244   /* We might have gone past the start bit, so reinitialize it.  */
         (start_bit == 0)
245   if (bi->elt1->indx != start_bit / BITMAP_ELEMENT_ALL_BITS)
246     start_bit = bi->elt1->indx * BITMAP_ELEMENT_ALL_BITS;     <===== BITMAP_ELEMENT_ALL_BITS == 128
247   
248   /* Initialize for what is now start_bit.  */
249   bi->word_no = start_bit / BITMAP_WORD_BITS % BITMAP_ELEMENT_WORDS;     <===== it's always 0
                                 <===== BITMAP_WORD_BITS == 64    BITMAP_ELEMENT_WORDS == 2

  regassign u32 %74 (constval u32 0)    <===== start_bit 初始值为0
  regassign u32 %63 (iread u32 <* <$bitmap_element_def>> 3 (regread ptr %267))    <=====  bi->elt1->indx
  ....
  brfalse @@65 (ne i32 u32 (regread u32 %63, constval u32 0))
  regassign u32 %74 (mul u32 (regread u32 %63, constval u32 128))
@@65   regassign u32 %20 (div u32 (regread u32 %74, constval u32 64))
  regassign u32 %21 (rem u32 (regread u32 %20, constval u32 2))      <====== 从哪条分支走，这里都能被折叠成0

245~246 分支合并,简化cfg  (原来的if判断无论哪条分支都是在保证  start_bit = bi->elt1->indx * BITMAP_ELEMENT_ALL_BITS  )
regassign u32 %74 (mul u32 (regread u32 %63, constval u32 128))
@@65   regassign u32 %20 (div u32 (regread u32 %74, constval u32 64))
  regassign u32 %21 (rem u32 (regread u32 %20, constval u32 2))