Chapter 7 (also 8 but not officially) (#48)

* #44 Start on chapter 7

* #44
Temp hack on printing to not break on cycles.
Parse while statement.

* #44 Remove unnecessary method

* #44 Remove unnecessary phi map

* #44 Phi should have existing nodes from both scopes as input

* #44 We need to only insert phis once for each name that is looked up. To do so,
we need to know when a named is looked up first time inside the loop body.
The phi must have its second input added later on after the loop is fully parsed.

* #44 Add phi also when update() is called.
Only add phi if name was known at loop start

* #44 Only create phi in loop body if var binding in head != body.
During finish() lookup in body directly.

* #44 Update the print output to make it similar to that in Xmilia's fork

* #44 Disable peephole for phis & region involved in while loop

* Fix broken tests

Remove "disablePeephole" set in some tests, and remaining set for other tests, so a bulk test-all fails because disablePeephole is true for all.
Reset disablePeephole in Node.reset
Switch HashSet for BitSet (touches all).

* Restructure Node class

Group printing, group node & edge manipulations, group optimizations.
Also, fix LoopRegion idom, it does not use Region idom but uses the original

* #44 Node printing

* #44 Node printing

* #44 Fix control edges in loop region
Change label of loop region
Fix the output count in dump

* #44 Revise dump output

* #44 When a LoopRegionNode is duplicated, ensure the dupe is also a LoopRegionNode

* #44 When a LoopScopeNode is merged, we merge the names set too

* #44 When a LoopScopeNode is merged, we merge the names set too

* #44 Fix print1() on RegionNode to use label()

* #44 Fix bug caused by incorrect comparison of nodes between body and head of loop; the check must be performed before any phis are created

* Revert "#44 Fix bug caused by incorrect comparison of nodes between body and head of loop; the check must be performed before any phis are created"

This reverts commit 5f729709855046c5c10904f08385fd1aec9a298a.

* #44 Fix bug caused by incorrect comparison of nodes between body and head of loop; the check must be performed before any phis are created

* #44 when we create a phi incrementally for a loop region, all scopes that are cloned from the original must get the same phi update

* #44 test case

* #44 test case

* Squashed commit of the following:

commit 15666e287e6bbc39b3fcc49939165a1e908d43b2
Author: Cliff Click <cliffc@acm.org>
Date:   Mon Nov 27 09:45:40 2023 -0800

    lazy/eager phis

    Eager phis by default, but can flip Parser.LAZY to switch to lazy phis.  Several tests golden answers change when flipping this flag due to minor node renumbering.
    Phi combines same_inputs and singleUniqueInput.
    Fancier basic-block-like printing.  Uses isMultiHead & isMultiTail which show up in many Nodes.
    Remove blanks in triple-quote strings.

commit 6ea70182a92b7db4fe5a1fdc2c4491842ef77ee5
Author: Cliff Click <cliffc@acm.org>
Date:   Sun Nov 26 11:13:22 2023 -0800

    Remove dead scopes

    Add a debug find

commit a52c06a47bf1e56866d8b4d043290838f053ad76
Author: Cliff Click <cliffc@acm.org>
Date:   Sun Nov 26 10:21:48 2023 -0800

    No constraints on backedges

    nicer loopy graphs

commit b5ab550098355f0b66bf24b40adf2e5ba4d86e4e
Author: Cliff Click <cliffc@acm.org>
Date:   Sun Nov 26 10:13:22 2023 -0800

    Fix regression

commit 30a7f5c734c686c02d3ca479914c9427b65cc469
Author: Cliff Click <cliffc@acm.org>
Date:   Sat Nov 25 21:51:47 2023 -0800

    minor cleanup

commit ad742b795ce0c93cfc82b2b11740261cfd923d02
Author: Cliff Click <cliffc@acm.org>
Date:   Sat Nov 25 21:47:05 2023 -0800

    Fixes lazy phi

    Scope now takes a 'loop' option on dupping; the dup'ed Scope is filled with "Lazy Phi" sentinels - markers to indicate a Phi is need if this variable is ever referenced.  If its never referenced, at the loop exit the "Lazy Phi" unwinds back to the original value.  Lookup and update calls lazily insert concrete Phis over "Lazy Phis" at first touch.

    Rename _allScopes to _xScopes its "active" not "all" scopes now.
    Remove try/finally block at end, because a compiler bug throws, then attempts a GraphViz on a broken graph, then throws again, hiding the original bug.
    Removed 1 extra layer of Scopes.
    Scope printing now prints in edge order; added a reversemap for printing.
    More touches to ascii printing.

