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diff --git a/chapter04/README.md b/chapter04/README.md index 8bf29f94..e71ff8a7 100644 --- a/chapter04/README.md +++ b/chapter04/README.md @@ -352,16 +352,21 @@ The peephole method does following: `peephole()` calls `deadCodeElim()` which is shown below. -## Dead Code Elimination(DCE) -We call DCE every time we lose a use of a Node; when replacing via peeps and exciting scopes. -DCE recursively deletes Nodes: +## Dead Code Elimination(DCE) -- `delUse` deletes a Node use; if the use count goes to zero recursively. -- `kill` deletes all inputs on self, while calling `delUse` to remove the matching use. +We call `kill` every time a Node makes the 1-user-to-0-users transition during +graph editing. This commonly happens when replacing via peeps and exiting +scopes. This recursive `kill` amounts to Dead Code Elimination in many other +compilers; here we call it in a very fine-grained way. Many Nodes will be +created by the parser, only to be killed before the very next lexeme is parsed. For more information please read the [appendix section](appendix/dce.md#appendix) on dead code elimination. +The suggestively called `deadCodeElim` function is a thin wrapper over `kill` +used by `peepholeOpt` to avoid prematurely killing a Node which only briefly +makes the 1->0 users transition. + ```java // m is the new Node, self is the old. // Return 'm', which may have zero uses but is alive nonetheless. |