On 9 September 2015 at 09:18, Mathieu Malaterre <malat@xxxxxxxxxx> wrote: >> Thanks for the link. However -in my case- the compile option >> `-fexcess-precision=standard` does produce the same code as >> `-ffloat-store`. At least for > > Meant to say `does not produce` The FAQ now says: "(whether this option has effect or not depends on other options, please read the manual entry for -fexcess-precision= carefully)." The manual says: "-fexcess-precision=standard is not implemented for languages other than C, and has no effect if -funsafe-math-optimizations or -ffast-math is specified. On the x86, it also has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable" > >> [...] >> double div; /* OPJ_FLOAT64 */ >> div = dd / dr; >> if (div >= thresh) >> [...] >> >> Is there a longer description for `-fexcess-precision=standard` which >> explains case of failures ? The manual says: "if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments)." The FAQ says: "Floating-point results may still depend on the optimization level and target architecture in some cases that are allowed by the ISO C standard. For instance, different sets of instructions may be used for code such as x*y+z depending on the target architecture and the optimization level, and this difference may change the results." I don't have a list of cases allowed by the standard. If you know more examples, we could add them to the answer. > From a clean debian/sid 32bits chroot I can no longer reproduce this. > In summary: > > 1. removed -ffast-math > 2. add -std=c99 > 3. Change the code for an explicit storage of division (div = dd / dr) > => issue solved ! The manual says: "This option is enabled by default for C if a strict conformance option such as -std=c99 is used." Thus, you are in fact using -fexcess-precision=standard. That only means that floating-point computations will follow ISO C rules. FAQ: "Floating-point results may still depend on the optimization level and target architecture". Cheers, Manuel.