* Remove flag to parseBlock

* Bugfix for lazy on empty loop body

Remove extra phi name-matching code

* Keep head alive until entire end_loop merging is done

* Create chapter 8 as a copy of chapter 7

* #44 Chapter 7 is now eager phis only (#46)

* #44 Chapter 7 is now eager phis only

* MOVE IR Printing to dedicated class to declutter Node. Remove Ary (#49)

* #44 Move IR printing to dedicated class to declutter Node, and remove Ary

* #44 refactor Node.delUse() to use common del()

* #44 Some cleanups

* #44 Some cleanups

* #44 Some cleanups

* #44 Some cleanups

* Revert "#44 Some cleanups"

This reverts commit 2702c338c8bdaa5ef2128000e26f1a8496baf34a.

* #44 Some cleanups

* #44 Some cleanups

* #44 Some cleanups

* #44 Fix typo

* #44 Fix markdown formatting

* #44 Remove line numbers from code snippets

---------

Co-authored-by: Cliff Click <cliffc@acm.org>

1 files changed, 227 insertions, 0 deletions

diff --git a/chapter07/README.md b/chapter07/README.md
new file mode 100644
index 00000000..c6bcdee4
--- /dev/null
+++ b/chapter07/README.md
@@ -0,0 +1,227 @@
+# Chapter 7
+
+In this chapter we introduce the `while` statement.
+
+Here is the [complete language grammar](docs/07-grammar.md) for this chapter.
+
+## Dealing With Back Edges
+
+The complication introduced by looping constructs is that variables flow back into the body of the loop on iteration.
+For example:
+
+```java
+while(arg < 10) {
+    arg = arg + 1;
+}
+return arg;
+```
+
+The variable `arg` is assigned a new value inside the body of the loop, and this then flows back into the body of the loop.
+
+In general, we will rewrite the looping construct as follows:
+
+```java
+loop_head:
+if( !(arg < 10) ) 
+    goto loop_exit;
+arg = arg + 1;
+goto loop_head;
+
+loop_exit:
+```
+
+Above is for illustration only, we do not have labels and `goto` statements in the language.
+
+From an SSA[^1] point of view, since `arg` flows back, it requires a `phi` node at the head. Conceptually we would like the outcome to be:
+
+```java
+// arg1 represents incoming arg
+//
+loop_head:
+arg2 = phi(arg1, arg3);
+if( !(arg2 < 10) ) 
+    goto loop_exit;
+arg3 = arg2 + 1;
+goto loop_head;
+
+loop_exit:
+```
+
+Notice that the phi for `arg2` refers to `arg3`, which is not known at the time we parse the `while` loop predicate. This is the crux of the problem that we need 
+to solve in order to successfully construct the Sea of Nodes graph, which is always in SSA form.
+
+Recall from [Chapter 5](../chapter05/README.md) that when parsing `if` statements, we clone the symbol tables as we go past the `if` predicate.
+Later we merge the two sets of symbol tables at a `Region` - creating phis for all names that encountered a change in definition within the two 
+branches of the `if` statement. In the case of the `if` statement, the phis are created at the merge point when we already know the definitions
+that are being merged.
+
+The essential idea for loop constructs is to eagerly create phis for all names in the symbol tables *before* we enter the loop's `if` condition,
+since we do not know which names will be redefined inside the body of the loop. When the loop terminates, we go back and remove unnecessary
+phis. We call this approach the "eager phi" approach.[^2]
+
+In [Chapter 8](../chapter08) we will implement a "lazy phi" approach that creates phis only when we encounter redefinition of a name.
+
+## New Node Types
+
+Our list of nodes remains the same as in [Chapter 5](../chapter05/README.md), however, we create a subtype of `Region` named `Loop` to better
+encapsulate some of the additional logic required. A key aspect of this is to temporarily disable peepholes of the `Region` and any phis
+created until we complete parsing the loop body. This is because our phis are not fully constructed until the loop end.
+
+## Detailed Steps
+
+1. We start by creating a new subclass of `Region`, the `Loop`. The `Loop` gets two control inputs, 
+   the first one is the entry point, i.e. the current binding to `$ctrl`, and second one (`null`) is a placeholder for the back edge that is 
+   set after loop is parsed. The absence of a back edge is used as an indicator to switch off peepholes of the region and
+   associated phis. 
+
+    ```java
+    ctrl(new LoopNode(ctrl(),null).peephole());
+    ```
+
+   The newly created region becomes the current control.
+
+2. We duplicate the current Scope node. This involves duplicating all the symbols at
+   every level with the scope, and creating phis for every symbol except the `$ctrl` binding.
