Autoincrement addressing modes

[Michael P. Hayes]

Jeffrey A Law writes:
 > 
 >   In message < 199802112304.XAA11969@phal.cygnus.co.uk >you write:
 >   > > At this stage I'm interested in what other common forms should be
 >   > > supported.
 >   > 
 >   > The SH doesn't have any exotic update forms, but it has another
 >   > quirk in autoincrement that gcc doesn't handle efficiently:
 >   > POST_INC is only available for reading, while PRE_DEC is only
 >   > available for writing.
 > The PA has similar constraints.

Does this asymmetry only apply to these update forms of addressing
modes?

Any ideas how to describe this to GCC?  I suppose if there was a flag
to differentiate memory reads from writes, then
GO_IF_LEGITIMATE_ADDRESS could reject the invalid forms.