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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. |