HCoop / hcoop / debian / mlton.git / blame
| Commit | Line | Data |
|---|---|---|
| 7f918cf1 CE |
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. |