Steven Noonan <ste...@uplinklabs.net> writes: > On Tue, Nov 10, 2015 at 10:52 AM, Markus Armbruster <arm...@redhat.com> wrote: >> Peter Maydell <peter.mayd...@linaro.org> writes: >> >>> On 10 November 2015 at 17:33, Paolo Bonzini <pbonz...@redhat.com> wrote: >>>> >>>> >>>> On 10/11/2015 16:57, Stefan Hajnoczi wrote: >>>>> clang 3.7.0 on x86_64 warns about the following: >>>>> >>>>> disas/arm.c:1782:17: warning: shifting a negative signed value is >>>>> undefined [-Wshift-negative-value] >>>>> imm |= (-1 << 7); >>>>> ~~ ^ >>>>> >>>>> Note that this patch preserves the tab indent in this source file >>>>> because the surrounding code still uses tabs. >>>>> >>>>> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> >>>> >>>> I would like to know a case where (except with ubsan) clang actually >>>> uses the optimization. >>>> >>>> If not, this is just error message theatre (which is not news for clang) >>>> and shouldn't have been part of -Wall. >>> >>> It could be they're attempting to warn us now about the possibility >>> that in a future version of clang they will start using this UB >>> to optimize with. >>> >>> http://stackoverflow.com/questions/22883790/left-shift-of-negative-values >>> reports that Intel's ICC will use this in dead-code-elimination >>> optimization. One day clang might do that too. >> >> Nice example of a compiler being gratuitously nasty. >> > > I don't read this warning as "clang will do crazy things with your > code eventually". Clang has always been very verbose when it comes to > undefined behavior, and I don't think that's really a bad thing to do. [...]
Misunderstanding? Clang's warning is at worst annoying, but nowhere near nasty. ICC concluding that code executing a left shift of a negative value must be unreachable is gratuitously nasty.
