diff options
Diffstat (limited to 'chapter06/README.md')
| -rw-r--r-- | chapter06/README.md | 15 |
1 files changed, 9 insertions, 6 deletions
diff --git a/chapter06/README.md b/chapter06/README.md index 3442b1cc..14e21bbc 100644 --- a/chapter06/README.md +++ b/chapter06/README.md @@ -1,10 +1,13 @@ # Chapter 6: Peepholes for If +[Previous: Chapter 5](../chapter05/README.md) | +[Next: Chapter 7](../chapter07/README.md) + # Table of Contents 1. [Type System Revision](#type-system-revision) 2. [Peephole of `if`](#peephole-of-if) -3. [`Region` and `Phi`](#Region-and-Phi) +3. [`Region` and `Phi`](#region-and-phi) 4. [Discussion](#discussion) 5. [Expanding Peephole Beyond Constant Expressions](#expanding-peephole-beyond-constant-expressions) 6. [Dominators](#dominators) @@ -48,7 +51,7 @@ The entire revised Lattice is shown below: parsing, the normal graph building machinery takes care of cleaning up dead nodes. * When we peephole an `If` node we test if the predicate is a constant. If so, - then one branch or the other dead, depending on if the constant is `0` or not. + then one branch or the other is dead, depending on if the constant is `0` or not. * When we add the `Proj` nodes, in this scenario, we already know one of the projections is dead. The peephole of the relevant `Proj` node replaces the `Proj` with a `Constant` of type "~Ctrl" indicating dead control. * At this point our dead code elimination would kill the `If` node since it has no uses @@ -62,7 +65,7 @@ The entire revised Lattice is shown below: * The parsing then continues as normal. * The other changes are when we reach the merge point with a dead control. * Again keeping with our strategy we merge as normal, including creating `Phi` nodes. -* The `Region` may have have one of its inputs as "~Ctrl". This will be seen +* The `Region` may have one of its inputs as "~Ctrl". This will be seen by the `Phi` which then optimizes itself. If the `Phi` has just one live input, the `Phi` peephole replaces itself with the remaining input. * Finally, at the end, when the `Region` node is peepholed, we see that it has only one live input and no Phi uses @@ -70,7 +73,7 @@ The entire revised Lattice is shown below: ## `Region` and `Phi` -One of the invariants we maintain is that the for each control input to a +One of the invariants we maintain is that for each control input to a `Region` every `Phi` has a 1-to-1 relationship with a data input. Thus, if a `Region` loses a control input, every `Phi`'s corresponding data input must be deleted. Conversely, we cannot collapse a Region until it has no dependent @@ -122,7 +125,7 @@ After peephole: ## Expanding Peephole Beyond Constant Expressions While our peephole of the `CFG` collapses dead code when the expression is -a compile time constant. it does not do as well as it could with following example: +a compile-time constant, it does not do as well as it could with the following example: ```java int a = 0; @@ -188,7 +191,7 @@ the outcome of the present `if` is fully determined. The code that does this check is in the [`compute` method of the `if`](https://github.com/SeaOfNodes/Simple/blob/main/chapter06/src/main/java/com/seaofnodes/simple/node/IfNode.java#L38-L45). The `idom` method in `Node` provides a default implementation of the depth based immediate dominator calculation. -`Region`'s [override this](https://github.com/SeaOfNodes/Simple/blob/main/chapter06/src/main/java/com/seaofnodes/simple/node/RegionNode.java#L57-L74) to implement the more complex search described above. +`Region` [overrides this](https://github.com/SeaOfNodes/Simple/blob/main/chapter06/src/main/java/com/seaofnodes/simple/node/RegionNode.java#L57-L74) to implement the more complex search described above. It turns out that to simplify the example above, we need a [further peephole in our `Add` node](https://github.com/SeaOfNodes/Simple/blob/main/chapter06/src/main/java/com/seaofnodes/simple/node/AddNode.java#L64-L67). |