Re: AW: AW: [avr-libc-dev] Additional instruction patterns for or more efficient code using mixed QI/HI/SI expressions

[Denis Chertykov]

Björn Haase <address@hidden> writes:

> Hy Denis,
> 
> Thank's for reviewing the code fragments
> 
> >Some your new patterns are right some are wrong. I suggest to produce
> >the work step by step with small changes.
> 
> Consider my suggestions to be the sketch of a basic idea. My suggestion
> is to concentrate on the change in the two "define_expand"s and the
> move patterns first. Possibly one might add the compare patterns as well.
> 
> >PS: Righ now I think that better to rewrite many insns for
> >    HI,SI,DI modes as splitters to QI mode (naitive AVR) insns.
> >    If you want to help then better to concentrate on this.
> 
> I agree with you in that this would be a much better solution. The only
> possible disadvantage is, in my opinion, that debugging of optimized code
> would be
> allmost impossible.

Don't worry about debugging. IMHO compiler so smart to resolve such
problems. Many ports (i386,arm, ...) uses splitting.

> 
> Before writing so many combined patterns, I have also thought about
> using UNSPECs, eg. for defining adc, sbc, etc. and use splitting of the SI
> instructions.

First ! oly add and sub needed in borrow. xor,and,or,not,neg are very
simple. You can start with one of them.

UNSPEC's is not required for this.

Look at arm port:

(define_insn_and_split "*arm_adddi3"
  [(set (match_operand:DI          0 "s_register_operand" "=&r,&r")
        (plus:DI (match_operand:DI 1 "s_register_operand" "%0, 0")
                 (match_operand:DI 2 "s_register_operand" "r,  0")))
   (clobber (reg:CC CC_REGNUM))]
  "TARGET_ARM && !TARGET_CIRRUS"
  "#"
  "TARGET_ARM && reload_completed"
  [(parallel [(set (reg:CC_C CC_REGNUM)
                   (compare:CC_C (plus:SI (match_dup 1) (match_dup 2))
                                 (match_dup 1)))
              (set (match_dup 0) (plus:SI (match_dup 1) (match_dup 2)))])
   (set (match_dup 3) (plus:SI (ltu:SI (reg:CC_C CC_REGNUM) (const_int 0))
                               (plus:SI (match_dup 4) (match_dup 5))))]
  "
  {
    operands[3] = gen_highpart (SImode, operands[0]);
    operands[0] = gen_lowpart (SImode, operands[0]);
    operands[4] = gen_highpart (SImode, operands[1]);
    operands[1] = gen_lowpart (SImode, operands[1]);
    operands[5] = gen_highpart (SImode, operands[2]);
    operands[2] = gen_lowpart (SImode, operands[2]);
  }"
  [(set_attr "conds" "clob")
   (set_attr "length" "8")]
)


> 
> So far, I, however thought that there is no easy way for teaching
> the compiler to actually use them since this would require to get
> rid of all objects with DI/SI and HI mode after the RTL generation
> step.

I mean `splitting' as define_split not as define_expand.
May be better to enable such splitters only after reload pass.

> I thought, that this would make it necessary to add a completely new
> optimizer pass. For implementing this, one would need a deeper
> knowledge of gcc interna than I have.

No special passes !

Denis.