Re: [Qemu-devel] target-i386: guest variable shift by 0 provokes shift by -1

[Paolo Bonzini]

Il 18/03/2014 16:01, Peter Maydell ha scritto:
> By whether the backend or the frontend has trivial
> access to the value to be able to avoid doing bad
> things if it's out of range. If either do, they can
> do a translate-time check to avoid issues. If neither
> does then we need the host CPU architecture to be
> OK with the out of range value, or we need to insert
> an explicit mask op in the backend (which is likely
> to be a duplicate of an explicit mask op already
> inserted at the frontend to get the frontend-target
> mandated behaviour for out of range ops).

Isn't that exactly what SPARC does?  But the optimizer should be able to 
remove a redundant AND (not sure it is able to optimize & 255 & 31 to a 
single instruction, but it certainly can remove the second AND in "& 31 
& 255").

Paolo