Re: How to clean up i386 machine description?

[Richard Henderson <rth at cygnus dot com>]

On Mon, Nov 09, 1998 at 06:51:55PM +0100, Jan Hubicka wrote:
> Well, I was thinking about implementation of this idea. But it should IMO
> require large amount of attributes and complex testing. Many tests can not
> be done in the md file conditions so special predicates needs to be wrote.
> This should result in not very well readable code.

Instruction attributes (rather than predicates) can be much more
powerful than you are giving them credit.

> Actually many alternative solutions exist. For instance in add case
> we should write special patterns for incrementation and decrementation.
> The handling of different opcode for eax should be done using multiple
> alternatives.

Special patterns for inc/dec is ok, but which_alternative cannot
handle implementation specific instruction selection.

> Also in some cases using attrubutes should be impossible. For example
> in many cases the non-prefix opcode is emited when cc0_probably_useless
> function is true. This decision is cotext depenedent.

There must be another way to solve this.  I will have to think
on this further.


r~