Re: [PATCH v2] arm64: Add ASM modifier for xN register operands

[Will Deacon]

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