diff --git a/lib/grn_io.h b/lib/grn_io.h
index 6de63bd..fa2990e 100644
--- a/lib/grn_io.h
+++ b/lib/grn_io.h
@@ -107,6 +107,8 @@ struct _grn_io {
   uint32_t count;
   uint8_t flags;
   uint32_t *lock;
+  uint32_t segment_size_mask;
+  uint8_t segment_size_shift;
 };
 
 GRN_API grn_io *grn_io_create(grn_ctx *ctx, const char *path,
diff --git a/lib/io.c b/lib/io.c
index 9913c8a..c792fef 100644
--- a/lib/io.c
+++ b/lib/io.c
@@ -161,6 +161,22 @@ grn_io_compute_file_size(uint32_t version)
   }
 }
 
+static uint8_t
+grn_io_compute_shift(uint32_t size)
+{
+  uint8_t shift = 0;
+  while (((uint64_t)1 << shift) < size) {
+    shift++;
+  }
+  return (((uint64_t)1 << shift) == size) ? shift : 0;
+}
+
+static uint32_t
+grn_io_compute_mask(uint32_t shift)
+{
+  return ((uint32_t)1 << shift) - 1;
+}
+
 static inline uint32_t
 grn_io_max_segment(grn_io *io)
 {
@@ -219,6 +235,8 @@ grn_io_create_tmp(uint32_t header_size, uint32_t segment_size,
         io->flags = GRN_IO_TEMPORARY;
         io->lock = &header->lock;
         io->path[0] = '\0';
+        io->segment_size_shift = grn_io_compute_shift(segment_size);
+        io->segment_size_mask = grn_io_compute_mask(io->segment_size_shift);
         return io;
       }
       GRN_GFREE(io);
@@ -320,6 +338,8 @@ grn_io_create(grn_ctx *ctx, const char *path, uint32_t header_size, uint32_t seg
             io->count = 0;
             io->flags = flags;
             io->lock = &header->lock;
+            io->segment_size_shift = grn_io_compute_shift(segment_size);
+            io->segment_size_mask = grn_io_compute_mask(io->segment_size_shift);
             grn_io_register(io);
             return io;
           }
@@ -592,6 +612,8 @@ grn_io_open(grn_ctx *ctx, const char *path, grn_io_mode mode)
             io->count = 0;
             io->flags = header->flags;
             io->lock = &header->lock;
+            io->segment_size_shift = grn_io_compute_shift(segment_size);
+            io->segment_size_mask = grn_io_compute_mask(io->segment_size_shift);
             if (!array_init(io, io->header->n_arrays)) {
               grn_io_register(io);
               return io;
@@ -987,10 +1009,19 @@ grn_io_win_map(grn_io *io, grn_ctx *ctx, grn_io_win *iw, uint32_t segment,
 {
   uint32_t nseg, segment_size = io->header->segment_size;
   if (offset >= segment_size) {
-    segment += offset / segment_size;
-    offset = offset % segment_size;
+    if (io->segment_size_shift) {
+      segment += offset >> io->segment_size_shift;
+      offset = offset & io->segment_size_mask;
+    } else {
+      segment += offset / segment_size;
+      offset = offset % segment_size;
+    }
+  }
+  if (io->segment_size_shift) {
+    nseg = (offset + size + segment_size - 1) >> io->segment_size_shift;
+  } else {
+    nseg = (offset + size + segment_size - 1) / segment_size;
   }
-  nseg = (offset + size + segment_size - 1) / segment_size;
   if (!size || !ctx || segment + nseg > io->header->max_segment) { return NULL; }
   iw->ctx = ctx;
   iw->diff = 0;