subroutine MPP_REDUCE_( a, pelist )
!find the max of scalar a the PEs in pelist (all PEs if this argument is omitted)
!result is also automatically broadcast to all PEs
      MPP_TYPE_, intent(inout) :: a
      integer, intent(in), optional :: pelist(0:)
      integer :: n
!work holds pWrk array + 1 word for symmetric copy of a
      MPP_TYPE_ :: work(SHMEM_REDUCE_MIN_WRKDATA_SIZE+1)
      pointer( ptr, work )
      integer :: words
      character(len=8) :: text

      if( .NOT.module_is_initialized )call mpp_error( FATAL, 'MPP_REDUCE: You must first call mpp_init.' )
      n = get_peset(pelist); if( peset(n)%count.EQ.1 )return

      if( debug .and. (current_clock.NE.0) )call SYSTEM_CLOCK(start_tick)
!allocate space from the stack for pwrk and b
      ptr = LOC(mpp_stack)
      words = size(work(:))*size(transfer(work(1),word))
      if( words.GT.mpp_stack_size )then
          write( text, '(i8)' )words
          call mpp_error( FATAL, 'MPP_REDUCE user stack overflow: call mpp_set_stack_size('//text//') from all PEs.' )
      end if
      mpp_stack_hwm = max( words, mpp_stack_hwm )
      
      work(1) = a
      call SHMEM_REDUCE_( work, work, 1, peset(n)%start, peset(n)%log2stride, peset(n)%count, work(2), sync )
      call mpp_sync(pelist)
      a = work(1)
      if( debug .and. (current_clock.NE.0) )call increment_current_clock( EVENT_ALLREDUCE, MPP_TYPE_BYTELEN_ )
      return
    end subroutine MPP_REDUCE_