How to clean up i386 machine description?

[Jan Hubicka]

> Running a very close second in importance is to give the scheduler
> an accurate description of how the instruction to be emitted should
> be scheduled.  It does little good to tell the scheduler that the
> instruction is of one type when for some case something entirely
> different is emitted. 
Yes, this is one of my largest nightmares. Especially the hi mode
patterns are bad.
I am trying to split patterns where possible....
> 
> One of Jan's patches attempts to add issue information for the
> pentium to the paterns.  However, the approach he took is not
> particularly accurate, and I fear not to be scalable to even 3 
There is many inaccuraties, but it works quite well for pentium,
since we are lucky and instruction usually have same characteristics
(except the HI mode patterns). My attributes are able to recognize some
trivial cases like when lea instruction is emmited.
I am slowly working on improving this situation..

But for example how can I write attribute condition testing whether
input constant is 0x123 or so?
> As a consequence of a bit of nagging from some quarters, I've been
> thinking about this a lot recently.  The only way I can see to get
> this right is to move all of the final instruction selection logic
> out of the large output code blocks, and put it somewhere visible
> to the scheduler -- namely, tied to attribute type.  Furthermore,
> the instruction output patterns must be based exclusively on the
> external selection code.  Otherwise we can't prove convergance and
> will have lost out on that hard work.
It sounds interesting.. Maybe I should try to do this job for some patterns
later...
But to be sure what do you mean, can you give some trivial example? :)
> 
> This is a huge amount of work, amounting to a near complete rewrite
> of the .md file.  I can't see that anything less will result in
> anything other than mass confusion though.
Well I know :)) 

Honza
> 
> 
> r~

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