On 2015-11-05 22:23:01 +0100, Markus Trippelsdorf wrote: > On 2015.11.05 at 21:52 +0100, David Brown wrote: > > There has been some discussions going on in the comp.lang.c newsgroup > > about how far compilers are allowed to go regarding optimisation using > > their knowledge of undefined behaviour (i.e., if a piece of code has > > undefined behaviour, the compiler can assume that the user does not care > > about the result in that case, and can therefore do whatever it wants in > > order to generate faster code). > > The compiler just assumes that undefined behavior will not happen and > optimizes accordingly. That's undefined by the C standard and this is useful for some optimizations, but it would be great if GCC could, as an extension, provide builtins with some well-defined behavior in cases that would be UB by the C standard. Let me give an example. I have a code that must run as fast as possible. There is a 64-bit integer division. A division by 0 is possible, but occurs with a very low probability (something like 1/2^64, I assume). When it occurs, anything that would yield an abnormal termination is acceptable (no other behavior is acceptable). I could do a test, but this would slow down the code, while the x86 division instruction generates a trap in case of division by 0, so that such an instruction could be used alone (with no associated test). So, such a builtin would be very useful. -- Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
