On Fri, Apr 28, 2017 at 03:43:56PM +0100, Ard Biesheuvel wrote: > On 28 April 2017 at 15:33, Mark Rutland <mark.rutland@xxxxxxx> wrote: > > On Fri, Apr 28, 2017 at 08:18:52AM +0100, Ard Biesheuvel wrote: > OK, good. That is a departure from previous behavior of Clang, which > was causing build errors before due to the fact that msr/mrs > instructions involving 32-bit values must still use x registers. > > > * If an operand type is 64 bits in size, clang will not produce a warning > > regarding the operand size. > > > > * If an x or w modifier is used, clang will not produce a warning > > regarding the operand size, regardless of whether it matches the > > register size. Clang is happy for %wN to be used on a pointer type. > > > > * If an operand type is less than 64 bits in size, and neither an x or > > w modifier is used, clang will produce a warning as above. > > > > * If an operand type is greater than 64 bits in size, clang encounters > > an internal error. > > > > Given that, I think we *should not* use the x modifier to suppress this > > warning, as I think for those cases we have a potential bug as outlined > > in my prior reply. > > > > Instead, we should use a temporary 64-bit variable (or cast input > > operands to 64-bit), which avoids that and makes clang happy. > > > > Yes, I think that makes sense. Likewise, we could even raise a feature request against GCC because these warnings actually sound useful. Thanks for getting to the bottom of this. Will