+
+    ```java
+    // Make a new Scope for the body.
+    _scope = _scope.dup(true);
+    ```
+   
+   Note that the `dup` call is given an argument `true`. This triggers creating phis. The code
+   that creates the phis in the `dup()` method is shown below.
+
+    ```java
+    // boolean loop=true if this is a loop region
+    
+    String[] reverse = reverse_names();
+    dup.add_def(ctrl());      // Control input is just copied
+    for( int i=1; i<nIns(); i++ ) {
+       if ( !loop ) { dup.add_def(in(i)); }
+       else {
+          // Loop region
+          // Create a phi node with second input as null - to be filled in
+          // by endLoop() below
+          dup.add_def(new PhiNode(reverse[i], ctrl(), in(i), null).peephole());
+          // Ensure our node has the same phi in case we created one
+          set_def(i, dup.in(i));
+       }
+    }
+    ```
+   
+   Inside the `else` block both the duplicated scope and the original scope get the same phi node.
+
+3. Next we set up the `if` condition, very much like we do with regular ifs.
+
+    ```java
+    // Parse predicate
+    var pred = require(parseExpression(), ")");
+    // IfNode takes current control and predicate
+    IfNode ifNode = (IfNode)new IfNode(ctrl(), pred).<IfNode>keep().peephole();
+    // Setup projection nodes
+    Node ifT = new ProjNode(ifNode, 0, "True" ).peephole();
+    ifNode.unkeep();
+    Node ifF = new ProjNode(ifNode, 1, "False").peephole();
+    ```
+
+4. We set the control token to the `True` projection node `ifT`, but before that we make another clone of 
+   the current scope. 
+
+    ```java
+    // The exit scope, accounting for any side effects in the predicate
+    var exit = _scope.dup();
+    exit.ctrl(ifF);
+    ```
+
+   The new scope is saved as the exit scope that will live after the loop ends, therefore `$ctrl` in the exit scope is 
+   set to the `False` projection. The exit scope captures any side effects of the loop's predicate.
+
+5. We now set the control to the `True` projection and parse the loop body.
+
+    ```java
+    // Parse the true side, which corresponds to loop body
+    ctrl(ifT);              // set ctrl token to ifTrue projection
+    parseStatement();       // Parse loop body
+    ```
+
+6. After the loop body is parsed, we go back and process all the phis we created earlier.
+   
+    ```java
+    // The true branch loops back, so whatever is current control gets
+    // added to head loop as input
+    head.endLoop(_scope, exit);
+    ```
+
+   The `endLoop` method sets the second control of the loop region to the control from the back edge.
+   It then goes through all the phis and sets the second data input on the phi to the corresponding entry
+   from the loop body; phis that were not used are peepholed and get replaced by the original input.
+
+    ```java
+    Node ctrl = ctrl();
+    ctrl.set_def(2,back.ctrl());
+    for( int i=1; i<nIns(); i++ ) {
+       PhiNode phi = (PhiNode)in(i);
+       assert phi.region()==ctrl && phi.in(2)==null;
+       phi.set_def(2,back.in(i));
+       // Do an eager useless-phi removal
+       Node in = phi.peephole();
+       if( in != phi )
+          phi.subsume(in);
+    }
+    ```
+
+7. Finally, both the original scope (head) we started with, and the duplicate created for the body are killed.
+   At exit the false control is the current control, and exit scope is set as the current scope.
+
+### Visualization
+
+The example quoted above is shown below at an intermediate state:
+
+![Graph1](./docs/07-graph1.svg)
+
+* Three Scopes are shown, reading clockwise, the loop head, exit and the body.
+
+The final graph looks like this:
+
+![Graph2](./docs/07-graph2.svg)
+
+## More Complex Examples
+
+### Nested Loops
+
+```java
+int sum = 0;
+int i = 0;
+while(i < arg) {
+    i = i + 1;
+    int j = 0;
+    while( j < arg ) {
+        sum = sum + j;
+        j = j + 1;
+    }
+}
+return sum;
+```
+
+![Graph2](./docs/07-graph3.svg)
+
+### Loop With Nested If
+
+```java
+int a = 1;
+int b = 2;
+while(a < 10) {
+    if (a == 2) a = 3;
+    else b = 4;
+}
+return b;
+```
+
+![Graph2](./docs/07-graph4.svg)
+
+
+[^1]: Cytron, R. et al (1991).
+    Efficiently computing static single assignment form and the control dependence graph, in ACM Transactions on Programming Languages and Systems, 13(4):451-490, 1991.
+
+[^2]: Click, C. (1995).
+    Combining Analyses, Combining Optimizations, 103.
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