"Moore, Catherine" <catherine_mo...@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
