On 2011-11-02 15:31:42 +1100, leon zadorin wrote: > Under certain conditions GCC is able to perform compile-time folding > of floating point expressions. This is great. > > Sometimes, however, the resulting (or indeed intermediate) expressions > may evaluate to a non-finite value (e.g. NaN, Inf) -- at compile-time. > > During a traditional run-time execution of floating point arithmetic > there are various ways to detect the case of values going into > something like an overflow (individual guarding of each > possibly-violating expression via 'isfinite()'; or via reading the FPU > status register, which hardware itself may flag appropriately; or via > the 'fetestexcept' et al wrappers). > > I was wondering if there was a way to ask GCC to optionally emit some > kind of a diagnostic (e.g. a warning of some kind) when it does > floating point calculations at compile-time and when such compile-time > calculations yield in a non-finite number. In ISO C99, I don't think the compiler is allowed to replace a floating expression with an underflow or overflow by its result at compile time. 6.6#4 of ISO C99 says: "Each constant expression shall evaluate to a constant that is in the range of representable values for its type." and an underflow or overflow means that the result is not in the range of representable values, so that the rules for constant expressions must not be applied. Now, I don't know what C++ allows (in your case) and I haven't tested how GCC behaves (there may be a bug...). -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <http://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <http://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / Arénaire project (LIP, ENS-Lyon)
