> On Mar 21, 2023, at 3:51 PM, Jeff Law <jeffreya...@gmail.com> wrote: > > > > On 3/21/23 13:01, Qing Zhao wrote: > >> The code previously worked now has some issue since we added some new stuff >> into standard, and the compiler added some new transformation based on this >> new stuff. Should the compiler issue some warnings to warn the users about >> such change? Then the user will go to the new standard to get more info? > It is not at all uncommon to have old code no longer work due to a compiler > update. > > This can happen if the compiler changes the default version of the language > it's supporting (say c99 to c11) or fixes a missed-error bug (more common in > C++) or the optimizers just get smarter and code which was always buggy, but > worked by accident no longer works. > > When we can reasonably give diagnostics, we try to. Improvements in this > space are appreciated :-)
For this specific case, the C standard (C99 and later) explicitly mentioned that the fp-contract might “undermine predictability”, “can even decrease accuracy” at the same time to allow it. http://port70.net/%7Ensz/c/c99/n1256.html#note78 78) This license is specifically intended to allow implementations to exploit fast machine instructions that combine multiple C operators. As contractions potentially undermine predictability, and can even decrease accuracy for containing expressions, their use needs to be well-defined and clearly documented. In GCC, the fp-contract is on by default, but the documentation of it doesn’t mention such warning at all. I think it will be helpful to add explicit warning in the -ffp-contract documentation. If you agree on this, I can come up with a patch to add such warning for -ffp-contract. Qing > > Jeff >
