Re: redundant divmodsi4 not optimized away

[Greg McGary <greg at mcgary dot org>]

On 04/28/10 05:58, Michael Matz wrote:

> On Tue, 27 Apr 2010, Greg McGary wrote:
>    
> > (define_insn "*udivmodsi4_libcall"
> >    [(set (reg:SI 4)
> >      (udiv:SI (reg:SI 1)
> >           (reg:SI 2)))
> >     (set (reg:SI 1)
> >      (umod:SI (reg:SI 1)
> >           (reg:SI 2)))
> >     (clobber (reg:SI 2))
> >     (clobber (reg:SI 3))
> >     (clobber (reg:CC CC_REGNUM))
> >     (clobber (reg:SI RETURN_POINTER_REGNUM))]
> >    ""
> >    "call\\t__udivmodsi4"
> >    [(set_attr "length"    "4")])
> >      
> So, this pattern uses r2 and clobbers r2+r3.  Two calls in a row can't be
> eliminated because the execution of one destroys one operand of the other
> as far as GCC knows, and the necessary copies to reload the correct value
> into r2 before the second call might confuse combine/CSE/DCE/whatever.  At
> least that would be my theory to start from :)
>    

The libcall insn above appears only after reload, as the result of a 
split.  All the CSE passes occur before reload when the insn pattern is 
this:

  [(set (match_operand:SI 0 "gen_reg_operand" "=r")
    (udiv:SI (match_operand:SI 1 "gen_reg_operand" "r")
         (match_operand:SI 2 "gen_reg_operand" "r")))
   (set (match_operand:SI 3 "gen_reg_operand" "=r")
    (umod:SI (match_dup 1)
         (match_dup 2)))
   (clobber (reg:SI 1))
   (clobber (reg:SI 2))
   (clobber (reg:SI 3))
   (clobber (reg:SI 4))
   (clobber (reg:CC CC_REGNUM))
   (clobber (reg:SI RETURN_POINTER_REGNUM))]

G