From a749c244bea3e2774d453165cbd18ed52863a8f5 Mon Sep 17 00:00:00 2001
From: Cliff Click <cliffc@acm.org>
Date: Fri, 31 May 2024 16:24:00 -0700
Subject: peep dead CFG diamond (#83)

---
 chapter10/src/main/java/com/seaofnodes/simple/node/Node.java   |  2 +-
 .../src/main/java/com/seaofnodes/simple/node/RegionNode.java   | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

(limited to 'chapter10/src/main/java/com')

diff --git a/chapter10/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter10/src/main/java/com/seaofnodes/simple/node/Node.java
index 5c9b283e..bb698860 100644
--- a/chapter10/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter10/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -358,7 +358,7 @@ public abstract class Node {
     private Node deadCodeElim(Node m) {
         // If self is going dead and not being returned here (Nodes returned
         // from peephole commonly have no uses (yet)), then kill self.
-        if( m != this && isUnused() ) {
+        if( m != this && isUnused() && !isDead() ) {
             // Killing self - and since self recursively kills self's inputs we
             // might end up killing 'm', which we are returning as a live Node.
             // So we add a bogus extra null output edge to stop kill().
diff --git a/chapter10/src/main/java/com/seaofnodes/simple/node/RegionNode.java b/chapter10/src/main/java/com/seaofnodes/simple/node/RegionNode.java
index c4bd1572..42501118 100644
--- a/chapter10/src/main/java/com/seaofnodes/simple/node/RegionNode.java
+++ b/chapter10/src/main/java/com/seaofnodes/simple/node/RegionNode.java
@@ -35,6 +35,7 @@ public class RegionNode extends Node {
     @Override
     public Node idealize() {
         if( inProgress() ) return null;
+        // Delete dead paths into a Region
         int path = findDeadInput();
         if( path != 0 &&
             // Do not delete the entry path of a loop (ok to remove the back
@@ -61,6 +62,15 @@ public class RegionNode extends Node {
         // If down to a single input, become that input
         if( nIns()==2 && !hasPhi() )
             return in(1);       // Collapse if no Phis; 1-input Phis will collapse on their own
+
+        // If a CFG diamond with no merging, delete: "if( pred ) {} else {};"
+        if( !hasPhi() &&       // No Phi users, just a control user
+            in(1) instanceof ProjNode p1 &&
+            in(2) instanceof ProjNode p2 &&
+            p1.in(0)==p2.in(0) &&
+            p1.in(0) instanceof IfNode iff )
+            return iff.ctrl();
+
         return null;
     }
 
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