Re: Need advice: x86 redudant compare to zero

[Andrew Pinski <pinskia at physics dot uc dot edu>]

On Oct 13, 2005, at 7:41 PM, Evan Cheng wrote:

> Hi,
>
> Here is a bits of code from bzip2:
>
> #define mswap(zz1, zz2)         { int zztmp = zz1; zz1 = zz2; zz2 = 
> zztmp; }
>
> void foo(int unLo, int unHi, int ltLo, int *ptr, char *block, int med, 
> int d, int n) {
>
>   while (1) {
>     if (unLo > unHi) break;
>     n = ((int)block[ptr[unLo]+d]) - med;
>     if (n == 0) {
>       mswap(ptr[unLo], ptr[ltLo]);
>       ltLo++; unLo++; continue;
>     };
>     if (n >  0) break;
>     unLo++;
>   }
> }


PowerPC describes the pattern were we don't need the result of n after a
compare as:
(define_insn ""
  [(set (match_operand:CC 0 "cc_reg_operand" "=x,?y")
        (compare:CC (minus:P (match_operand:P 1 "gpc_reg_operand" "r,r")
                             (match_operand:P 2 "gpc_reg_operand" 
"r,r"))
                    (const_int 0)))
   (clobber (match_scratch:P 3 "=r,r"))]
  "TARGET_POWERPC"
  "@
   subf. %3,%2,%1
   #"
  [(set_attr "type" "fast_compare")
   (set_attr "length" "4,8")])


So something like (not tested):

(define_insn ""
  [(set (reg FLAGS_REG)
        (compare (minus:P (match_operand:SI 1 "nonimmediate_operand" 
"0,0")
                           (match_operand:SI 2 "general_operand" 
"ri,rm"))
                    (const_int 0)))
   (clobber (match_scratch:SI 1 "=r"))]
  "ix86_match_ccmode (insn, CCGOCmode)
    && ix86_binary_operator_ok (MINUS, SImode, operands)"
  "sub{l}\t{%2, %0|%0, %2}"
  [(set_attr "type" "alu")
   (set_attr "mode" "SI")])

Maybe adding a pattern like that, will work?

You most likely would like to "steal" other patterns like that from the 
rs6000
backend.

Thanks,
Andrew Pinski