NonMoving: Eliminate integer division in nonmovingBlockCount

Perf showed that the this single div was capturing up to 10% of samples
in nonmovingMark. However, the overwhelming majority of cases is looking
at small block sizes. These cases we can easily compute explicitly,
allowing the compiler to turn the division into a significantly more
efficient division-by-constant.

While the increase in source code looks scary, this all optimises down
to very nice looking assembler. At this point the only remaining
hotspots in nonmovingBlockCount are due to memory access.

1 files changed, 20 insertions, 4 deletions

diff --git a/rts/sm/NonMoving.h b/rts/sm/NonMoving.h
index 0f9cfa0e48..fdeee19d96 100644
--- a/rts/sm/NonMoving.h
+++ b/rts/sm/NonMoving.h
@@ -161,13 +161,29 @@ INLINE_HEADER unsigned int nonmovingSegmentBlockSize(struct NonmovingSegment *se
     return 1 << seg->block_size;
 }
 
-// How many blocks does the given segment contain? Also the size of the bitmap.
-INLINE_HEADER unsigned int nonmovingSegmentBlockCount(struct NonmovingSegment *seg)
+// How many blocks does a segment with the given block size have?
+INLINE_HEADER unsigned int nonmovingBlockCount(uint8_t log_block_size)
 {
-  unsigned int sz = nonmovingSegmentBlockSize(seg);
   unsigned int segment_data_size = NONMOVING_SEGMENT_SIZE - sizeof(struct NonmovingSegment);
   segment_data_size -= segment_data_size % SIZEOF_VOID_P;
-  return segment_data_size / (sz + 1);
+  unsigned int blk_size = 1 << log_block_size;
+  // N.B. +1 accounts for the byte in the mark bitmap.
+  return segment_data_size / (blk_size + 1);
+}
+
+// How many blocks does the given segment contain? Also the size of the bitmap.
+INLINE_HEADER unsigned int nonmovingSegmentBlockCount(struct NonmovingSegment *seg)
+{
+  // We compute the overwhelmingly common size cases directly to avoid a very
+  // expensive integer division.
+  switch (seg->block_size) {
+    case 3:  return nonmovingBlockCount(3);
+    case 4:  return nonmovingBlockCount(4);
+    case 5:  return nonmovingBlockCount(5);
+    case 6:  return nonmovingBlockCount(6);
+    case 7:  return nonmovingBlockCount(7);
+    default: return nonmovingBlockCount(seg->block_size);
+  }
 }
 
 // Get a pointer to the given block index