Re: FW: [PATCH] [MIPS] microMIPS gcc support

[Richard Sandiford <rdsandiford@googlemail.com>]

"Moore, Catherine" <Catherine_Moore@mentor.com> writes:
> Hi Richard,
> I've attached an example patch for the add pattern that tries to
> identify the short microMIPS variants.  In your last review, you
> mentioned that you would like to see the register requirements modeled
> in the patterns.  Do you have any comments on the approach that I'm
> taking?  Thanks,

Looks good, thanks.  I was a bit surprised that we want to hide this
completely from the register allocators (by adding "!" to all of
the micromips forms) but I can see that being too honest about the
alternatives might lead to poor decisions.  That's definitely something
that could be retuned later if someone wants to.

A few bikesheddy comments:

- Please use "Y..." constraints for constants (as with "Yb", "Yh", etc.).
  I was hoping to keep "Y..." for constants and "Z..." for memory stuff. 

- You used a mixture of predicates and out-of-line functions to do
  the matching.  I think we should use the predicate approach across
  the board, because it's hard to predict which ones will come in
  useful in future.

- Predicates should always check the code though.  E.g.:

  (define_predicate "umips_addius5_imm"
    (and (match_code "const_int")
         (match_test "IN_RANGE (INTVAL (op), -8, 7)")))

- In general, please try to make the names of the predicates as generic
  as possible.  There's nothing really add-specific about the predicate
  above.  Or microMIPS-specific either really: some of these predicates
  are probably going to be useful for MIPS16 too.

  The existing MIPS16 functions follow the convention:

      "n" if negated (optional)
    + "s" or "u" for signed vs. unsigned
    + "imm"
    + number of significant bits
    + "_"
    + multiplication factor or, er, "b" for "+1"...

  It might be nice to have a similar convention for microMIPS.
  The choices there are a bit more exotic, so please feel free to
  diverge from the MIPS16 one above; we can switch MIPS16 over once
  the microMIPS one is settled.  In fact, a new convention that's
  compact enough to be used in both predicate and constraint names
  would be great.  E.g. for the umips_addius5_imm predicate above,
  a name like "Ys5" would be easier to remember than "Zo"/"Yo".

  That said，I realise things like umips_addiur2_imm_p are a bit
  hard to describe this way and might need to stay instruction-specific.

- We already have a "ks" constraint for the stack pointer.

- I like the choice of "u" for M16_REGS.  Kind of lucky that that letter's
  still free. :-) As you've probably noticed though, we're running desparately
  short of contraint letters, so if you also need names for other less
  frequently-used classes, it might be better to use "k..."  for those.

Thanks,
Richard