[Bug rtl-optimization/56711] New: spectaculary bad code generated for __uint128_t

[felix-gcc at fefe dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56711

             Bug #: 56711
           Summary: spectaculary bad code generated for __uint128_t
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: felix-gcc@fefe.de


Consider this function:

size_t scan_ulong(const char* src,unsigned long int* dest) {
  register const char *tmp=src;
  register unsigned long int l=0;
  register unsigned char c;
  while ((c=*tmp-'0')<10) {
    __uint128_t x=(__uint128_t)l*10+c;
    if ((unsigned long)x != x) break;
    l=(unsigned long)x;
    ++tmp;
  }
  if (tmp-src) *dest=l;
  return tmp-src;
}

I'm compiling this with gcc -Os -c test.c on an amd64-linux box.
The code gcc generates is 92 bytes long, the one from clang only 65.  What is
happening here?  What are all that code doing that gcc is generating there?