[Bug rtl-optimization/53558] New: Register allocator doesn't tie return value register to output variable.

[ramana at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53558

             Bug #: 53558
           Summary: Register allocator doesn't tie return value register
                    to output variable.
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: ram...@gcc.gnu.org


Given the testcase below GCC generates extra moves to return value registers on
ARM v7-a linux-gnueabi . Disabling the smax insn pattern or the "generic" 3
operand condition move to doesn't really help 

This also appears to be a problem on other architectures given the register
allocator fails to merge the copy of the result value in to the result  .The
*arm_smax_insn pattern looks like the following. Debuggging the register
allocator showed that param 0,1, and 2 are in r1, r2 and r0 . However there is
probably an easier fit with r0, r2 and r0 given r0 and r2 die as well in this
instruction. I might be missing something here. 

(define_insn "*arm_smax_insn"
  [(set (match_operand:SI          0 "s_register_operand" "=r,r")
        (smax:SI (match_operand:SI 1 "s_register_operand"  "%0,?r")
                 (match_operand:SI 2 "arm_rhs_operand"    "rI,rI")))
   (clobber (reg:CC CC_REGNUM))]
  "TARGET_ARM"
  "@
   cmp\\t%1, %2\;movlt\\t%0, %2
   cmp\\t%1, %2\;movge\\t%0, %1\;movlt\\t%0, %2"
  [(set_attr "conds" "clob")
   (set_attr "length" "8,12")]
)






typedef unsigned int uint32_t;
typedef unsigned long long uint64_t;
typedef unsigned long uintptr_t;
typedef unsigned short uint16_t;
void f2(char *d, char const *s, int flags)
{
  uint32_t tmp0, tmp1;

  if (flags & 1)
    tmp0 = *s++;

  if (flags & 2)
    {
      uint16_t *ss = (void *)s;
      tmp1 = *ss++;
      s = (void *)ss;
    }

  if (flags & 1)
    *d++ = tmp0;

  if (flags & 2)
    {
      uint16_t *dd = (void *)d;
      *dd++ = tmp1;
      d = (void *)dd;
    }
}