Small peep to drop simple store-updates out of loops

3 files changed, 46 insertions, 0 deletions

diff --git a/chapter23/src/main/java/com/seaofnodes/simple/node/LoadNode.java b/chapter23/src/main/java/com/seaofnodes/simple/node/LoadNode.java
index 0a9dae59..06226df0 100644
--- a/chapter23/src/main/java/com/seaofnodes/simple/node/LoadNode.java
+++ b/chapter23/src/main/java/com/seaofnodes/simple/node/LoadNode.java
@@ -52,6 +52,7 @@ public class LoadNode extends MemOpNode {
 
     @Override
     public Type compute() {
+        if( ptr()._type==Type.TOP ) return Type.TOP; // Dead pointer, not a type error.
         if( !(mem()._type instanceof TypeMem mem) )
             return _declaredType; // No memory yet?  Declared type
         assert !_declaredType.isFRef();
diff --git a/chapter23/src/main/java/com/seaofnodes/simple/node/MemPhiNode.java b/chapter23/src/main/java/com/seaofnodes/simple/node/MemPhiNode.java
index 7e54f50f..bada20ac 100644
--- a/chapter23/src/main/java/com/seaofnodes/simple/node/MemPhiNode.java
+++ b/chapter23/src/main/java/com/seaofnodes/simple/node/MemPhiNode.java
@@ -36,6 +36,11 @@ public class MemPhiNode extends PhiNode {
                 setDef(i,CodeGen.CODE.add(mem.alias(_alias)));
                 return this;
             }
+        if( nIns()==3 && region() instanceof LoopNode loop && !loop.inProgress() &&
+            in(2) instanceof StoreNode st && st.mem()==this ) {
+            Node sink = st.sink(this);
+            if( sink!=null ) return sink;
+        }
         return super.idealize();
     }
 
diff --git a/chapter23/src/main/java/com/seaofnodes/simple/node/StoreNode.java b/chapter23/src/main/java/com/seaofnodes/simple/node/StoreNode.java
index 5d3e10e5..603a0607 100644
--- a/chapter23/src/main/java/com/seaofnodes/simple/node/StoreNode.java
+++ b/chapter23/src/main/java/com/seaofnodes/simple/node/StoreNode.java
@@ -109,6 +109,46 @@ public class StoreNode extends MemOpNode {
         return null;
     }
 
+    // Phi(mem, Store(phi,ptr,off,val)) -> Store(mem,ptr,off,Phi(load(mem),val)).
+    // With no observers of the backedge store, only the last value matters.
+    // The entry load preserves the field even when the loop takes zero trips.
+    Node sink(MemPhiNode phi) {
+        phi.addDepForwards(this);
+        if( nOuts()!=1 ) return null;
+        phi.addDep(ptr());
+        // Start with a fixed field of an allocation dominating the loop.
+        // In particular, do not sink stores through a loop-varying address.
+        if( !(ptr() instanceof ProjNode p) || !(p.in(0) instanceof NewNode obj) ||
+            !(ptr()._type instanceof TypeMemPtr tmp) || tmp._obj.isAry() ||
+            !(off() instanceof ConstantNode) || err()!=null ) return null;
+        LoopNode loop = (LoopNode)phi.region();
+        // Bulk splitting identifies precise slices by Region/alias. Finish it
+        // before replacing this memory point with a Store.
+        for( Node use : loop._outputs )
+            if( use instanceof BulkMemPhiNode ) {
+                phi.addDepForwards(use);
+                return null;
+            }
+        phi.addDep(obj);
+        CFGNode ctrl = loop.entry();
+        while( ctrl!=null && ctrl!=obj.cfg0() ) {
+            phi.addDep(ctrl);
+            ctrl=ctrl.idom(phi);
+        }
+        if( ctrl==null ) return null;
+
+        Node mem = phi.in(1);
+        Node init = new LoadNode(_loc,_name,_alias,_declaredType,mem,ptr(),off()).peephole();
+        Node value = CodeGen.CODE.add(new PhiNode(_name,_declaredType,loop,init,val()).peephole());
+        Node sink = new StoreNode(_loc,_name,_alias,_declaredType,mem,ptr(),off(),value,_init);
+        // Break the memory cycle, killing the old store. Keep the Phi alive
+        // even if that store was its last use, until our caller replaces it.
+        phi.keep();
+        phi.setDef(2,mem);
+        phi.unkeep();
+        return sink;
+    }
+
     // Check that "mem" has no uses except "this"
     private boolean checkOnlyUse(Node mem) {
         if( mem.nOuts()==1 ) return true;