Re: How to clean up i386 machine description?

[Michael Hayes <m dot hayes at elec dot canterbury dot ac dot nz>]

Colin Douglas Howell writes:

 > This gives me a interesting thought.  We have several requirements
 > here.  One is to write the instruction patterns so that we know what
 > type of instruction has been selected for scheduling and output
 > purposes.  A second is to tighten the operand predicates so we
 > minimize the amount of work done by reload.  Finally, we must write
 > the patterns so that reload can do a good job of fixing up operands at
 > minimal cost.  Since in many cases the type of instruction we will
 > select depends greatly on the operands, and since reload is modifying
 > the operands, these requirements may be in conflict.
 > 
 > Suppose we solve them by replacing each pattern with three sets of
 > patterns, one to be used before reload, one during reload, and one
 > after.

Hmmm, this looks like a pattern explosion!  I haven't been following
this thread too closely so I might have the wrong end of the stick.
I gather you're wanting to tighten up the operand predicates for the
x86 and are having trouble with the weird combination of valid
operands.  

If this is the case, what I suggest you do is to use the extra
condition field of the patterns to reject invalid combinations of the
operands.  I did this with the C4x target where I wanted to minimise
the amount of reloading.  On the x86 you'll probably find that the
predicates match almost anything but that there are limited
combinations of valid operands.

Michael.