grn_io: don't use division in grn_io_win_map()

[s-yata]

Description

In some cases, grn_io_win_map() is called many times and the cost of an integer division may affect the performance.

• groonga/lib/io.c

  Lines 989 to 993 in 9c8a355

  	if (offset >= segment_size) {
  	segment += offset / segment_size;
  	offset = offset % segment_size;
  	}
  	nseg = (offset + size + segment_size - 1) / segment_size;

  if (offset >= segment_size) {
    segment += offset / segment_size;
    offset = offset % segment_size;
  }
  nseg = (offset + size + segment_size - 1) / segment_size;

Hint

segment_size is generally a power of two and divisions can be replaced with a bitwise-shift or bitwise-and.

[s-yata]

I've compared a division based implementation and a bitwise-shift based implementation, but I could not find significant differences.

[s-yata]

The attached files are the patches I tested.

• avoid-division.txt
• bitwise-shift.txt

[s-yata]

Rejected!