https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118465
Uroš Bizjak <ubizjak at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Paolo Bonzini from comment #1)
> Note %rbx is used, via %bh, to extract bits 8-15 of %r9:
>
> movl %r9d, %ebx
> movzbl %r9b, %edi
> movzbl %bh, %ebx
> movl %ebx, %r9d
>
> This could be just
>
> movzbl %r9b, %edi
> shrl $8, %r9
> movzbl %r9b, %r9d
>
> But if it's indeed due to stack alignment (didn't check), GCC is not wrong
> in noticing it can use %ebx.
%rbx is calee-saved register (so, preserved accross function calls).
These push/pop instructions are there to implement the above requirement.
