Re: [PING][RFC][PATCH] RL78 - Add predicates to reduce code bloat when accessing volatile memory.

[DJ Delorie <dj at redhat dot com>]

> My understanding of 'volatile' is that even the resulting code I gave as 
> an example is not actually volatile-safe:
> 
> mov    a, !240
> or    a, #32
> mov    !240, a
> 
> as the value stored in address 240 could change just before and/or just 
> after the 'or', and get overwritten by the second move.

That's not "volatile", that's "atomic".  Volatile would mean, for
example, that if the C source showed one reference to the variable,
the assembler would have exactly one reference to the variable.

If, for example, you had an SImode integer, some patterns might read
the value more than once, which would violate the volatility.  Those
patterns must not use the new predicates.

> As I understand it, one of the reasons for having the virtual pass was 
> to make the instruction set appear completely orthogonal.

The reason is that the RL78 ISA is too unorthogonal for gcc to handle
at all.  I tried, it didn't work.

> Why then, when every instruction can use every virtual register in
> any addressing mode, does it think that:
> 
> movw    r10, #240
> movw    ax, r10
> movw    hl, ax
> mov    a, [hl]
> or    a, #32
> mov    [hl], a
> 
> is more volatile-safe?

Again, volatile != atomic.  As long as the assembler references the
variables exactly the same number of times as the C source says to,
it's volatile-safe.