On 06/30/2016 04:15 PM, Paolo Bonzini wrote: > > > ----- Original Message ----- >> From: "John Snow" <[email protected]> >> To: [email protected] >> Cc: [email protected], [email protected], [email protected], "peter >> maydell" <[email protected]>, >> "John Snow" <[email protected]> >> Sent: Thursday, June 30, 2016 9:36:36 PM >> Subject: [PATCH] Remove left shifts of negative signed integers >> >> Another exercise in placating Clang's increasingly strict -Werror mode. >> Technically, this is undefined behavior. In practice, -N<<M is the same >> as -(N<<M). >> >> Signed-off-by: John Snow <[email protected]> > > There's been discussions on this in the past; sorry but this is a > super-duper NACK. > > GCC correctly puts this warning under -Wextra, and promises not to ever > make use of this facet of undefined behavior. The only correct patch > is the one that disables the warning for clang, and possibly adds > -fwrapv. In GCC, -fwrapv correctly silences ubsan's left-shift > and signed-overflow warnings. In Clang, this is reported at > https://llvm.org/bugs/show_bug.cgi?id=25552. It's a heavy hammer > but it's the safest options as compiler evolve. > > Paolo >
Where /exactly/ do you propose we canonicalize the clang invocation you would like to support? --js
