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diff --git a/chapter02/README.md b/chapter02/README.md index 444664dd..8799863f 100644 --- a/chapter02/README.md +++ b/chapter02/README.md @@ -42,6 +42,18 @@ We extend the set of nodes by adding following additional node types. | Div | Data | Divide a value by another | Two data nodes, values are divided, order matters | Result of the division | | Minus | Data | Negate a value | One data node, value is negated | Result of the unary minus | + +## *Value* equality vs *Reference* equality + +In much of Simple, Nodes are looked up (`find()` calls) via *reference* +equality. This is by far the most common case. In [Chapter +9](../chapter09/README.md) we introduce *value* equality for the first time. +In both cases the choice of value-vs-reference equality is intentional: it is +*never* correct to "just pick one or the other kind of equality". When in +doubt check the context: only *Global Value Numbering* uses value equality; +everywhere we mean reference equality. + + ## Peephole Optimizations Nodes in the graph can be peephole-optimized. The graph is viewed through a @@ -141,6 +153,7 @@ starting in [Chapter 4](../chapter04/README.md) and [Chapter There are other important properties of the Lattice that we discuss in [Chapter 4](../chapter04/README.md) and [Chapter 10](../chapter10/README.md), such as the "meet" and "join" operators and their rules. + ## Nodes Pre Peephole Optimization The following visual shows how the graph looks like pre-peephole optimization: @@ -152,10 +165,12 @@ The following visual shows how the graph looks like pre-peephole optimization: * The edges from Constants to Start are shown in dotted lines as these are not true control edges * We label each edge with its position in the node's list of inputs. + ## Post-peephole  + ## Demonstration To demonstrate how the optimisation works, let's consider the following code: ``` |