[patch] h8300.md: Optimize stack pushes.

[Kazu Hirata]

Hi Alexandre,

> > However, as far as the hardware is concerned, the stack pointer
> > address is only required to be a multiple of 2, not 4, so we can push
> > 2 bytes, reg:HI, and then subtract 2 from the stack pointer.
> 
> What if an interrupt occurs, or a signal is delivered, between the two
> instructions?  Then the stack pointer will be misaligned.

I don't think that would be a problem unless the interrupt service
routine starts digging into the stack upward and tries to do something
with it.  The actual alignment for the stack is 2 bytes.
PUSH_ROUNDING implies 4 byte alignment on the surface, but that's is
only needed to ensure that a caller can talk to a callee.

Kazu Hirata