| 1 | Shrink |
| 2 | ====== |
| 3 | |
| 4 | <:Shrink:> is a rewrite pass for the <:SSA:> and <:SSA2:> |
| 5 | <:IntermediateLanguage:>s, invoked from every optimization pass (see |
| 6 | <:SSASimplify:> and <:SSA2Simplify:>). |
| 7 | |
| 8 | == Description == |
| 9 | |
| 10 | This pass implements a whole family of compile-time reductions, like: |
| 11 | |
| 12 | * `#1(a, b)` => `a` |
| 13 | * `case C x of C y => e` => `let y = x in e` |
| 14 | * constant folding, copy propagation |
| 15 | * eta blocks |
| 16 | * tuple reconstruction elimination |
| 17 | |
| 18 | == Implementation == |
| 19 | |
| 20 | * <!ViewGitFile(mlton,master,mlton/ssa/shrink.sig)> |
| 21 | * <!ViewGitFile(mlton,master,mlton/ssa/shrink.fun)> |
| 22 | * <!ViewGitFile(mlton,master,mlton/ssa/shrink2.sig)> |
| 23 | * <!ViewGitFile(mlton,master,mlton/ssa/shrink2.fun)> |
| 24 | |
| 25 | == Details and Notes == |
| 26 | |
| 27 | The <:Shrink:> pass is run after every <:SSA:> and <:SSA2:> |
| 28 | optimization pass. |
| 29 | |
| 30 | The <:Shrink:> implementation also includes functions to eliminate |
| 31 | unreachable blocks from a <:SSA:> or <:SSA2:> program or function. |
| 32 | The <:Shrink:> pass does not guarantee to eliminate all unreachable |
| 33 | blocks. Doing so would unduly complicate the implementation, and it |
| 34 | is almost always the case that all unreachable blocks are eliminated. |
| 35 | However, a small number of optimization passes require that the input |
| 36 | have no unreachable blocks (essentially, when the analysis works on |
| 37 | the control flow graph and the rewrite iterates on the vector of |
| 38 | blocks). These passes explicitly call `eliminateDeadBlocks`. |
| 39 | |
| 40 | The <:Shrink:> pass has a special case to turn a non-tail call where |
| 41 | the continuation and handler only do `Profile` statements into a tail |
| 42 | call where the `Profile` statements precede the tail call. |