doc/guide/src/Zone.adoc

   1 Zone
   2 ====
   3
   4 <:Zone:> is an optimization pass for the <:SSA2:>
   5 <:IntermediateLanguage:>, invoked from <:SSA2Simplify:>.
   6
   7 == Description ==
   8
   9 This pass breaks large <:SSA2:> functions into zones, which are
  10 connected subgraphs of the dominator tree.  For each zone, at the node
  11 that dominates the zone (the "zone root"), it places a tuple
  12 collecting all of the live variables at that node.  It replaces any
  13 variables used in that zone with offsets from the tuple.  The goal is
  14 to decrease the liveness information in large <:SSA:> functions.
  15
  16 == Implementation ==
  17
  18 * <!ViewGitFile(mlton,master,mlton/ssa/zone.fun)>
  19
  20 == Details and Notes ==
  21
  22 Compute strongly-connected components to avoid put tuple constructions
  23 in loops.
  24
  25 There are two (expert) flags that govern the use of this pass
  26
  27 * `-max-function-size <n>`
  28 * `-zone-cut-depth <n>`
  29
  30 Zone splitting only works when the number of basic blocks in a
  31 function is greater than `n`.  The `n` used to cut the dominator tree
  32 is set by `-zone-cut-depth`.
  33
  34 There is currently no attempt to be safe-for-space.  That is, the
  35 tuples are not restricted to containing only "small" values.
  36
  37 In the `HOL` program, the particular problem is the main function,
  38 which has 161,783 blocks and 257,519 variables -- the product of those
  39 two numbers being about 41 billion.  Now, we're not likely going to
  40 need that much space since we use a sparse representation.  But even
  41 1/100th would really hurt.  And of course this rules out bit vectors.