On 25/11/2015 18:44, Peter Maydell wrote: > > Ubsan also has warnings for undefined behavior of left shifts. Checks for > > left shift overflow and left shift of negative numbers, unfortunately, > > cannot be silenced without also silencing the useful ones about out-of-range > > shift amounts. -fwrapv ought to shut them up, but doesn't yet > > (https://llvm.org/bugs/show_bug.cgi?id=25552; I am taking care of fixing > > the same issues in GCC). Luckily ubsan is optional, and the easy > > workaround is to use -fsanitize-recover. > > We still haven't had any response from the LLVM/clang folks that > this interpretation of the meaning of -fwrapv is their interpretation > of it, have we? (I can't see any comments on the referenced bug.)
Reasonably, they will have to follow what GCC does, independent of -fwrapv. GCC has now promised to not exploit << undefined behavior, even without -fwrapv. So at this point, -fwrapv is only required to placate ubsan---which it will do for GCC as soon as my other patch is approved (I talked on IRC with one of the GCC-ubsan authors and he said he was okay). clang with ubsan remains broken, but that's no worse than before. Paolo
