On 17 November 2015 at 10:57, Paolo Bonzini <pbonz...@redhat.com> wrote: > > > On 17/11/2015 11:55, Peter Maydell wrote: >> If you pass clang -fwrapv then this causes -fsanitize=undefined to >> no longer complain about signed integer overflows from addition. >> However the sanitizer will still complain about left shifts of >> negative values. The conclusion I draw is that clang (as per >> the documentation) applies fwrapv only to addition &c, not to >> shifts. > > Ok, I'll open a bug for this. It's probably unintended, they've already > fixed -fwrapv once for pointers.
Thanks. Other things that would need to be fixed for -fwrapv to apply to shifts: * clang need to document this * gcc need to document this (this is a stronger statement than what they currently have since it is a guarantee not to change the semantics in the future if -fwrapv is set) * -fwrapv in clang should suppress -Wshift-negative-value * ideally, test cases in both the clang and gcc test suites to defend the 2s-complement signed shift semantics thanks -- PMM
