The other option v1, uses memory storage, and because of that, it cannot lazy expand the expression at the function call, so because you cannot store 1/3 perfectly in 64 bits, after multiplying it, it always results in a number lower 1.

On Wed, Mar 7, 2018 at 11:54 AM, degski via Boost-users mailto:boost-users@lists.boost.org> wrote:

On 6 March 2018 at 20:43, Nathan Ernst via Boost-users mailto:boost-users@lists.boost.org> wrote:

This does feel like a floating-point optimization switch, though, at least to me.

Additionally, the optimization level it self could interfere with the order of calculation, leading to possibly different results.

degski

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