RTL context in machine description files

[Jeff Hammond <j.hammond AT cputech dot com>]

I adding a processor that is similar to an existing one 
(1750a) so I am modifying the relevant files.  This
is for a compiler only, I only need gcc to generate
the assembly code.

In the machine description file how do you know what 
to expect in the operands array?  I know that this is 
partly determined by the constriants in match_operand.
However not all 'm' constraints generate the same
rtx's.  I figure that some of this is determined by what
the compiler it trying to do at the time.  How do I find
out what the range of possibilities are and what each
one means?

Thanks,

Jeff Hammond 

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