On 2016.02.27 at 15:10 -0800, Paul E. McKenney via llvm-dev wrote: > On Sat, Feb 27, 2016 at 11:16:51AM -0800, Linus Torvalds wrote: > > On Feb 27, 2016 09:06, "Paul E. McKenney" <paul...@linux.vnet.ibm.com> > > wrote: > > > > > > > > > But we do already have something very similar with signed integer > > > overflow. If the compiler can see a way to generate faster code that > > > does not handle the overflow case, then the semantics suddenly change > > > from twos-complement arithmetic to something very strange. The standard > > > does not specify all the ways that the implementation might deduce that > > > faster code can be generated by ignoring the overflow case, it instead > > > simply says that signed integer overflow invoked undefined behavior. > > > > > > And if that is a problem, you use unsigned integers instead of signed > > > integers. > > > > Actually, in the case of there Linux kernel we just tell the compiler to > > not be an ass. We use > > > > -fno-strict-overflow > > That is the one! > > > or something. I forget the exact compiler flag needed for "the standard is > > as broken piece of shit and made things undefined for very bad reasons". > > > > See also there idiotic standard C alias rules. Same deal. > > For which we use -fno-strict-aliasing.
Do not forget -fno-delete-null-pointer-checks. So the kernel obviously is already using its own C dialect, that is pretty far from standard C. All these options also have a negative impact on the performance of the generated code. -- Markus
