Hans-Peter Nilsson wrote:
> On Wed, 15 Feb 2006, DJ Delorie wrote:
> I wrote:
>
>>>Anyway, at least keep a way to add reg+reg and reg+integer, load and
>>>store of memory and load of integer and address without condition
>>>code effects and your port has a chance to avoid the related bloat.
>>
>>At least, move/load/store shouldn't touch flags.
>
>
> I may have listed more operations than necessary, but the
> important bit is that you need to form arbitrary addresses in
> the stack frame without touching flags. If for any const_int N,
> (plus reg N) is a valid address for moves to and from memory
> that doesn't touch flags, then I suppose you don't *need* an
> "add" that doesn't touch flags.
Move/Load/Store without flag is no problem. But for add, to allow
multiword add, carry is needed and I can't make it optionnal.
But as you said, I could make the load/store take 3 args, either
load rD, rB(rA)
or
load rD, imm4(rA)
with imm4 being between -16 and 15.
Another thing for memory. I can't make 8 bits access, the memory is 16
bits wide and I can't change that, so 8 bits access will have to be done
in sw.
Also, I could make the address given a word address or a byte address
(but then I would just drop the LSB since i don't support unaligned
access ... and the immediate in load/store would be each even between
-32 and 30).
DJ Delorie wrote:
> You have to think about what kind of constants are going to be common
> in your software, and plan accordingly.
I can see several types of immediates:
1) Complete arbitrary constants like filter coefficients, stuff like that.
2) Small positive/negative integers: like to increment or walk in a array
3) Single bits or grouped bits anywhere in the word (to set/test bits)
4) Power of N - 1 : To do modulo / masking.
For the class 1, not much to do about it ... Those will have to be
loaded with several operations ...
To handle class 2/3/4 in the operation taking an immediate (and that are
not mov), I was thinking of allowing a 4 bits immediate, that could be
placed in any nibble, and the nibbles on the left could either be filled
with 1 or 0, and the nibbles on the right could also be filled with 1 or
0 (independently).
So for ex 0x003f would be possible (3 in second nibble, 0-filled on the
left and 1 filled on the right). 0xfff1 also, but not 0x0370 for example ...
Now the problem is to well describe to gcc what can be taken as an
immediate and what can't ...
Anyway, thanks for youe advices ! I may not be able to follow all of
them because I also have hw constraint but I do appreciate them. It may
get sometime before I actually come to start the work on gcc (first I
have to actually do the hw and port binutils ;)
Sylvain
