Re: [Qemu-devel] [PATCH] util/cacheinfo: Fix warning generated by clang

[Richard Henderson]

On 07/01/2017 03:30 PM, Peter Maydell wrote:
> On 1 July 2017 at 23:20, Richard Henderson <address@hidden> wrote:
> > On 06/30/2017 08:39 AM, Pranith Kumar wrote:
> > > Clang generates the following warning on aarch64 host:
> > >
> > >     CC      util/cacheinfo.o
> > > /home/pranith/qemu/util/cacheinfo.c:121:48: warning: value size does not
> > > match register size specified by the constraint and modifier
> > > [-Wasm-operand-widths]
> > >           asm volatile("mrs\t%0, ctr_el0" : "=r"(ctr));
> > >                                                  ^
> > > /home/pranith/qemu/util/cacheinfo.c:121:28: note: use constraint modifier
> > > "w"
> > >           asm volatile("mrs\t%0, ctr_el0" : "=r"(ctr));
> > >                              ^~
> > >                              %w0
> >
> >
> > That is an absolutely stupid warning.  There's long precedent for the
> > compiler choosing the prefix for you based on the type of the argument.
>
> Isn't that the problem? The type of the argument says "32 bits"
> but the instruction here really wants 64 bits (MRS takes Xn, not Wn).

The warning is telling me to use %w to force Wn.  So if the assembler really 
doesn't like Wn, the warning is a bit more than confusing.

Perhaps it ought to be telling me to use %x to force Xn in spite of the type?


r~